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Metabolic Diseases of the Liver
69: 1108-1120, 1975 Copyright© 1975 by The Williams & Wilkins Co.
Vol. 69, No.5 Prin ted in U.S.A.
GASTROENTEROLOGY
WORK GROUP V
Metabolic Diseases of the Liver I. Pathophysiology of liver diseases A. Bilirubin metabolism and hepatic transport B. Ascites and renal failure C. Hepatic coma D. Portal hypertension E . Hepatic cholesterol metabolism F. Liver function tests II. Bile Secretion and cholestasis III. Ethanol and alcoholic liver disease IV. Nutrition A. Vitamins B . Proteins C. Trace elements V. Genetic defects A. Hemochromatosis B. Copper and the liver C. Hepatic storage diseases D. Porphyria VI. Pediatric liver disease A. Liver disease in infants B. Neonatal hyperbilirubinemia VII. Hepatic drug metabolism VIII. Priorities
I. Pathophysiology of Liver Diseases A. Bilirubin Metabolism and Hepatic Transport Hyperbilirubinemia and jaundice are commonly observed features of acquired or congenital diseases of the liver and biliary tract. In addition, hyperbilirubinemia in the newborn period represents an important potential hazard for the development of bilirubin encephalopathy. There are major uncertainties regarding the mechanisms and control of pigment production and disposal in healthy individuals and in patients with various forms of jaundice. Moreover, the pathogenesis and preventon of bilirubin encephalopathy in the newborn period are incompletely understood. In the liver, bilirubin shares transport mechanisms with other organic anions, including products of intermediary me-
tabolism, and exogenously administered hormones, drugs , dyes, and carcinogens. A clearer comprehension of bilirubin metabolism at the level of the liver should reveal important information regarding plasma to bile transport of a variety of biologically important compounds. There are five areas in which research efforts are needed: (1) bilirubin formation; (2) transport of bilirubin in plasma ; (3) hepatic uptake of bilirubin and other organic anions; (4) hepatic endoplasmic reticulum; (5) interdisciplinary efforts. 1. Bilirubin formation. Although the sources of bilirubin and the major metabolic pathway for its formation are defined, little is known of the metabolic regulation of this pathway. Moreover, present methods for measuring the relative contribution of various heme pools to over-all production of bilirubin are inadequate and should be refined . Turnover of the various
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cytochromes in the microsomal fraction of solved and complex problems relating to the liver must be determined before biliru- hepatic handling of bilirubin. bin formation in the liver can be estimated. Most current and future research efforts The relationship between hepatic drug me- should aim for a better understanding of tabolism, conjugation of bilirubin, biliru- the biochemical mechanisms by which the bin formation, and transport in the liver liver transfers bilirubin and other organic has not been elucidated under physiologi- anions from the plasma into the bile. This cal conditions and in various diseases of understanding will make it possible to alter the liver. deranged hepatic transport mechanisms by 2. Transport of bilirubin in plasma. The pharmacological or dietary means. binding of bilirubin and other organic anB. Ascites and Renal Failure ions to albumin and to other transport proteins is of critical importance for the Ascites is the most common major metabolic fate and disposition of these chronic complication of advanced liver discompounds. Studies are lacking of the ease. Renal circulatory failure related to basic structure of the organic anion bind- liver disease is nearly always fatal and ing sites on albumin and the effect of nearly always occurs in association with various diseases on this binding mecha- ascites or with fulminant hepatic failure . While several factors have been implicated msm . 3. Hepatic uptake of bilirubin and other in the formation of ascites and of renal organic anions. The liver has the unique failure, the underlying pathogenetic mechproperty of extracting bilirubin and other anisms are poorly understood . Diseases of the liver, especially cirrhosis, organic anions from the plasma and either metabolizing or excreting them in the bile. rank among the commonest causes of At least two soluble prbtein fractions have death in the United States. An underbeen identified in the liver which seem to standing of the mechanisms which lead to be involved in this uptake, but their kinetic formation of ascites and renal failure is role awaits characterization and analysis. indispensable for treatment of fatal condiFurthermore, it is not known whether a tions of the liver. specific facilitated transport mechanism is In the pathogenesis of ascites, several present at the level of the hepatic plasma factors seem to play a role, including reduced plasma colloid osmotic pressure, membrane. 4. Hepatic endoplasmic reticulum. The elevated portal venous pressure, alteraenzyme UDP-glucuronyltransferase and tions in lymph flow from the liver, and other microsomal enzymes are involved in disorders of water and electrolyte metabothe conjugation of bilirubin with carbohy- lism. Treatment and prevention of ascites drate moieties. Whether one or several is difficult because of current ignorance enzymes are involved in the formation of about the relative importance of and interglucuronides is uncertain. Another area of relation between these factors . A major research effort should be research is the structure-function relationship of the endoplasmic reticulum of the mounted to better understand the relative liver in the transport of bilirubin and other importance of factors which are involved in organic anions. This is particularly impor- the formation of ascites. This requires a tant if we hope to understand the effect of more precise definition of the role of the barbiturates and related drugs on the liver in the regulation of fluid and electrolyte balance under normal conditions. It serum bilirubin level. 5. Interdisciplinary efforts. Many of the would be valuable to have an animal model problems listed above require the coopera- in which formation of ascites and renal tion of investigators with various back- failure could be studied under experimengrounds including physiology, cell biology, tal conditions. Total renal blood flow and biochemistry, pharmacology, and veteri- relative renal blood f1ow within the various nary medicine. It is evident that only a compartments of the kidney should be team approach can clarify various un- investigated during hepatic failure.
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Most of these studies require the cooperation of investigators with special competence in liver physiology, renal physiology, transport mechanisms, and hemodynamics. The understanding of pathogenetic mechanisms underlying ascites formation and renal failure in chronic liver disease will permit formulation of a rational approach to the treatment of these conditons. C. Hepatic Coma The pathogenesis of diffuse metabolic encephalopathy which frequently accompanies advanced liver disease is still unknown. Hepatic encephalopathy may occur with advanced chronic liver disease or may be a part of acute fulminant liver failure. Approximately one-third of patients with chronic liver failure develop hepatic encephalopathy during the terminal stage of their disease. In addition, acute fulminant hepatitis almost invariably leads to hepatic coma and has a mortality of about 80%.
Although three types of hepatic coma can be separated, namely acute fulminant, chronic recurrent, and hepatocerebral degeneration, the pathogenesis of all three is unknown. Ammonia is believed to play a major role in the development of coma, but the importance of other toxins has not been studied adequately. The recent suggestion that false neurotransmitters may play a role opens an exciting avenue for new investigation. The treatment of hepatic encephalopathy is still unsatisfactory, although the exclusion of proteins together with the use of antibiotics is beneficial. Various heroic measures, such as exchange transfusion, have been used but are of limited value. Studies of the pathogenesis of hepatic coma are urgently needed and should be directed toward investigating blood flow and oxygen consumption in the brain and cerebral metabolism , particularly utilization of glucose and short chain fatty acids in hepatic coma. Moreover, the possibility that false neurotransmitters accumulate in the brain should be investigated, perhaps as a first step by analyzing the cerebrospinal fluid. It would be most helpful to have an animal model in which controlled stud-
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ies could be carried out. Finally, the role of ammonia in the pathogenesis of hepatic coma requires further definition. The present therapeutic approach to hepatic encephalopathy is partially successful, but largely empirical. The value of protein restriction, antibiotics and lactulose should be defined and the role of precipitating factors elucidated. It is evident that progress in this area can be made only by an interdisciplinary approach which includes experts in intermediary metabolism, neurology, and physiology, in addition to experts in liver disease. Development of an artificial liver, analogous to the artificial kidney, may offer hope that survival of patients with acute hepatic coma may be improved. It is realized, however, that the technological problems involved in designing an artificial liver are extensive and preliminary experience with available prototypes has revealed a number of shortcomings which require correction and modification before this approach appears promising. D. Portal Hypertension Portal hypertension complicates most forms of chronic liver disease. Although increased resistance to flow of blood through the liver appears to be the major cause, other contributing factors have not been excluded. Esophageal varices resulting from portal hypertension are an important source of intestinal bleeding, and are believed to be the cause of death in a significant fraction of patients with alcoholic, postnecrotic, or cryptogenic cirrhosis. Moreover, portal hypertension is an important contributory factor to the formation of ascites and, perhaps thereby, to renal insufficiency in these patients. Although by definition chronic liver disease usually is associated with a deranged hepatic architecture, few studies have been performed to correlate hepatic vascular structure with portal hypertension . Most of the present research on portal hypertension is concerned with surgical treatment. The lack of a suitable animal model prevents conducting these studies under controlled conditions. Controlled clinical trials of the various
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surgical methods for the relief of portal hypertension are important and should be carried out in many clinical centers. In addition, it is essential to identify the nature of the vascular changes in the liver which lead to portal hypertension. This will require correlative studies between pathologists and physiologists and should be based on the development of newer and more sensitive methods for measuring hepatic blood flow . The long range goals should be directed towards the prevention of portal hypertension in the course of chronic liver disease. Moreover, clear criteria should be worked out which would permit better identification of patients who may benefit from surgical correction of portal hypertension.
E. Hepatic Cholesterol Metabolism Knowledge about the role of the intestinal tract and the liver in controlling hepatic cholesterol synthesis is incomplete . Moreover, in cholestasis, hepatic synthesis is greatly accelerated, suggesting that the normal feedback control of this metabolic pathway in inoperative. Many forms of acute and chronic liver disease are associated with increased plasma cholesterol levels. Since these patients incur an added risk for developing atherosclerosis, better comprehension of the mechanisms regulating plasma cholesterol are of prime importance. There is also evidence that hypercholesteremia of liver disease may be related to an increased incidence of gallstones, and this relationship should be defined. Intensive studies on the role of cholesterol and bile acids in the regulation of hepatic cholesterol synthesis are being pursued. There is evidence that hepatic cholesterol synthesis undergoes diurnal variation and is influenced by dietary factors. Efforts are being made to characterize the transport of cholesterol from the intestinal lumen to the plasma, including identification of possible cholesterol and sterol binding proteins in the cytoplasm of the liver and in the intestinal mucosa. The major effort in hepatic cholesterol metabolism should be directed toward elucidation of the control of cholesterol
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synthesis within the liver and the intestinal mucosa. This requires clarification of the role of sterol carrier proteins in the liver and gut and a definition of the role of bile acids in this transport. While most of the necessary methods for these studies are available, there remains a great need for cooperative efforts between specialists with expertise in protein synthesis, lipid metabolism, transport mechanisms and clinical liver disease. The major long range goal will be to identify the factors which regulate cholesterol synthesis under physiological conditions and to study the abnormalities in this regulatory mechanism in various forms of liver disease, including cholestasis. This information would allow development of a rational therapy.
F. Liver Function Tests Liver function tests are designed to evaluate the effect of various forms of liver disease on the functional state of the organ. While most tests reflect biochemical or physiological functions of the liver, they usually are insensitive and detect only gross injury to individual functions of the hepatic cell. Presently available laboratory approaches are inadequate to assess small degrees of hepatic dysfunction, and they do not permit identification of incipient liver disease or minor functional abnormalities. This is especially important in the preoperative work-up of patients undergoing elective surgery. Work over the last few years has produced little progress in this area in that batteries of existing tests are applied to many liver diseases without acquiring significant new information. Although some new tests have been developed, they usually represent variants of existing tests and generally add little in the direction of refined assessment. The correlation between hepatic histology, obtained using needle biopsy, and the functional assessment of the liver often is poor. The greatest need is for more sensitive and more specific liver function tests. These tests should be based on metabolic functions which are specific for the liver and whose derangement can be detected
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early and by relatively simple means of analysis. Ideal measurements of hepatic function should reflect not only individual metabolic function, but also should allow assessment of the number of liver cells available for a specific function. It is unknown, for example, whether decreased hepatic function, as measured with presently available tests, reflects reduced metabolic activity of all liver cells, or an absolute reduction in the number of normally functioning liver cells.
II. Bile Secretion and Cholestasis This subject requires an understanding of the mechanisms by which water, inorganic electrolytes, and a number of organic compounds are moved from the hepatic circulation into the bile. This information will also be of value to a variety of allied areas , including pharmacotherapeutics and the radiological visualization of bile ducts and the gallbladder. At present, four different mechanisms for secretion of various bile components have been identified. To a limited extent, it is possible to influence .these four secretory mechanisms with drugs. Abnormalities in these secretory mechanisms have been correlated with various clinical states, but morphology at the light-microscopic and electron-microscopic levels has provided little insight into the mechanism of bile formation. Elucidation of the sites and mechanisms of bile formation and their modification by disease is lacking. It will be necessary to relate bile secretion to various intracellular processes, including the function of the endoplasmic reticulum and the Golgi apparatus. The role of bile acids and other organic constituents of the bile on bile secretion has not been clarified. Finally, better methods should be developed for radiological visualization of the gallbladder and bile ducts for the diagnosis of diseases of the biliary tract. Micropuncture , microdissection, and perfusion techniques analogous to those which have been used successfully in the kidney, should be adapted for the liver. In addition, development of methods to culture normal nondividing liver cells would
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permit investigation of individual cell functions related to hepatic excretion. Advances in the aforementioned areas would increase understanding of the mechanisms by which bile is formed and would lead to a better appreciation of the abnormalities underlying various forms of acute and chronic liver disease.
III. Ethanol and Alcoholic Liver Disease There are two major questions about the effect of ethanol on the liver. (1) By what mechanisms does alcohol induce acute and chronic liver disease? (2) What is responsible for the apparent susceptibility of only a limited segment of the drinking population to severe alcohol-induced liver disease? Alcoholic liver disease is one of the major causes of morbidity and death in the United States. Current estimates are that there are 6.5 million alcoholics in the United States and about 8000 deaths per year attributable to alcoholic cirrhosis of the liver. Three types of alcoholic liver disease have been identified, namely fatty liver, alcoholic hepatitis, and cirrhosis. Progress has been made in identifying the source of the fat accumulating in the liver, but the relationship of this process to the development of hepatitis, necrosis, and eventually cirrhosis is not clear. Although dietary factors have been shown to play a role in the amount and type of lipids that accumulate in the liver of alcoholic patients, the relative role of malnutrition in the pathogenesis of alcoholic hepatitis and cirrhosis is uncertain. Animal models have not proven useful in the past; however, alcoholic hepatitis can be induced in baboons and may be a valuable model in future work. Much effort has been directed to identifying mechanisms by which alcohol is metabolized by the liver. While the role of alcohol dehydrogenase has been demonstrated beyond doubt, there is uncertainty to what extent alcohol also may be metabolized by a microsomal ethanoloxidizing system. This is important because of the demonstrated interdependence between alcohol and drug metabolism in the liver.
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Areas of need include identification of the characteristics of individuals who abuse alcohol and develop liver disease. Since this is only about 10% of all individuals who abuse alcohol, the biochemical and possibly genetic characteristics of this population subgroup should be studied and analyzed. Another major effort should be elucidation of the pathogenetic mechanism of alcoholic hepatitis; this study will require detailed information on the effect of alcohol on subcellular organelles of hepatocytes. Another important area is a description of the natural history of alcoholic hepatitis, particularly its regression on abstinence and its progression to cirrhosis on continued consumption of ethanol. Current and future therapeutic modalities should be investigated with regard to their effectiveness in reversing alcoholic hepatitis and in arresting its progression to cirrhosis. This will require better comprehension of both the metabolism of alcohol in the liver and of its effect on individual liver cell functions and structure.
IV. Nutrition
A . Vitamins Deficiency of individual vitamins or groups of vitamins may play a pathogenetic role in chronic liver disease. In addition, many patients with chronic liver disease develop and manifest signs and symptoms of vitamin deficiency. Major unsolved problems in this area are related to the lack of reliability, sensitivity and specificity of the present assay techniques for vitamins, both in tissues and in body fluids, and to the difficulty in assessing the ability of the liver to convert vitamin precursors into the biologically active form s of the vitamins. Another important area of uncertainty is the role of a deficiency or excess of vitamins in modifying the susceptibility of the liver to infectious or toxic agents . Finally, little is known about the effect which liver disease may have on basic vitamin requirements . Many patients with chronic liver disease, particularly due to alcoholism , manifest symptoms of vitamin deficiency. It is evident, however, that consumption of vi-
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tamins, and the costs for procuring them , by far exceed the amount that can be justified by demonstrable deficiency. It is vitally important, therefore, to establish proper base lines and a rational approach to vitamin therapy in patients who either have chronic liver disease or who are at high risk of toxic or infectious liver injury. Deficiency of folate and vitamin B, 2 has been demonstrated in many patients with chronic liver disease and particularly in alcoholics. Folic acid deficiency has been shown to induce morphological and functional alterations of the hepatocyte . Deficiency of riboflavin and nicotinic acid has been implicated in the development of fatty liver. Thiamine depletion , as reflected by peripheral neuropathy, encephalopathy, and alterations in carbohydrate metabolism , is frequently observed in chronic liver disease. Little information is available on the possible effects of pantothenate deficiency on liver metabolism . Deficiency of vitamin B 6 is known to interfere with protein and nucleic acid metabolism in the liver. The fat-soluble vitamin A is required for the functional stability of the lysosomal membrane which may have important bearing on some forms of toxic liver injury. Vitamin Dis converted to one of its active forms by the hepatic microsomal enzyme system . A deficiency of vita min K is frequently observed in chronic liver disease and results in defective synthesis of essential clotting factors. The major need for addit ional and more detailed information is related to the role of various water- and fat-soluble vitamins in the maintenance of normal liver function and structure. This requires not only more sensitive and more reliable assay methods at the microquantitative level but also a better understanding of the individual metabolic functions in which the various vitamins are involved. This information would permit us to assess the importance of individual vitamins in the prevention and repair of liver injury.
B. Proteins Availability of exogenous proteins is essential for the maintenance of proper liver function and structure . In addition, the
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liver is the site of active protein synthesis, including production of highly specialized protein fractions , and this synthetic process is impaired to a variable extent in most forms of liver disease. The interrelation between protein nutrition and liver disease is one of the major health problems affecting the world's population, particularly in the underdeveloped areas. A better understanding of the effect of protein malnutrition on liver function and of the role of the liver in the synthesis of various protein fractions would give us critical foreknowledge that we Jack at present. Synthesis of various protein fractions in the liver can be divided into two main components, namely proteins for intrahepatic function and proteins for export outside the liver, such as serum proteins . Despite impressive recent advances in the understanding of protein synthesis, little is known about the specificity of the liver in formation of individual protein fractions, nor about the mechanisms by which these proteins are released into the plasma . Moreover, the factors which regulate protein synthesis in the liver are understood incompletely. Various hormones have been demonstrated to play a role in this regulatory process. Although liver disease is known to affect the protein synthetic rate in the liver, the individual steps of this reduced synthesis are poorly understood and it is, for example, unclear why most patients with chronic liver disease exhibit impaired ability to synthesize albumin . The need for a basic and balanced supply of amino acids has been demonstrated as a fundamental requirement for normal protein synthesis in the liver, but it is unknown how this requirement changes with development of chronic or acute liver disease. Finally, it has been found that protein malnutrition may enhance or reduce the susceptibility of the liver to injury by various toxins and infections. The greatest need for obtaining answers to some of the preceding questions is in the area of technology. While individual cell components can be isolated and studied in detail, there presently are no suitable means for determining the basic amino acid requirements of individual liver cells .
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Available animal models are suitable only to a limited extent. On the other hand, it is evident that investigation of protein turnover and degradation , and nitrogen balance studies which were fashionable in the past, are yielding relatively little useful information and should be abandoned.
3. Trace elements The liver serves as a storage organ for most of the essential trace elements, including iron, zinc, copper, manganese, selenium, cobalt, and molybdenum. Many of these trace elements are essential co-factors for enzyme systems that are most active in the liver. Deficiency of one or several of these trace elements may lead to severe liver injury. On the other hand, excessive accumulation of some of these elements, notably iron and copper, is associated with well defined disease states. Zinc deficiency in animals and man has been demonstrated, but its relationship to liver injury is unknown . Since zinc is indispensable for the activity of alcohol and glutamic dehydrogenases, zinc deficiency is suspected of playing a contributory role in the development of alcoholic liver disease. Manganese activates various metal enzyme complexes which are essential for intermediary metabolism , but the role of manganese deficiency or excess in producing or preventing liver injury is unclear. Selenium deficiency produces liver necrosis in rats and pigs, but the role of selenium in human hepatic metabolism is unclear. Development of methodology for the microassay of tissues and body fluids for trace elements probably is the most urgent need in this area. While the atomic absorption spectrophotometer offers a new approach to this problem, this method is not suitable at present for several of the trace metals. With these refined methods, the role of trace metals in various forms of liver disease and in suitable animal models should be investigated . A proper understanding of the role of deficiency or excess of trace elements as contributory or primary causes of liver disease is essential for proper management of patients with hepatic disease. In addi-
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tion, this information will provide the baseline for dietary supplementation with trace metals for the prevention of liver injury in susceptible individuals.
V. Genetic Defects A. Hemochromatosis Hemochromatosis is a familial metabolic disorder in which the basic defect appears to be the absorption of dietary iron in excess of the normal body needs. Idiopathic genetically transmitted hemochromatosis is a relatively rare disease and present evidence strongly suggests that the disease may be reversible by removal of excess 1ron. Most attention has been directed toward an understanding of the regulation of iron absorption from the normal gut. Despite numerous observations about hemochromatosis, a complete understanding of the mechanism of iron absorption and its regulation is still lacking. It has not been possible to produce a suitable animal model characterized by iron overload. Cobalt and manganese share a common pathway of absorption with iron, but study of these two metals has not resulted in useful information regarding the mechanism or control of iron absorption. An understanding of the pathogenesis of hemochromatosis requires two bodies of information, namely, one concerning the mechanism of intestinal iron absorption and one involving the mechanism by which iron produces cell toxicity, particularly in the liver. This information is indispensable for devising a rational method for the prevention of hemochromatosis. Equally important is the development of relatively simple and reliable tests for detecting hemochromatosis before it has progressed to the stage of irreversible damage. B. Copper and the liver Copper is an essential co-factor for a number of important enzymes in intermediary metabolism. An inherited disorder of defective copper absorption (Menke's kinky-hair syndrome) has recently been described. Accumulation of copper and chronic copper toxicity is a hallmark of
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Wilson's disease (hepatolenticular degeneration), which is inherited as a recessively transmitted trait. This is a rare inherited disease which affects between 1000 and 2000 individuals in the United States. Because of the chronic disability of advanced Wilson's disease, the cost for the management of these patients is excessive . On the other hand, early diagnosis and treatment may greatly improve the prognosis a,nd retard the development of disability, resulting in a very substantial saving to society. It has been demonstrated conclusively that copper is essential for the normal structure and function of the liver and that the liver plays a central role in the homeostasis of copper metabolism. Moreover, the human liver is susceptible to copper poisoning when the trace metal is present in excess. The mechanism of copper absorption in the intestinal tract is incompletely understood , but it is evident that under physiological conditions, the liver maintains a zero balance by excreting excess copper, in part through the biliary tract, and in part as secretion of copper containing proteins. Removal of excess copper from the liver can be accomplished by various chelating agents, including openicillamine. The following information is essential for a more rational approach to the prevention and treatment of Wilson 's disease. First, an understanding of the mechanism and regulation of copper absorption in the intestine. Second, more detailed information on the factors which regulate copper excretion in the bile and copper retention in the liver in Wilson 's disease. Third, detailed information of the mechanisms by which excessive copper in the liver produces cellular injury. It would be most helpful to discover an animal model in which copper metabolism could be studied under controlled conditions.
C. Hepatic Storage Diseases Hepatic storage diseases are genetically determined and usually involve excessive accumulation of lipids or carbohydrates. The following are lipid storage diseases. Fabry's disease, characterized by deficiency of a-galactosidase activity which re-
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suits in accumulation of a galactosylgalactosyl ceramide. Tay-Sachs disease, of which there are at least three subtypes, all genetically determined, and all resulting in accumulation of gangliosides in the liver. Metachromatic leukodystrophy, characterized by arylsulfatase A deficiency. Mucosulfatidosis, which is believed to be due to deficiency of arylsulfatases A, B, and C. Cholesterol ester storage disease and Wolman 's disease, which are characterized by deficiency of lysosomal acid lipase activity. The following are hepatic carbohydrate storage diseases: M ucopolysaccharidosis. At least three different types of this condition have been recognized, including Hurler's disease, Hunter's disease, and San Filippo syndrome. Glycopen storape disease. Most forms of glycogenosis prominently affect the liver. This is particularly true with regard to the classical types I, II, III, VI, VIII, IX, and X. While most of these genetically transmitted storage diseases are relatively rare, recent evidence suggests that in several instances empirical treatment has resulted in substantial improvement or in retardation of the course of the disease. For most of these storage diseases, genetically determined enzyme deficiencies have been either demonstrated or are suspected. Some of these conditions are due to lysosomal defects, whereas others represent deficiencies in the activity of enzymes involved in intermediary metabolism . The greatest need in this area is for improved diagnostic methods, including prenatal diagnosis. Relatively simple chemical or cytological methods for diagnosis are indispensable for prevention and treatment of this group of diseases. In some instances, replacement of the missing lysosomal enzyme has yielded promising, albeit limited, results. This approach should be pursued with the most modern experimental tools. Since most of these conditions are inher-
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ited as a recessive trait, a better understanding of the fundamental biochemical deficiency will provide the basis for the detection of asymptomatic carriers of the disease. Detection of carriers is indispensable for genetic counseling, which in most instances is the only practical means for reducing the incidence of this group of diseases. D. Porphyria Porphyria represents a group of diseases, most of which are genetically determined, in which excessive amounts of porphyrins and porphyrin precursors are excreted in the urine and stool. Intermittent acute porphyria, hereditary coproporphyria, and variegate porphyria are three dominantly inherited hepatic types of porphyria, which are associated with abdominal and neurological manifestations and may be fatal in acute attacks. For porphyria cutanea tarda, a genetic basis has not been demonstrated unequivocally. While hepatic porphyria is relatively rare, its importance lies in the fact that many patients with the inherited disease are clinically asymptomatic, but acute and potentially fatal attacks may be precipitated by trivial amounts of commonly used drugs and hormones. The clinical manifestations, course, and chemical findings of the hepatic porphyrias have been well characterized, but to date, only in one form (acute intermittent porphyria) has the un derlying enzymatic defect been elucidated. In this dominantly inherited disease, uroporphyrinogen I synthetase activity is significantly reduced which probably accounts for the alarming effect of various drugs, nutritional, and hormonal factors in precipitating acute and at times fatal attacks. Detection of carriers of acute intermittent porphyria by screening all family members of patients with the disease by enzyme assay in circulating erythrocytes or fibroblast culture is essential for reducing the incidence of attacks and fatalities. The following problems need urgent clarification in porphyria: First, identification of the enzymatic defect in hereditary coproporphyria, variegate porphyria, and porphyria cutanea tarda must be made.
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Second, elucidation is needed of the various hormonal and nutritional factors which account for the fact that many of the patients with the inherited disease will remain chemically latent and clinically asymptomatic. Third, detailed information is required on the mechanisms by which the excessive porphyrins or porphyrin precursors or the defect in heme synthesis result in life-threatening abdominal and neurological manifestations. Solution of these problems requires development of new methods which permit kinetic characterization of the various sequential enzymes in heme biosynthesis. The information gained through the preceding studies would permit identification of patients with the inherited defect before it becomes clinically manifest. An understanding of the various nutritional, hormonal, and chemical factors which aggravate the defect in heme synthesis would allow us to devise preventive measures which would greatly reduce or eliminate the development of acute attacks in patients with inherited porphyria.
VI. Pediatric Liver Disease A. Liver Disease in Infants Liver disease in newborns, infants, and children frequently reflects developmental abnormalities or hepatic injury that occur during gestation or shortly after birth. The importance of liver disease in the infant and child lies in the fact that its recognition may be possible at a stage when rational treatment may result in a reversal of the pathogenetic process before irreparable liver damage has been sustained . Current research in pediatric liver disease is almost entirely descriptive. Information on bile acid metabolism, bile formation, and various genetically determined deficiencies is fragmentary. In some special instances, such as galactosemia and a-1-antitrypsin deficiency, the enzymatic defects have been elucidated in part. The most urgent research needs are a better understanding of the morphological and functional development of the hepatobiliary system during gestation and after birth. In particular, the metabolic and
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enzymatic adaptation of the liver at birth is poorly understood and little is known about the factors which bring about this major metabolic change. Moreover, detailed information is needed on the interrelationship between the placenta and the liver .before birth. Research on most of these problems will entail a necessary interdisciplinary approach between physiologists, cell biologists, embryologists, pediatricians, and individuals with experience in gastroenterology. The long range goals are a detailed understanding of the sequential developmental adaptation which the liver undergoes from gestation to adult life. On the basis of this information, a more rational approach to the treatment and prevention of pediatric liver disease will be possible. B. Neonatal Hyperbilirubinemia Unconjugated hyperbilirubinemia during the immediate neonatal period occurs in almost all infants, but only a few exhibit serum bilirubin levels which are high enough to induce bilirubin encephalopathy. The recognition of the potential hazard of a high unconjugated serum bilirubin level has led to the development of a number of corrective measures, including exchange transfusion, which have substantially reduced the incidence of bilirubin encephalopathy. On the other hand, with the improved methods of care for premature infants and for newborns with other complications, the incidence of significant hyperbilirubinemia in these special risk infants has increased. There still is a substantial disability and mortality resulting from bilirubin encephalopathy that has not been prevented despite the available corrective or preventive measures. While exchange transfusion is of immense help in the management and prevention of hyperbilirubinemia resulting from hemolytic disease, neonatal jaundice in infants without hemolysis is more difficult to manage. Limited success has been reported with exchange transfusion, phototherapy, and treatment of the mother during the last days of gestation with phenobarbital or related compounds . Although the importance of the binding of bilirubin
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to albumin is generally recognized, the factors which permit transfer of pigment from the plasma into the brain are still incompletely understood. One of the most important areas which demands attention is the effectiveness, potential danger, and mechanism of action of phototherapy. It has been well established that prolonged illumination of the icteric infant with visible light substantially reduces the serum bilirubin level. However, the mechanism by which this occurs is still poorly understood . It also has been demonstrated that treatment of the pregnant mother with compounds which induce hepatic drug metabolism results in a substantially lower serum bilirubin level in the newborn infant. The relationship between these drugs given to the mother and the increased transport efficiency of the liver in the newborn infant is unclear. Finally, the manner in which bilirubin damages the brain cells has not been demonstrated. The long range goals of research in this area are the prevention or reduction of hyperbilirubinemia of the newborn . A secondary goal is a better understanding of the factors which facilitate transfer of bilirubin into the central nervous system; identification of such factors will permit elaboration of measures to prevent bilirubin from gaining access to the brain in infants in whom hyperbilirubinemia has developed. VII. Hepatic Drug Metabolism The liver is the major site for the biotransformation and inactivation of drugs, hormones, and toxins. The rate of this biological process may be controlled by acute and chronic administration of one or several hormones and metabolites. Over 400 different exogenous chemicals, in addition to numerous endogenous compounds, are metabolized by the microsomal enzyme system of the liver; it is evident, therefore, that a detailed understanding of this enzymatic apparatus is essential for a rational approach to hormone and drug therapy. This is particularly important for long
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term treatment with single drugs or with combinations of various agents. Studies during the last few years have provided much information on the enzymatic mechanisms of the liver which are involved in the biotransformation of these compounds. Several in vitro systems are available for estimating the ability of the hepatocyte to transform drugs. However, the various hormonal, nutritional, and metabolic regulatory mechanisms are incompletely understood and only fragmentary information is available on the effect which a given drug has on the rate of metabolism of another drug. The effect of acute and chronic liver disease on these processes is almost unknown. The most important lack of information relates to the enzymatic mechanisms involved in the biotransformation of drugs and their metabolic regulation. This requires the development of new techniques which permit serial assay of these enzymes in small samples of liver tissue. Another area of importance is the correlation between enzymatic activity as measured in liver tissue with the actual kinetics of drug metabolism in the intact organism. Finally, information is needed to predict the effect of one drug on the metabolism of another drug in the intact organism as well as on the level of the hepatic enzyme mechanisms. The long range goal of these studies is the collection of sufficient information to adequately predict the kinetics and pharmacological effectiveness of a given drug that is administered on a repeated basis. It should become possible to predict the effect of one drug on the rate of metabolism of another drug when both are given simultaneously or sequentially. VIII. Priorities
Those areas of metabolic liver disease research which appear most promising are discussed below. Alcohol-induced liver disease is one of our major health problems. There are about 6.5 million chronic alcoholics in the United States, and about 8000 deaths per
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year are attributable to cirrhosis of the liver induced by ethanol abuse. Experimental work during the last few years has greatly clarified the biochemical mechanisms by which alcohol is metabolized in the liver and the relationship of these mechanisms to other essential liver functions, including the inactivation of drugs, hormones, and toxins. Alcohol alone causes an increase in liver fat, but the degree of this hepatic steatosis is partly dependent on nutritional factors. The relationship of hepatic fat to the development of alcoholic hepatitis and portal cirrhosis is unclear, as only a minority of chronic alcoholics develop these severe complications. Recent observations suggest new approaches to this problem. It recently has been shown that an experimental model, resembling human alcoholic hepatitis, can be produced in baboons fed alcohol over a 9-month period. The availability of this animal model should permit detailed studies on the relationship between alcohol intake and liver necrosis and should allow evaluation of protective or therapeutic measures. A second important observation has been the finding that alcohol stimulates hepatic fibroblastic activity, resulting in increased elaboration and deposition of collagen fibers and extracellular ground substance. This may provide an explanation for some of the histological changes of alcoholic hepatitis and portal cirrhosis and may allow identification of individuals who are susceptible to alcoholic liver disease. Liver injury, including cholestasis and hepatitis, caused by commonly available drugs or widely used toxins, is reported with increasing frequency. Some of these compounds often cause hepatic dysfunction if they are taken in sufficiently large doses or over prolonged periods. More commonly with hepatotoxic drugs, only a small percentage of individuals exhibit liver disease. The cause of this difference in susceptibility is unknown, but genetic or immunological factors are suspected and clearly are of importance in identifying individuals at risk. Of great significance is the fact that most drugs, hormones, and toxins are metabolized in the liver; liver toxicity may be
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caused by drug metabolites rather than by the parent compounds. The recent demonstration that such drug metabolites may bind covalently to essential macromolecules of the liver cell points to an important new approach to the problem of drug-induced liver injury. Transient unconjugated hyperbilirubinemia occurs in most newborn infants, but only a small percentage of them have pigment levels high enough to develop bilirubin encephalopathy (kernicterus). Until recently, exchange transfusion was the only means of managing severe hyperbilirubinemia. Two new preventive or therapeutic modalities have become available, which are simpler and frequently more effective. Exposure of infants to cool visible light has been shown to lower the serum bilirubin level effectively. The light is believed to induce photochemical oxidation of the pigment to more water-soluble derivatives which are nontoxic and easily excreted. Treatment of the mother during the last few days of pregnancy with phenobarbital or related compounds significantly lowers the infant's serum bilirubin level during the first days of life. The drug is able to cross the placental barrier and penetrate the fetal liver, where it presumably induces the hepatic transport mechanism involved in the excretion of bilirubin. Wilson's disease is a relatively rare inherited abnormality of copper metabolism, characterized by severe injury to the liver a:nd the central nervous system. Recent progress in the understanding of copper metabolism permitted development of a promising therapeutic and preventive regimen, involving use of copper chelating compounds, which in many instances allows patients with this disease to lead a normal life. In acute intermittent porphyria, the recent discovery of the primary genetically determined enzyme defect made it possible to develop a relatively simple biochemical test for the detection of asymptomatic carriers of this disease. This permits identification of individuals who have the inherited trait and therefore may be susceptible to drug-induced precipitation of acute neuropsychiatric attacks.
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Vol . 69, No.5
List of Persons Who Contributed Statements Used In Preparing This Report Work Group Memb ers Rudi Schmid, Chairman Burton Combes Carroll M . Leevy UrsA. Meyer Robert K. Ockner Steven Schenker M. Michael Thaler
Other Contributors Irwin M. Arias George Hug Anand a S. Prasad Telfer B. Reynolds
Irmin Sternlieb William H. J. Summerskill Louis R. Weintraub
Vol. 69, No.5 Prin ted in U.S.A.
GASTROENTEROLOGY
WORK GROUP V
Metabolic Diseases of the Liver I. Pathophysiology of liver diseases A. Bilirubin metabolism and hepatic transport B. Ascites and renal failure C. Hepatic coma D. Portal hypertension E . Hepatic cholesterol metabolism F. Liver function tests II. Bile Secretion and cholestasis III. Ethanol and alcoholic liver disease IV. Nutrition A. Vitamins B . Proteins C. Trace elements V. Genetic defects A. Hemochromatosis B. Copper and the liver C. Hepatic storage diseases D. Porphyria VI. Pediatric liver disease A. Liver disease in infants B. Neonatal hyperbilirubinemia VII. Hepatic drug metabolism VIII. Priorities
I. Pathophysiology of Liver Diseases A. Bilirubin Metabolism and Hepatic Transport Hyperbilirubinemia and jaundice are commonly observed features of acquired or congenital diseases of the liver and biliary tract. In addition, hyperbilirubinemia in the newborn period represents an important potential hazard for the development of bilirubin encephalopathy. There are major uncertainties regarding the mechanisms and control of pigment production and disposal in healthy individuals and in patients with various forms of jaundice. Moreover, the pathogenesis and preventon of bilirubin encephalopathy in the newborn period are incompletely understood. In the liver, bilirubin shares transport mechanisms with other organic anions, including products of intermediary me-
tabolism, and exogenously administered hormones, drugs , dyes, and carcinogens. A clearer comprehension of bilirubin metabolism at the level of the liver should reveal important information regarding plasma to bile transport of a variety of biologically important compounds. There are five areas in which research efforts are needed: (1) bilirubin formation; (2) transport of bilirubin in plasma ; (3) hepatic uptake of bilirubin and other organic anions; (4) hepatic endoplasmic reticulum; (5) interdisciplinary efforts. 1. Bilirubin formation. Although the sources of bilirubin and the major metabolic pathway for its formation are defined, little is known of the metabolic regulation of this pathway. Moreover, present methods for measuring the relative contribution of various heme pools to over-all production of bilirubin are inadequate and should be refined . Turnover of the various
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Nov ember 1975
METABOLIC LIVER DISEASES
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cytochromes in the microsomal fraction of solved and complex problems relating to the liver must be determined before biliru- hepatic handling of bilirubin. bin formation in the liver can be estimated. Most current and future research efforts The relationship between hepatic drug me- should aim for a better understanding of tabolism, conjugation of bilirubin, biliru- the biochemical mechanisms by which the bin formation, and transport in the liver liver transfers bilirubin and other organic has not been elucidated under physiologi- anions from the plasma into the bile. This cal conditions and in various diseases of understanding will make it possible to alter the liver. deranged hepatic transport mechanisms by 2. Transport of bilirubin in plasma. The pharmacological or dietary means. binding of bilirubin and other organic anB. Ascites and Renal Failure ions to albumin and to other transport proteins is of critical importance for the Ascites is the most common major metabolic fate and disposition of these chronic complication of advanced liver discompounds. Studies are lacking of the ease. Renal circulatory failure related to basic structure of the organic anion bind- liver disease is nearly always fatal and ing sites on albumin and the effect of nearly always occurs in association with various diseases on this binding mecha- ascites or with fulminant hepatic failure . While several factors have been implicated msm . 3. Hepatic uptake of bilirubin and other in the formation of ascites and of renal organic anions. The liver has the unique failure, the underlying pathogenetic mechproperty of extracting bilirubin and other anisms are poorly understood . Diseases of the liver, especially cirrhosis, organic anions from the plasma and either metabolizing or excreting them in the bile. rank among the commonest causes of At least two soluble prbtein fractions have death in the United States. An underbeen identified in the liver which seem to standing of the mechanisms which lead to be involved in this uptake, but their kinetic formation of ascites and renal failure is role awaits characterization and analysis. indispensable for treatment of fatal condiFurthermore, it is not known whether a tions of the liver. specific facilitated transport mechanism is In the pathogenesis of ascites, several present at the level of the hepatic plasma factors seem to play a role, including reduced plasma colloid osmotic pressure, membrane. 4. Hepatic endoplasmic reticulum. The elevated portal venous pressure, alteraenzyme UDP-glucuronyltransferase and tions in lymph flow from the liver, and other microsomal enzymes are involved in disorders of water and electrolyte metabothe conjugation of bilirubin with carbohy- lism. Treatment and prevention of ascites drate moieties. Whether one or several is difficult because of current ignorance enzymes are involved in the formation of about the relative importance of and interglucuronides is uncertain. Another area of relation between these factors . A major research effort should be research is the structure-function relationship of the endoplasmic reticulum of the mounted to better understand the relative liver in the transport of bilirubin and other importance of factors which are involved in organic anions. This is particularly impor- the formation of ascites. This requires a tant if we hope to understand the effect of more precise definition of the role of the barbiturates and related drugs on the liver in the regulation of fluid and electrolyte balance under normal conditions. It serum bilirubin level. 5. Interdisciplinary efforts. Many of the would be valuable to have an animal model problems listed above require the coopera- in which formation of ascites and renal tion of investigators with various back- failure could be studied under experimengrounds including physiology, cell biology, tal conditions. Total renal blood flow and biochemistry, pharmacology, and veteri- relative renal blood f1ow within the various nary medicine. It is evident that only a compartments of the kidney should be team approach can clarify various un- investigated during hepatic failure.
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Most of these studies require the cooperation of investigators with special competence in liver physiology, renal physiology, transport mechanisms, and hemodynamics. The understanding of pathogenetic mechanisms underlying ascites formation and renal failure in chronic liver disease will permit formulation of a rational approach to the treatment of these conditons. C. Hepatic Coma The pathogenesis of diffuse metabolic encephalopathy which frequently accompanies advanced liver disease is still unknown. Hepatic encephalopathy may occur with advanced chronic liver disease or may be a part of acute fulminant liver failure. Approximately one-third of patients with chronic liver failure develop hepatic encephalopathy during the terminal stage of their disease. In addition, acute fulminant hepatitis almost invariably leads to hepatic coma and has a mortality of about 80%.
Although three types of hepatic coma can be separated, namely acute fulminant, chronic recurrent, and hepatocerebral degeneration, the pathogenesis of all three is unknown. Ammonia is believed to play a major role in the development of coma, but the importance of other toxins has not been studied adequately. The recent suggestion that false neurotransmitters may play a role opens an exciting avenue for new investigation. The treatment of hepatic encephalopathy is still unsatisfactory, although the exclusion of proteins together with the use of antibiotics is beneficial. Various heroic measures, such as exchange transfusion, have been used but are of limited value. Studies of the pathogenesis of hepatic coma are urgently needed and should be directed toward investigating blood flow and oxygen consumption in the brain and cerebral metabolism , particularly utilization of glucose and short chain fatty acids in hepatic coma. Moreover, the possibility that false neurotransmitters accumulate in the brain should be investigated, perhaps as a first step by analyzing the cerebrospinal fluid. It would be most helpful to have an animal model in which controlled stud-
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ies could be carried out. Finally, the role of ammonia in the pathogenesis of hepatic coma requires further definition. The present therapeutic approach to hepatic encephalopathy is partially successful, but largely empirical. The value of protein restriction, antibiotics and lactulose should be defined and the role of precipitating factors elucidated. It is evident that progress in this area can be made only by an interdisciplinary approach which includes experts in intermediary metabolism, neurology, and physiology, in addition to experts in liver disease. Development of an artificial liver, analogous to the artificial kidney, may offer hope that survival of patients with acute hepatic coma may be improved. It is realized, however, that the technological problems involved in designing an artificial liver are extensive and preliminary experience with available prototypes has revealed a number of shortcomings which require correction and modification before this approach appears promising. D. Portal Hypertension Portal hypertension complicates most forms of chronic liver disease. Although increased resistance to flow of blood through the liver appears to be the major cause, other contributing factors have not been excluded. Esophageal varices resulting from portal hypertension are an important source of intestinal bleeding, and are believed to be the cause of death in a significant fraction of patients with alcoholic, postnecrotic, or cryptogenic cirrhosis. Moreover, portal hypertension is an important contributory factor to the formation of ascites and, perhaps thereby, to renal insufficiency in these patients. Although by definition chronic liver disease usually is associated with a deranged hepatic architecture, few studies have been performed to correlate hepatic vascular structure with portal hypertension . Most of the present research on portal hypertension is concerned with surgical treatment. The lack of a suitable animal model prevents conducting these studies under controlled conditions. Controlled clinical trials of the various
November 1975
METABOLIC LIVER DISEASES
surgical methods for the relief of portal hypertension are important and should be carried out in many clinical centers. In addition, it is essential to identify the nature of the vascular changes in the liver which lead to portal hypertension. This will require correlative studies between pathologists and physiologists and should be based on the development of newer and more sensitive methods for measuring hepatic blood flow . The long range goals should be directed towards the prevention of portal hypertension in the course of chronic liver disease. Moreover, clear criteria should be worked out which would permit better identification of patients who may benefit from surgical correction of portal hypertension.
E. Hepatic Cholesterol Metabolism Knowledge about the role of the intestinal tract and the liver in controlling hepatic cholesterol synthesis is incomplete . Moreover, in cholestasis, hepatic synthesis is greatly accelerated, suggesting that the normal feedback control of this metabolic pathway in inoperative. Many forms of acute and chronic liver disease are associated with increased plasma cholesterol levels. Since these patients incur an added risk for developing atherosclerosis, better comprehension of the mechanisms regulating plasma cholesterol are of prime importance. There is also evidence that hypercholesteremia of liver disease may be related to an increased incidence of gallstones, and this relationship should be defined. Intensive studies on the role of cholesterol and bile acids in the regulation of hepatic cholesterol synthesis are being pursued. There is evidence that hepatic cholesterol synthesis undergoes diurnal variation and is influenced by dietary factors. Efforts are being made to characterize the transport of cholesterol from the intestinal lumen to the plasma, including identification of possible cholesterol and sterol binding proteins in the cytoplasm of the liver and in the intestinal mucosa. The major effort in hepatic cholesterol metabolism should be directed toward elucidation of the control of cholesterol
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synthesis within the liver and the intestinal mucosa. This requires clarification of the role of sterol carrier proteins in the liver and gut and a definition of the role of bile acids in this transport. While most of the necessary methods for these studies are available, there remains a great need for cooperative efforts between specialists with expertise in protein synthesis, lipid metabolism, transport mechanisms and clinical liver disease. The major long range goal will be to identify the factors which regulate cholesterol synthesis under physiological conditions and to study the abnormalities in this regulatory mechanism in various forms of liver disease, including cholestasis. This information would allow development of a rational therapy.
F. Liver Function Tests Liver function tests are designed to evaluate the effect of various forms of liver disease on the functional state of the organ. While most tests reflect biochemical or physiological functions of the liver, they usually are insensitive and detect only gross injury to individual functions of the hepatic cell. Presently available laboratory approaches are inadequate to assess small degrees of hepatic dysfunction, and they do not permit identification of incipient liver disease or minor functional abnormalities. This is especially important in the preoperative work-up of patients undergoing elective surgery. Work over the last few years has produced little progress in this area in that batteries of existing tests are applied to many liver diseases without acquiring significant new information. Although some new tests have been developed, they usually represent variants of existing tests and generally add little in the direction of refined assessment. The correlation between hepatic histology, obtained using needle biopsy, and the functional assessment of the liver often is poor. The greatest need is for more sensitive and more specific liver function tests. These tests should be based on metabolic functions which are specific for the liver and whose derangement can be detected
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early and by relatively simple means of analysis. Ideal measurements of hepatic function should reflect not only individual metabolic function, but also should allow assessment of the number of liver cells available for a specific function. It is unknown, for example, whether decreased hepatic function, as measured with presently available tests, reflects reduced metabolic activity of all liver cells, or an absolute reduction in the number of normally functioning liver cells.
II. Bile Secretion and Cholestasis This subject requires an understanding of the mechanisms by which water, inorganic electrolytes, and a number of organic compounds are moved from the hepatic circulation into the bile. This information will also be of value to a variety of allied areas , including pharmacotherapeutics and the radiological visualization of bile ducts and the gallbladder. At present, four different mechanisms for secretion of various bile components have been identified. To a limited extent, it is possible to influence .these four secretory mechanisms with drugs. Abnormalities in these secretory mechanisms have been correlated with various clinical states, but morphology at the light-microscopic and electron-microscopic levels has provided little insight into the mechanism of bile formation. Elucidation of the sites and mechanisms of bile formation and their modification by disease is lacking. It will be necessary to relate bile secretion to various intracellular processes, including the function of the endoplasmic reticulum and the Golgi apparatus. The role of bile acids and other organic constituents of the bile on bile secretion has not been clarified. Finally, better methods should be developed for radiological visualization of the gallbladder and bile ducts for the diagnosis of diseases of the biliary tract. Micropuncture , microdissection, and perfusion techniques analogous to those which have been used successfully in the kidney, should be adapted for the liver. In addition, development of methods to culture normal nondividing liver cells would
Vol. 69, No . 5
permit investigation of individual cell functions related to hepatic excretion. Advances in the aforementioned areas would increase understanding of the mechanisms by which bile is formed and would lead to a better appreciation of the abnormalities underlying various forms of acute and chronic liver disease.
III. Ethanol and Alcoholic Liver Disease There are two major questions about the effect of ethanol on the liver. (1) By what mechanisms does alcohol induce acute and chronic liver disease? (2) What is responsible for the apparent susceptibility of only a limited segment of the drinking population to severe alcohol-induced liver disease? Alcoholic liver disease is one of the major causes of morbidity and death in the United States. Current estimates are that there are 6.5 million alcoholics in the United States and about 8000 deaths per year attributable to alcoholic cirrhosis of the liver. Three types of alcoholic liver disease have been identified, namely fatty liver, alcoholic hepatitis, and cirrhosis. Progress has been made in identifying the source of the fat accumulating in the liver, but the relationship of this process to the development of hepatitis, necrosis, and eventually cirrhosis is not clear. Although dietary factors have been shown to play a role in the amount and type of lipids that accumulate in the liver of alcoholic patients, the relative role of malnutrition in the pathogenesis of alcoholic hepatitis and cirrhosis is uncertain. Animal models have not proven useful in the past; however, alcoholic hepatitis can be induced in baboons and may be a valuable model in future work. Much effort has been directed to identifying mechanisms by which alcohol is metabolized by the liver. While the role of alcohol dehydrogenase has been demonstrated beyond doubt, there is uncertainty to what extent alcohol also may be metabolized by a microsomal ethanoloxidizing system. This is important because of the demonstrated interdependence between alcohol and drug metabolism in the liver.
Nov ember 1975
METABOLIC LIVER DISEASES
Areas of need include identification of the characteristics of individuals who abuse alcohol and develop liver disease. Since this is only about 10% of all individuals who abuse alcohol, the biochemical and possibly genetic characteristics of this population subgroup should be studied and analyzed. Another major effort should be elucidation of the pathogenetic mechanism of alcoholic hepatitis; this study will require detailed information on the effect of alcohol on subcellular organelles of hepatocytes. Another important area is a description of the natural history of alcoholic hepatitis, particularly its regression on abstinence and its progression to cirrhosis on continued consumption of ethanol. Current and future therapeutic modalities should be investigated with regard to their effectiveness in reversing alcoholic hepatitis and in arresting its progression to cirrhosis. This will require better comprehension of both the metabolism of alcohol in the liver and of its effect on individual liver cell functions and structure.
IV. Nutrition
A . Vitamins Deficiency of individual vitamins or groups of vitamins may play a pathogenetic role in chronic liver disease. In addition, many patients with chronic liver disease develop and manifest signs and symptoms of vitamin deficiency. Major unsolved problems in this area are related to the lack of reliability, sensitivity and specificity of the present assay techniques for vitamins, both in tissues and in body fluids, and to the difficulty in assessing the ability of the liver to convert vitamin precursors into the biologically active form s of the vitamins. Another important area of uncertainty is the role of a deficiency or excess of vitamins in modifying the susceptibility of the liver to infectious or toxic agents . Finally, little is known about the effect which liver disease may have on basic vitamin requirements . Many patients with chronic liver disease, particularly due to alcoholism , manifest symptoms of vitamin deficiency. It is evident, however, that consumption of vi-
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tamins, and the costs for procuring them , by far exceed the amount that can be justified by demonstrable deficiency. It is vitally important, therefore, to establish proper base lines and a rational approach to vitamin therapy in patients who either have chronic liver disease or who are at high risk of toxic or infectious liver injury. Deficiency of folate and vitamin B, 2 has been demonstrated in many patients with chronic liver disease and particularly in alcoholics. Folic acid deficiency has been shown to induce morphological and functional alterations of the hepatocyte . Deficiency of riboflavin and nicotinic acid has been implicated in the development of fatty liver. Thiamine depletion , as reflected by peripheral neuropathy, encephalopathy, and alterations in carbohydrate metabolism , is frequently observed in chronic liver disease. Little information is available on the possible effects of pantothenate deficiency on liver metabolism . Deficiency of vitamin B 6 is known to interfere with protein and nucleic acid metabolism in the liver. The fat-soluble vitamin A is required for the functional stability of the lysosomal membrane which may have important bearing on some forms of toxic liver injury. Vitamin Dis converted to one of its active forms by the hepatic microsomal enzyme system . A deficiency of vita min K is frequently observed in chronic liver disease and results in defective synthesis of essential clotting factors. The major need for addit ional and more detailed information is related to the role of various water- and fat-soluble vitamins in the maintenance of normal liver function and structure. This requires not only more sensitive and more reliable assay methods at the microquantitative level but also a better understanding of the individual metabolic functions in which the various vitamins are involved. This information would permit us to assess the importance of individual vitamins in the prevention and repair of liver injury.
B. Proteins Availability of exogenous proteins is essential for the maintenance of proper liver function and structure . In addition, the
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liver is the site of active protein synthesis, including production of highly specialized protein fractions , and this synthetic process is impaired to a variable extent in most forms of liver disease. The interrelation between protein nutrition and liver disease is one of the major health problems affecting the world's population, particularly in the underdeveloped areas. A better understanding of the effect of protein malnutrition on liver function and of the role of the liver in the synthesis of various protein fractions would give us critical foreknowledge that we Jack at present. Synthesis of various protein fractions in the liver can be divided into two main components, namely proteins for intrahepatic function and proteins for export outside the liver, such as serum proteins . Despite impressive recent advances in the understanding of protein synthesis, little is known about the specificity of the liver in formation of individual protein fractions, nor about the mechanisms by which these proteins are released into the plasma . Moreover, the factors which regulate protein synthesis in the liver are understood incompletely. Various hormones have been demonstrated to play a role in this regulatory process. Although liver disease is known to affect the protein synthetic rate in the liver, the individual steps of this reduced synthesis are poorly understood and it is, for example, unclear why most patients with chronic liver disease exhibit impaired ability to synthesize albumin . The need for a basic and balanced supply of amino acids has been demonstrated as a fundamental requirement for normal protein synthesis in the liver, but it is unknown how this requirement changes with development of chronic or acute liver disease. Finally, it has been found that protein malnutrition may enhance or reduce the susceptibility of the liver to injury by various toxins and infections. The greatest need for obtaining answers to some of the preceding questions is in the area of technology. While individual cell components can be isolated and studied in detail, there presently are no suitable means for determining the basic amino acid requirements of individual liver cells .
Vol. 69, No . 5
Available animal models are suitable only to a limited extent. On the other hand, it is evident that investigation of protein turnover and degradation , and nitrogen balance studies which were fashionable in the past, are yielding relatively little useful information and should be abandoned.
3. Trace elements The liver serves as a storage organ for most of the essential trace elements, including iron, zinc, copper, manganese, selenium, cobalt, and molybdenum. Many of these trace elements are essential co-factors for enzyme systems that are most active in the liver. Deficiency of one or several of these trace elements may lead to severe liver injury. On the other hand, excessive accumulation of some of these elements, notably iron and copper, is associated with well defined disease states. Zinc deficiency in animals and man has been demonstrated, but its relationship to liver injury is unknown . Since zinc is indispensable for the activity of alcohol and glutamic dehydrogenases, zinc deficiency is suspected of playing a contributory role in the development of alcoholic liver disease. Manganese activates various metal enzyme complexes which are essential for intermediary metabolism , but the role of manganese deficiency or excess in producing or preventing liver injury is unclear. Selenium deficiency produces liver necrosis in rats and pigs, but the role of selenium in human hepatic metabolism is unclear. Development of methodology for the microassay of tissues and body fluids for trace elements probably is the most urgent need in this area. While the atomic absorption spectrophotometer offers a new approach to this problem, this method is not suitable at present for several of the trace metals. With these refined methods, the role of trace metals in various forms of liver disease and in suitable animal models should be investigated . A proper understanding of the role of deficiency or excess of trace elements as contributory or primary causes of liver disease is essential for proper management of patients with hepatic disease. In addi-
No vember 1975
METABOLIC LIVER DISEASES
tion, this information will provide the baseline for dietary supplementation with trace metals for the prevention of liver injury in susceptible individuals.
V. Genetic Defects A. Hemochromatosis Hemochromatosis is a familial metabolic disorder in which the basic defect appears to be the absorption of dietary iron in excess of the normal body needs. Idiopathic genetically transmitted hemochromatosis is a relatively rare disease and present evidence strongly suggests that the disease may be reversible by removal of excess 1ron. Most attention has been directed toward an understanding of the regulation of iron absorption from the normal gut. Despite numerous observations about hemochromatosis, a complete understanding of the mechanism of iron absorption and its regulation is still lacking. It has not been possible to produce a suitable animal model characterized by iron overload. Cobalt and manganese share a common pathway of absorption with iron, but study of these two metals has not resulted in useful information regarding the mechanism or control of iron absorption. An understanding of the pathogenesis of hemochromatosis requires two bodies of information, namely, one concerning the mechanism of intestinal iron absorption and one involving the mechanism by which iron produces cell toxicity, particularly in the liver. This information is indispensable for devising a rational method for the prevention of hemochromatosis. Equally important is the development of relatively simple and reliable tests for detecting hemochromatosis before it has progressed to the stage of irreversible damage. B. Copper and the liver Copper is an essential co-factor for a number of important enzymes in intermediary metabolism. An inherited disorder of defective copper absorption (Menke's kinky-hair syndrome) has recently been described. Accumulation of copper and chronic copper toxicity is a hallmark of
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Wilson's disease (hepatolenticular degeneration), which is inherited as a recessively transmitted trait. This is a rare inherited disease which affects between 1000 and 2000 individuals in the United States. Because of the chronic disability of advanced Wilson's disease, the cost for the management of these patients is excessive . On the other hand, early diagnosis and treatment may greatly improve the prognosis a,nd retard the development of disability, resulting in a very substantial saving to society. It has been demonstrated conclusively that copper is essential for the normal structure and function of the liver and that the liver plays a central role in the homeostasis of copper metabolism. Moreover, the human liver is susceptible to copper poisoning when the trace metal is present in excess. The mechanism of copper absorption in the intestinal tract is incompletely understood , but it is evident that under physiological conditions, the liver maintains a zero balance by excreting excess copper, in part through the biliary tract, and in part as secretion of copper containing proteins. Removal of excess copper from the liver can be accomplished by various chelating agents, including openicillamine. The following information is essential for a more rational approach to the prevention and treatment of Wilson 's disease. First, an understanding of the mechanism and regulation of copper absorption in the intestine. Second, more detailed information on the factors which regulate copper excretion in the bile and copper retention in the liver in Wilson 's disease. Third, detailed information of the mechanisms by which excessive copper in the liver produces cellular injury. It would be most helpful to discover an animal model in which copper metabolism could be studied under controlled conditions.
C. Hepatic Storage Diseases Hepatic storage diseases are genetically determined and usually involve excessive accumulation of lipids or carbohydrates. The following are lipid storage diseases. Fabry's disease, characterized by deficiency of a-galactosidase activity which re-
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suits in accumulation of a galactosylgalactosyl ceramide. Tay-Sachs disease, of which there are at least three subtypes, all genetically determined, and all resulting in accumulation of gangliosides in the liver. Metachromatic leukodystrophy, characterized by arylsulfatase A deficiency. Mucosulfatidosis, which is believed to be due to deficiency of arylsulfatases A, B, and C. Cholesterol ester storage disease and Wolman 's disease, which are characterized by deficiency of lysosomal acid lipase activity. The following are hepatic carbohydrate storage diseases: M ucopolysaccharidosis. At least three different types of this condition have been recognized, including Hurler's disease, Hunter's disease, and San Filippo syndrome. Glycopen storape disease. Most forms of glycogenosis prominently affect the liver. This is particularly true with regard to the classical types I, II, III, VI, VIII, IX, and X. While most of these genetically transmitted storage diseases are relatively rare, recent evidence suggests that in several instances empirical treatment has resulted in substantial improvement or in retardation of the course of the disease. For most of these storage diseases, genetically determined enzyme deficiencies have been either demonstrated or are suspected. Some of these conditions are due to lysosomal defects, whereas others represent deficiencies in the activity of enzymes involved in intermediary metabolism . The greatest need in this area is for improved diagnostic methods, including prenatal diagnosis. Relatively simple chemical or cytological methods for diagnosis are indispensable for prevention and treatment of this group of diseases. In some instances, replacement of the missing lysosomal enzyme has yielded promising, albeit limited, results. This approach should be pursued with the most modern experimental tools. Since most of these conditions are inher-
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ited as a recessive trait, a better understanding of the fundamental biochemical deficiency will provide the basis for the detection of asymptomatic carriers of the disease. Detection of carriers is indispensable for genetic counseling, which in most instances is the only practical means for reducing the incidence of this group of diseases. D. Porphyria Porphyria represents a group of diseases, most of which are genetically determined, in which excessive amounts of porphyrins and porphyrin precursors are excreted in the urine and stool. Intermittent acute porphyria, hereditary coproporphyria, and variegate porphyria are three dominantly inherited hepatic types of porphyria, which are associated with abdominal and neurological manifestations and may be fatal in acute attacks. For porphyria cutanea tarda, a genetic basis has not been demonstrated unequivocally. While hepatic porphyria is relatively rare, its importance lies in the fact that many patients with the inherited disease are clinically asymptomatic, but acute and potentially fatal attacks may be precipitated by trivial amounts of commonly used drugs and hormones. The clinical manifestations, course, and chemical findings of the hepatic porphyrias have been well characterized, but to date, only in one form (acute intermittent porphyria) has the un derlying enzymatic defect been elucidated. In this dominantly inherited disease, uroporphyrinogen I synthetase activity is significantly reduced which probably accounts for the alarming effect of various drugs, nutritional, and hormonal factors in precipitating acute and at times fatal attacks. Detection of carriers of acute intermittent porphyria by screening all family members of patients with the disease by enzyme assay in circulating erythrocytes or fibroblast culture is essential for reducing the incidence of attacks and fatalities. The following problems need urgent clarification in porphyria: First, identification of the enzymatic defect in hereditary coproporphyria, variegate porphyria, and porphyria cutanea tarda must be made.
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METABOLIC LIVER DISEASES
Second, elucidation is needed of the various hormonal and nutritional factors which account for the fact that many of the patients with the inherited disease will remain chemically latent and clinically asymptomatic. Third, detailed information is required on the mechanisms by which the excessive porphyrins or porphyrin precursors or the defect in heme synthesis result in life-threatening abdominal and neurological manifestations. Solution of these problems requires development of new methods which permit kinetic characterization of the various sequential enzymes in heme biosynthesis. The information gained through the preceding studies would permit identification of patients with the inherited defect before it becomes clinically manifest. An understanding of the various nutritional, hormonal, and chemical factors which aggravate the defect in heme synthesis would allow us to devise preventive measures which would greatly reduce or eliminate the development of acute attacks in patients with inherited porphyria.
VI. Pediatric Liver Disease A. Liver Disease in Infants Liver disease in newborns, infants, and children frequently reflects developmental abnormalities or hepatic injury that occur during gestation or shortly after birth. The importance of liver disease in the infant and child lies in the fact that its recognition may be possible at a stage when rational treatment may result in a reversal of the pathogenetic process before irreparable liver damage has been sustained . Current research in pediatric liver disease is almost entirely descriptive. Information on bile acid metabolism, bile formation, and various genetically determined deficiencies is fragmentary. In some special instances, such as galactosemia and a-1-antitrypsin deficiency, the enzymatic defects have been elucidated in part. The most urgent research needs are a better understanding of the morphological and functional development of the hepatobiliary system during gestation and after birth. In particular, the metabolic and
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enzymatic adaptation of the liver at birth is poorly understood and little is known about the factors which bring about this major metabolic change. Moreover, detailed information is needed on the interrelationship between the placenta and the liver .before birth. Research on most of these problems will entail a necessary interdisciplinary approach between physiologists, cell biologists, embryologists, pediatricians, and individuals with experience in gastroenterology. The long range goals are a detailed understanding of the sequential developmental adaptation which the liver undergoes from gestation to adult life. On the basis of this information, a more rational approach to the treatment and prevention of pediatric liver disease will be possible. B. Neonatal Hyperbilirubinemia Unconjugated hyperbilirubinemia during the immediate neonatal period occurs in almost all infants, but only a few exhibit serum bilirubin levels which are high enough to induce bilirubin encephalopathy. The recognition of the potential hazard of a high unconjugated serum bilirubin level has led to the development of a number of corrective measures, including exchange transfusion, which have substantially reduced the incidence of bilirubin encephalopathy. On the other hand, with the improved methods of care for premature infants and for newborns with other complications, the incidence of significant hyperbilirubinemia in these special risk infants has increased. There still is a substantial disability and mortality resulting from bilirubin encephalopathy that has not been prevented despite the available corrective or preventive measures. While exchange transfusion is of immense help in the management and prevention of hyperbilirubinemia resulting from hemolytic disease, neonatal jaundice in infants without hemolysis is more difficult to manage. Limited success has been reported with exchange transfusion, phototherapy, and treatment of the mother during the last days of gestation with phenobarbital or related compounds . Although the importance of the binding of bilirubin
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to albumin is generally recognized, the factors which permit transfer of pigment from the plasma into the brain are still incompletely understood. One of the most important areas which demands attention is the effectiveness, potential danger, and mechanism of action of phototherapy. It has been well established that prolonged illumination of the icteric infant with visible light substantially reduces the serum bilirubin level. However, the mechanism by which this occurs is still poorly understood . It also has been demonstrated that treatment of the pregnant mother with compounds which induce hepatic drug metabolism results in a substantially lower serum bilirubin level in the newborn infant. The relationship between these drugs given to the mother and the increased transport efficiency of the liver in the newborn infant is unclear. Finally, the manner in which bilirubin damages the brain cells has not been demonstrated. The long range goals of research in this area are the prevention or reduction of hyperbilirubinemia of the newborn . A secondary goal is a better understanding of the factors which facilitate transfer of bilirubin into the central nervous system; identification of such factors will permit elaboration of measures to prevent bilirubin from gaining access to the brain in infants in whom hyperbilirubinemia has developed. VII. Hepatic Drug Metabolism The liver is the major site for the biotransformation and inactivation of drugs, hormones, and toxins. The rate of this biological process may be controlled by acute and chronic administration of one or several hormones and metabolites. Over 400 different exogenous chemicals, in addition to numerous endogenous compounds, are metabolized by the microsomal enzyme system of the liver; it is evident, therefore, that a detailed understanding of this enzymatic apparatus is essential for a rational approach to hormone and drug therapy. This is particularly important for long
Vol . 69, No.5
term treatment with single drugs or with combinations of various agents. Studies during the last few years have provided much information on the enzymatic mechanisms of the liver which are involved in the biotransformation of these compounds. Several in vitro systems are available for estimating the ability of the hepatocyte to transform drugs. However, the various hormonal, nutritional, and metabolic regulatory mechanisms are incompletely understood and only fragmentary information is available on the effect which a given drug has on the rate of metabolism of another drug. The effect of acute and chronic liver disease on these processes is almost unknown. The most important lack of information relates to the enzymatic mechanisms involved in the biotransformation of drugs and their metabolic regulation. This requires the development of new techniques which permit serial assay of these enzymes in small samples of liver tissue. Another area of importance is the correlation between enzymatic activity as measured in liver tissue with the actual kinetics of drug metabolism in the intact organism. Finally, information is needed to predict the effect of one drug on the metabolism of another drug in the intact organism as well as on the level of the hepatic enzyme mechanisms. The long range goal of these studies is the collection of sufficient information to adequately predict the kinetics and pharmacological effectiveness of a given drug that is administered on a repeated basis. It should become possible to predict the effect of one drug on the rate of metabolism of another drug when both are given simultaneously or sequentially. VIII. Priorities
Those areas of metabolic liver disease research which appear most promising are discussed below. Alcohol-induced liver disease is one of our major health problems. There are about 6.5 million chronic alcoholics in the United States, and about 8000 deaths per
November 1975
METABOLIC LIVER DISEASES
year are attributable to cirrhosis of the liver induced by ethanol abuse. Experimental work during the last few years has greatly clarified the biochemical mechanisms by which alcohol is metabolized in the liver and the relationship of these mechanisms to other essential liver functions, including the inactivation of drugs, hormones, and toxins. Alcohol alone causes an increase in liver fat, but the degree of this hepatic steatosis is partly dependent on nutritional factors. The relationship of hepatic fat to the development of alcoholic hepatitis and portal cirrhosis is unclear, as only a minority of chronic alcoholics develop these severe complications. Recent observations suggest new approaches to this problem. It recently has been shown that an experimental model, resembling human alcoholic hepatitis, can be produced in baboons fed alcohol over a 9-month period. The availability of this animal model should permit detailed studies on the relationship between alcohol intake and liver necrosis and should allow evaluation of protective or therapeutic measures. A second important observation has been the finding that alcohol stimulates hepatic fibroblastic activity, resulting in increased elaboration and deposition of collagen fibers and extracellular ground substance. This may provide an explanation for some of the histological changes of alcoholic hepatitis and portal cirrhosis and may allow identification of individuals who are susceptible to alcoholic liver disease. Liver injury, including cholestasis and hepatitis, caused by commonly available drugs or widely used toxins, is reported with increasing frequency. Some of these compounds often cause hepatic dysfunction if they are taken in sufficiently large doses or over prolonged periods. More commonly with hepatotoxic drugs, only a small percentage of individuals exhibit liver disease. The cause of this difference in susceptibility is unknown, but genetic or immunological factors are suspected and clearly are of importance in identifying individuals at risk. Of great significance is the fact that most drugs, hormones, and toxins are metabolized in the liver; liver toxicity may be
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caused by drug metabolites rather than by the parent compounds. The recent demonstration that such drug metabolites may bind covalently to essential macromolecules of the liver cell points to an important new approach to the problem of drug-induced liver injury. Transient unconjugated hyperbilirubinemia occurs in most newborn infants, but only a small percentage of them have pigment levels high enough to develop bilirubin encephalopathy (kernicterus). Until recently, exchange transfusion was the only means of managing severe hyperbilirubinemia. Two new preventive or therapeutic modalities have become available, which are simpler and frequently more effective. Exposure of infants to cool visible light has been shown to lower the serum bilirubin level effectively. The light is believed to induce photochemical oxidation of the pigment to more water-soluble derivatives which are nontoxic and easily excreted. Treatment of the mother during the last few days of pregnancy with phenobarbital or related compounds significantly lowers the infant's serum bilirubin level during the first days of life. The drug is able to cross the placental barrier and penetrate the fetal liver, where it presumably induces the hepatic transport mechanism involved in the excretion of bilirubin. Wilson's disease is a relatively rare inherited abnormality of copper metabolism, characterized by severe injury to the liver a:nd the central nervous system. Recent progress in the understanding of copper metabolism permitted development of a promising therapeutic and preventive regimen, involving use of copper chelating compounds, which in many instances allows patients with this disease to lead a normal life. In acute intermittent porphyria, the recent discovery of the primary genetically determined enzyme defect made it possible to develop a relatively simple biochemical test for the detection of asymptomatic carriers of this disease. This permits identification of individuals who have the inherited trait and therefore may be susceptible to drug-induced precipitation of acute neuropsychiatric attacks.
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Vol . 69, No.5
List of Persons Who Contributed Statements Used In Preparing This Report Work Group Memb ers Rudi Schmid, Chairman Burton Combes Carroll M . Leevy UrsA. Meyer Robert K. Ockner Steven Schenker M. Michael Thaler
Other Contributors Irwin M. Arias George Hug Anand a S. Prasad Telfer B. Reynolds
Irmin Sternlieb William H. J. Summerskill Louis R. Weintraub