The Effects of Heredity and Environment on Copper Metabolism

Symposium on Trace Elements

The Effects of Heredity and Environment on Copper Metabolism 1. Herbert Scheinberg, M.D. *

Copper is an essential element that carries the potential of toxicity as well. Since the environment does not uniformly supply just the amount of copper physiologically required, hereditary mechanisms have evolved to avoid the Scylla of copper deficiency, and the Charybdis of toxicity. In this paper the evidence for the essentiality and toxicity of copper is summarized. The genetic and environmental interactions of copper are compared with those of a metal that is toxic but not essential. Essentiality The existence of a number of specific mammalian copper-proteins and of rare instances of acquired or inherited human copper deficiency attest to the essentiality of copper. Table 1 lists 16 specific mammalian proteins that have been isolated and shown to contain copper as an integral part of the molecule. The significance of these copper-proteins is discussed below. Copper deficiency is an unusual clinical disorder in human beings because almost any diet contains several milligrams of copper and the adult requirement for this metal is satisfied by a total body content of little more than 100 mg.5 In infants with diarrhea or malabsorption, and a diet consisting almost exclusively of milk, a syndrome of neutropenia and anemia, correctable by the administration of copper, is occasionally encountered. 2 The dietary-or environmental-supply of copper is very generous, often exceeding 5 mg per day. Very little more dietary iron is able to supply a sufficiency of this metal despite the fact that its total body content is several grams - more than an order of magnitude greater than the body's content of copper. *Professor of Medicine, Albert Einstein College of Medicine; Visiting Physician, Bronx Municipal Hospital Center and The Hospital of the Albert Einstein College of Medicine, Bronx, N ew York Supported in part by grant AM·1059 from the National Institute of Arthritis, Metabolism, and Digestive Diseases and by the Foundation for the Study of Wilson's Disease, Inc.

Medical Clinics of North America- VoL 60, No. 4, July 1976

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Table 1. Mammalian Copper Proteins ISOLATED FROM PROTEIN

Albocuprein I Albocuprein 11 Ceruloplasmin Cytochrome c oxidase 3,4-Dihydroxyphenylethylamine-f3-hydroxylase Dopamine f3-hydroxylase Ferroxidase 11 Hepatomitochondrocuprein L-6-D (metallothionein) Lysyl oxidase Mitochondrial monoamine oxidase Pink copper protein Plasma/serum monoamine oxidase Superoxide dismutase (cytocuprein) Cerebrocuprein Erythrocuprein Hemocuprein Hepatocuprein Tryptophan-2,3-dioxygenase Tyrosinase

Species

Organ or Tissue

Man Man Numerous including man Numerous Cattle Cattle Man Man, Cattle Man, Cattle Chicken Man, Rat, Cattle Man Man, Rabbit, Pig

Heart, liver, etc. Adrenals Adrenals Serum Liver Liver Cartilage Liver, brain Erythrocytes Plasma/serum

Man Man Man Man Rat Man

Brain Erythrocytes Blood Liver Liver Skin, eye

Brain Brain Plasma

Acquired copper deficiency is seen considerably more frequently in sheep and cattle foraging on copper-deficient pastures and not receiving mineral supplementation of their feed. Menkes' syndrome, a sex-linked and uniformly fatal infantile disorder, represents a genetic defect in the absorption or transport of copper. The metabolic abnormality is complex, since the severe mental deficiency, abnormalities of hair, micrognathus, and deficiencies of copper-proteins that characterize this disease are apparently not preventable by the administration of either oral or parenteral copper. 3 -Table 2 summarizes what is known of the biochemistry and physiology of copper deficiency in a number of species. Toxicity The toxicity of copper, relatively common in animals (Table 3) is unusual in man. In India suicide is not infrequently induced successfully by the ingestion of gram quantities of copper, usually as the sulfate. 1 Other forms of acquired copper toxicity are uncommon and almost always clinically trivial. Nausea, vomiting, diarrhea, and crampy abdominal pain are seen when about 10 mg of cupric ion are ingested, particularly if the ions are not protein bound. Outbreaks are generally limited to the interaction of acidic liquids with copper plumbing or vessels. Several examples that have been reported in the literature involve citrus juices or drinks, or carbonated beverages that have been mixed in, or allowed to stand for several hours in contact with, vessels, plumbing, or check-valves made of copperll(Table 4).

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Table 2.

Correlation of Physiological and Chemical Manifestations of Copper Deficiency

PHYSIOLOGY

CHEMISTRY

Nervous system Ataxia

~ CYTOCHROME OXIDASE

Mental deficiency Hypogeusia

? ~ CYTOCHROME ?

Bone marrow Anemia Neutropenia Skeletal system Osteoporosis, fractures Cardiovascular system Congestive heart failure Aortic rupture

SPECIES

and

~

myelin

Sheep Man Man

OXIDASE

?~

CERULOPLASMIN

?~

L YSYL OXIDASE

Man, pig Man

?

and abnormal collagen

Cattle, rats Pig, fowl

~ CYTOCHROME OXIDASE ~ LYSYL OXIDASE

Man and others

and abnormal elastin

Reproductive system Low fertility

?

Cattle, rats

Gastrointestinal system Diarrhea

?

Cattle

Integument Achromotrichia Abnormal hair structure ~ Tensile strength of skin

Table 3.

~ TYROSINASE and ~ Melanin Abnormal keratin ~ L YSYL OXIDASE and abnormal collagen

Sheep Rats

Pathophysiology of Copper Toxicity in Animals

CAUSE

SPECIES

EFFECT

Ingestion of excess of Cu and/or deficiency of Mo

Sheep, cattle (ruminants)

Diarrhea, hepatic necrosis, hemolysis, nephropathy, death

Ingestion of > 250 ppm Cucontaining feed

Swine, rats (nonruminants)

Hepatic necrosis

Cu from fertilizer run-off or molluscicides

Minnows, salmon, trout, flounder, snails, worms

Fatty liver, renal necrosis, gill abnormalities, death

Molluscs

Cardiorespiratory depression, death

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Table 4.

HERBERT SCHEINBERG

Pathophysiology of Copper Toxicity in Man

CAUSE

EFFECT

Ingestion $1 gm > 10 gm

Vomiting, gastritis Shock, hepatic necrosis, hemolysis, renal toxicity, coma, death

Genetic defect

Hepatic and cerebral degeneration, renal dysfunction, corneal pigmentation, hemolysis, amenorrhea, spontaneous abortion

Except for such occurrences, significant copper toxicity in human beings occurs only in individuals who have inherited a pair of specific abnormal "Wilson's disease genes."7 At birth these children are indistinguishable from normal neonates in their low serum concentrations of ceruloplasmin and high hepatic concentrations of copper. Unlike normal infants, however, the patients-to-be maintain these concentrations throughout childhood and adult life so that in them, the physiologic increase in serum ceruloplasmin and decrease in hepatic copper do not occur. They appear to have inherited a defect in the homeostatic mechanism by which a zero, or close to zero, balance of copper is normally maintained. A fraction of dietary copper is apparently excreted through lysosomes of hepatocytes into the bile, and it is this route and mode of excretion that seems to be defective in Wilson's disease.lO Copper accumulates in the liver as these patients age. If biopsy samples are examined, even at 2 to 3 years of age, the excessive accumulations of copper can generally be shown to have produced inflammatory changes, fatty depositon, necrosis of cells, pigmentary changes, and an excess of fibrous tissue as the process moves relentlessly toward postnecrotic cirrhosis. s Ultimately the capacity of the liver to bind these high concentrations of copper, and to remain relatively immune to its toxic effects, is overcome. Structural changes in the liver become more pronounced and functional abnormalities become evident, most generally as elevated transaminase levels. If copper is suddenly released from the liver hemolytic anemia may supervene, probably as a reaction to high serum concentrations of free, or loosely albumin-bound, copper.4 If copper leaves the liver more gradually it is deposited ubiquitously throughout the body and produces manifestations of its toxic potential in susceptible cells (Table 5). Table 5. Tissue Copper Analyses of a Normal Control Subject and a Patient with Wilson's Disease MICROGRAMS COPPER/GRAM TISSUE

Liver Kidney Muscle Spinal cord Cortex (gray) Basal ganglion

(dry weight)

Normal Subject

Patient

12 10 5 12 16 25

532 47 10 109 327 225

COPPER METABOLISM

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The central nervous system is ultimately as disastrously affected by copper toxicity as the liver. A variety of anatomic and functional divisions of the brain are involved. Neurologic disease of many varieties may result: resting tremors, intention tremors, choreoathetosis, drooling, dysarthria, and disturbances in gait and coordination. Sensory changes and abnormal reflexes are rare, but there may be serious behavioral, neurotic or psychotic changes. When these effects become manifest in adolescence, as they frequently do, deterioration of previously competent school work may be the first sign. 7 Figure 1 shows an anatomic defect caused by copper. Copper deposits in the cornea-Kayser-Fleischer rings, or crescents-are of no pathologic significance in that they do not interfere with vision. They are, however, of the greatest diagnostic value since they are always present in patients with Wilson's disease who manifest neurologic signs or symptoms. They may be seen in asymptomatic patients as well. Copper less commonly causes cataracts - usually of the sunflower variety - which can impair vision. Copper deposits in the kidney produce abnormalities of glomerular and tubular function. These are generally of no clinical importance, but occasionally lead to defects in acidification and to nephrocalcinosis. The resulting clinical picture may vary greatly depending on which organs or tissues are most seriously affected. Hemolytic anemia or hepatitis may come, and disappear and their basic cause-excessive copper-may not be suspected. In young women, a malfunctioning liver may produce sufficient hormonal imbalance to result in primary amenorrhea, and multiple miscarriages. A sudden massive-and perhaps fatal-hemorrhage from esophageal varices may be the first manifestation of Wilson's disease. Neurologic or psychiatric disorders may lead the patient to a physi-

Figure 1. Cavity in the putamen of a 41 year old woman with an eight year history of progressively severe tremors, rigidity, speech difficulties, drooling, a perpetual grin, and Kayser-Fleischer rings before onset of DL-penicillamine therapy. There was marked neurological improvement, together with psychological deterioration during the first 2 years of therapy. Death was caused by hepatic insufficiency.

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cian-or even to hospitalization in a mental institution-without any clinical or historical evidence of liver disorder. Occasionally routine ophthalmologic examination may uncover Kayser-FlE';ischer rings, leading to the diagnosis of previously unsuspected Wilson's disease. The diagnosis may also be made in patients who are completely asymptomatic, but whose deficiency of ceruloplasmin and excess of hepatic copper are sought for because of their blood relationship to a patient with Wilson's disease. 9 Although there is still uncertainty as to the precise pathophysiologic mechanism leading to the chronic copper toxicity of Wilson's disease, there is no doubt whatever that copper toxicity is the etiology of the disorder. Abundant proof has been obtained, over the past 18 years, by the administration to patients with the disease of penicillamine ({3,{3dimethylcysteine), a drug able to chelate tissue copper and to excrete it via the urine. Quantitative studies have demonstrated that regular administration of penicillamine can keep patients in negative copper balance for years. At the same time neurologic effects, hepatic dysfunction, or psychiatric disorders generally improve, and frequently to complete normality.u Kayser-Fleischer rings disappear or become less prominent with continued treatment. 1O Asymptomatic patients remain so indefinitely. Discussion The homeostatic abnormalities that characterize Menkes' syndrome and Wilson's disease constitute a negative demonstration of a genetic, physiological buffer system designed to avoid both deficiency and toxicity of copper in the face of wide variations in its dietary supply. Indeed, both human copper deficiency, and toxicity are extremely rare just because these two mechanisms are so extremely effective, and because hereditary defects in them are so unusual. Wilson's disease has an approximate worldwide prevalence of 1 in 200,000. As an autosomal recessive gene-produced disease, this corresponds to a prevalance of heterozygous individuals-in whom Wilsonian chronic copper toxicity never occurs-of 1 in 200. Wilson's disease would be a thousand times more common if its gene were dominant or if we were not diploid. Why did such homeostatic mechanisms develop in the case of copper and yet not in the case of such other metals as, for example, mercury? The mercuric ion and cupric ion have a number of siInilarities in their toxic potential (Table 6). Yet ingestion of very small quantities of the mercuric ion can produce serious or fatal toxicity-and copper, as we have seen, rarely does so, and requires much larger amounts. This striking difference is attributable, I believe, to the fact that copper is essential to health and life, while mercury is not. The evolutionary process apparently found virtues in copper for a number of physiologic purposes - but none in mercury. Organisms chose diets that could supply copper. There evolved biochemical mechanisms that facilitate the absorption and transport of copper, and others that promote the excretion of any excessive amounts of copper that may be ingested. Further,

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Table 6. Comparison of Metabolic Aspects of.Copper and Mercury

Toxic species Chemical basis of toxicity Essentiality Active absorption Active excretion Sites of absorption

Specific metalloproteins Excretory pathways Organs affected

Industrial hazard Toxic serum concentration of free ionic species Therapeutic agents

COPPER

MERCURY

Cu++ Combination with SH Yes Yes Yes GI tract Burned skin Lungs Uterine mucosa Yes GI tract Kidney (minimal)

Hgt, Hg++ Combination with SH for Hg++ No No No GI tract Skin Lungs

Uver Brain Kidney Eye Negligible

> 10 ILg/I00 ml Penicillamine B.A.L.

No GI tract Kidney Lungs (Hg t ) (minimal) GI tract Kidney Brain Eye Skin Appreciable (Hg t ) > 20 ILg/I00 ml Penicillamine N.acetyl penicillamine B.A.L.

copper always functions physiologically as an integral part of a specific protein molecule-cytochrome oxidase or tyrosinase, for example. Nonessential mercury, in dual contrast, neither stimulated mechanisms to prevent accumulation, nor the synthesis of proteins that would bind and thereby detoxify the mercuric ion. Mercuric ions with as much, or more, affinity for proteins as cupric ions circulate in the free divalent state and its combinations with proteins are nonspecific and highly injurious. Iron, zinc and cobalt are also heavy metals and essential cations. They are present either as an integral part of proteins or, in the case of cobalt, of a somewhat smaller organic molecule. Heavy metals that are not essential include cadmium, beryllium, manganese, nickel, chromium, and vanadium. All the latter are toxic. Thus, in contrast to essential heavy metals that are toxic only in unusual circumstances, nonessential metals are generally toxic. REFERENCES 1. Chuttani, H. K., Gupta, P. S., Gulati, S., et al.: Acute copper sulfate poiSOning. Amer.

J.

Med., 39:849-854, 1965. 2. Cordano, A., Placko, R. P., and Graham, G. G.: Hypocupremia and neutropenia in copper deficiency. Blood, 28:280-283, 1966. 3. Danks, D. M., Stevens, B. J., Campbell, P. E., et al.: Menkes' kinky·hair syndrome. Lan· cet, 1:1100-1103, 1972.

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4. Deiss, A., Lee, G. R., and Cartwright, G. E.: Hemolytic anemia in Wilson's disease. Ann. Intern. Med., 73:413-418, 1970. 5. Scheinberg, I. H., and Sternlieb, I.: Metabolism of trace metals, In Bondy, P. K., ed.: Duncan's Diseases of Metabolism, Vol. 2. 6th ed., 1969. Philadelphia, W. B. Saunders Co., 1969. 6. Scheinberg, I. H., and Sternlieb, I.: The long term management of hepatolenticular degeneration (Wilson's disease). Amer. J. Med., 29:316-333, 1960. 7. Scheinberg, I. H., and Sternlieb, I.: Wilson's disease. Ann. Rev. Med., 16:119-134, 1965. 8. Sternlieb, I.: Evolution of the hepatic lesion in Wilson's disease (hepatolenticular degeneration). pp. 511-525. In Popper, H., and Schaffner, F., eds.: Progress in Liver Dis eases, Vol. IV. New York, Grune & Stratton, 1972. 9. Sternlieb, I., and Scheinberg, I. H.: Prevention of Wilson's disease in asymptomatic patients. New Eng. J. Med., 278:352-359, 1968. 10. Sternlieb, I., van den Hamer, C. J. A., Morell, A. G., et al.: Lysosomal defect of hepatic copper excretion in Wilson's disease (Hepatolenticular degeneration). Gastroenterology, 64:99-105, 1973. 11. United States Department of Health, Education, and Welfare. Center for Disease Control. Acute copper poisoning-Pennsylvania. Morbidity and Mortality Weekly Report, Vol. 24, No. 11, p. 99, 1975. Division of Genetic Medicine Albert Einstein College of Medicine 1300 Morris Park Avenue Bronx, New York 10461