Sugar metabolism in the crystalline lens

SURVEY OF OPHTHALMOLOGY

CURRENT

VOLUME 23

l

NUMBER 1 JULY-AUGUST

1978

l

RESEARCH

EDWARD COTLIER, EDITOR

Sugar

Metabolism

Crystalline

in the

Lens

LEO T. CHYLACK, JR., M.D. AND HONG-MING

CHENG, O.D., Ph.D.

Howe Laboratory of Ophthalmology, Harvard Medical School and Massachusetts Eye & Ear in$rmary, Boston, Massachusetts

Research on the sugar metabolism of the crystalline lens, past and present, is reviewed. The chief energy source in the lens is the Em~en-Meyerhof pathway;

.4bstract.

respiration and oxidative phosphorylation become more important as the lens ages. The function of the a-giycerophosphate cycle is not fully understood. The mechanisms involved in cataract formation, including those of hypoglycemic cataract and osmotic cataracts, are discussed. Sugar cataracts can be delayed or prevented with such aldose reductase inhibitors as flavonoids. By inhibiting aldose reductase, the formation and accumulation of sugar alcohols is stopped. This approach may be useful as a medical therapy for human diabetic senile cataracts. (Surv Ophthalmol 23: 26-34, 1978) aldose reductase inhibitors cataract Key words. cataractogenesis crystalline lens diabetes hypoglycemia metabolism osmotic stress l

l

l

l

l

l

l

S

ugar metabolism, a general concept comprising the several fates of glucose in the mammalian cell, has attracted renewed interest in the study of cataract formation. Recently, glucose deprivation, leading to the loss of a key glycolytic enzyme (hexokinase), has been identified as the initial step in the evolution of the hypoglycemic cataract. Glucose, gaiactose or xylose excess leads to cataract formation in experimental animals and man, and the mechanism of this form of osmotic cataract has been thoroughly documented and substantiated. In fact, it is the successful medical intervention to prevent the so-called “sugar cataract” that has lead to the recent optimism that a medical approach to senile cataracts in diabetics may be in the

offing. It is the purpose of this paper to review the significant advances in research on glucose metabolism in the lens with emphasis on those pathways that offer us the greatest insights into the mechanism of cataractogenesis and possible medical therapy of human cataract. Those pathways of sugar metabolism which are not now related to any of the above will be deemphasized or omitted. Background During this century, our understanding of lens metabolism has grown immensely; prior to 1928, the lens was regarded as an inert bag of protein. However, in 1928, Kronfeld and Bothman4B showed that the lens was able to 26

CURRENT

RESEARCH

produce lactic acid in the presence of glucose. Nine years later, Sullmana4 found acidsoluble organic phosphates (glycolytic intermediates) in bovine lenses incubated with inorganic phosphate and various sugars. During the next twenty years, little work was done on phosphorus metabolism of the lens, but in 1952, Nordmann and MandeP7 published results conclusively demonstrating practically all of the phosphorylated intermediates of the Embden-Myerhof pathway in the lens. Shortly thereafter, Green et ~12~ documented the presence of all the enzymes and intermediates of anaerobic glycolysis in the lens. The existence of anaerobic glycolysis in the lens seemed well established. The role of aerobic pathways, such as the citric acid cycle and the cytochrome system of oxidative phosphorylation, was judged to be less important than anaerobic glycolysis in terms of overall energy production, largely due to the relatively small amount of CO, which is produced via the metabolism of C-6 of glucose. Epithelial mitochondria do not have access to a large portion of the pyruvate generated in the cortex. There, pyruvate is reduced to lactate and diffuses out of the lens.@ Glutamate, however, can enter the citric acid cycle as cu-oxoglutarate.Bs Aerobic pathways are found in mitochondria, and these organelles, limited to the epithelium and very superficial cortex, are extremely scarce in the lens. In spite of this, the highest levels of ATP are found in the epithelium.20 In 1961, Kinoshita et aIs demonstrated with calf lens that Na+ and K+ contents, amino acid incorporation into protein, and ATP levels could be sustained at normal levels in the absence of oxygen as long as sufficient glucose was present. Many other studies have supported the belief that the principal source of energy in the lens is the Embden-Meyerhof pathway.28~34~3s~38 Even cation transport, a process localized to the epithelium, does not require oxygen to maintain ionic homeostasis in the lens.’ This is surprising in view of the concentration of lens mitochondria in the epithelium. There are species differences; man and chicken are susceptible to dinitrophenol, a cataractogenic oxidative which uncouples compound phosphorylation from the cytochrome system.50 It was not until 1973 that Trayhurn and van Heyningen,B6~6s*s7 in a study of the bovine lens, clearly demonstrated a significant role for aerobic metabolism of amino

27

acids in lens homeostasis. It appears that the aerobic mechanism assumes relatively greater significance in overall lens homeostasis with increasing age.‘O In spite of the small portion of total lens volume occupied by the epithelium (0.1%) this tissue is responsible for approximately 30% of the ATP produced by the older lens.B8 Another possible link between anaerobic glycolysis and aerobic metabolism might be the a-glycerophosphate cycle.6o a-Glycerophosphate is formed by reduction of dihydroxyacetone phosphate (an intermediate of the Embden-Meyerhof pathway). This substance can then enter the mitochondrion, and be oxidized by NAD with the transfer of its electrons into the cytochrome system and oxidative phosphorylation. a-Glycerophosphate dehydrogenase is found both in the cytosol as well as in the mitochondrion. The dihydroxyacetone phosphate diffuses out of the mitochondrion to reenter the glycolytic sequence. The principal evidence in support of this is the observation that more a-glycerophosphate is formed anaerobically than aerobically. A schematic presentation of the metabolic pathways is shown in Fig. 1. Sugar Metabolism by the Lens Glucose is transported into the lens via facilitated transport.22~3S~4B@‘~53 This does not appear to be the “bottleneck.” In 1955, Harri? showed that fluoride, iodoacetate and acetoacetate inhibit glycolysis and lead to a drop in glucose transport; inhibitors of respiratory enzymes (CN-, salicylate, arsenite, dinitrophenol and anoxia) stimulate the uptake of glucose via the Pasteur effect. These data suggest that the rate of glucose transport is a function and not a regulator of metabolic rate. Early work suggested that insulin had little effect on lens metabolism. In 1958, Giles and Harri? showed that labeled insulin did not enter aqueous humor. Earlier work by Farkas and Pattersonls and ROSS”*suggested that while insulin did not stimulate glucose entry into the intact lens, it might increase uptake into the decapsulated lens. A study reported in 1977 documented the presence of insulin in the aqueous humor using a radioimmunoassay,*I5so the effect of this hormone on lens metabolism may have to be reconsidered. In 1955, Green et ~12~ found that the glycolytic rate in the lens was limited by the amount of hexokinase present. While not

28

CHYLACK, CHENG

Surv Ophthalmol 23 (1) July-August. 1976

Glucose--B-phosphate

Pentose Phosphate Shunt f21 I Fructose-1.6-dtohosphate

Phosphoenolpyruvate

(3) Giycerophosphate

Pyruvate-Acetyl 16)

1 Lactate

FIG. I. A simplified scheme of glucose metabolism including the Embden-Meyerhof pathway, agly~erophosphate cycle, sorbitol, pathway, pentose phosphate shunt and part of citric acid cycle: (I) hexokinase, (2) phosphofructokinase, (3) cytochrome (mitochondria), (4) aldose reductase, (5) pyruvate kinase, (6) lactate dehydrogenase, and (7) sorbitol dehydrogenase.

CoA

I Citric Acid Cycle

this phenomenon to the equating “bottleneck” observed by Harris em aP or Patterson (Patterson JW: personal communication), they clearly were observing one facet of the same phenomenon. In 1957, PirieG1suggested that hexokinase might be the pacemaker of lens glycolysis. Increasing ambient glucose concentration led to a marked increase in intracellular glucose without an increase in lactate formation. The extremely low specific activity of lens hexokinase in normal lens, as measured by Weckers and Sullman in 1938,76 Nordmann in 1954,58 Green and Solomon in 195g2” and van Heyningen in 196Y@is consistent with the ability of this enzyme to regulate introduction of glucose into the glycolytic pathways. More detailed studies of hexokinase’-lo have shown the lens to contain two of the five isozymes (I, Ha, Hb, III, IV) of hexokinase; they are types I and II in the classification scheme of Katzen and Schimke.ss They exist in soluble active and insoluble active and latent forms. Each isozyme has a different affinity for glucose and a characteristic heat lability; Type II is peculiarly unstable at body temperature (372°C) in the absence of its substrate, glucose.@ There is also some evidence suggesting that hexokinase may bind reversibly to lens mitochondria as a function of this glucose concentration (Chylack LT, Jr: unpublished observations). No longer can we regard the so-cafled “pacemaker” of lens glycolysis as a single enzyme effecting its regulatory influence by virtue of its low specific activity. Compartmentalization of enzyme between soluble and insoluble fractions may isolate enzyme from substrate;

differences in relative amounts of each isozyme in different areas of the lens may alter the rate of glucose phosphorylation; and thermal deactivation of the Type II isozyme may modify the amounts of enzyme available. All of these mechanisms may exert their effects simultaneously or individually, thereby modulating glycolytic rate. Hexokinase has been shown to be inhibited by its product, glucose-&phosphate (G-6-P).54 Inorganic phosphate can release the enzyme from the inhibitory effects of G-6-P;54 Green et aL2’ in 1955, demonstrated a stimulatory effect of inorganic phosphate on anaerobic lens glycolysis, but did not attribute this to release of G-6-P inhibition of hexokinase. They also demonstrated that ADP or AMP could increase lactic acid formation from G6-P or fructose-6-phosphate (F-6-P) but did not postulate a mechanism for this. This was an excellent demonstration of the rate limiting effect of phosphofructokinase (PFK) on anaerobic glycolysis. Lou and Kinoshita6” found that the amount of ATP normally found in the lens could almost completely inhibit lens phosphofructokinase. They also demonstrated a release of this inhibition by ADP and AMP and provided an explanation for the observations of Green et aLz5 Earlier investigators had observed that the rate of glucose utilization by the intact lens was less than the rate of the hexokinase reaction; this suggested that other reactions (i.e., PFK) might be participating in glycoiytic regulation. Recent work on lens PFKSp4has shown that this enzyme exists in two interconvertible forms in the lens, pH change bringing about a

CURRENT RESEARCH

change from one form into another. PFK-I is the dominant form at pH 7.05-7.40, while PFK-II predominates at pH 7.4-8.2. PFK-II is believed to be the form functioning in lens glycolysis; it is inhibited by high concentrations of ATP and this effect is enhanced at more acidic pH. F-6-P, ADP and AMP are potent deinhibitors of ATP-inhibited PFK. Other effectors, both positive and negative, are found in the lens: SO4 =, Pi, NH4 + and K + deinhibit, whereas Ca ++ further inhibits the enzyme. PFK also demonstrates the peculiar characteristic of cold-lability at acidic pH, even in the presence of SO4 = and Pi, both positive effectors. The inactivation is reversible in the presence of F-6-P at alkaline pH and can be prevented by including ATP or F6-P in the incubating media. There are some species differences in stabilization of PFK by ATP, SO4 =, and Pi. Rat lens is well preserved, while human lens PFK is incompletely protected by ATP? This suggests that acidity may be more detrimental to the maintenance of PFK stability in human lens than in rat lens. As with hexokinase, there are manifold ways in which PFK activity m a y alter glycolytic rate and act as a metabolic regulator. The most recent data suggest that, in addition to the above, PFK loses its capacity to phosphorylate F-6-P with increasing age. 6 This may, in part, account for the decreasing capacity of glycolysis with increasing age. Another rate limiting enzymatic step in glycolysis is pyruvate kinase. The rate of lactate production from fructose-I, 6diphosphate and phosphoenol pyruvate (PEP) are essentially identical, indicating no significant rate limiting steps between aldolase and pyruvate kinase. However, the rate of lactate production with PEP as substrate is only 1/6 that with pyruvate? These several points of potential metabolic control have been identified in lens as well as in many other tissues. The manner in which these mechanisms in the normal lens respond to or influence the cytoplasmic milieu remain to be determined. Also, the manner in which disordered metabolic regulation may lead to cataract formation remains to be defined.

Hypoglycemic Cataract Formation Neonatal hypoglycemia is an example of disordered glucose homeostasis in which the blood sugar may drop to 0-20 mg% for

29

several hours. In addition to the neurologic and other systemic manifestations of severe hypoglycemia, more than 20% of these children develop a characteristic lamellar and zonular cataract. 19,21,27,a1,~,5~,~,78 Recently, the mechanism of irreversible lens damage was found to be an irreversible deactivation of hexokinase, the enzyme which is intimately involved with glycolytic rate (and therefore, ATP production). 12,~3 The drop in serum glucose causes, in turn, a drop in aqueous humor glucose, the principal substrate of lens hexokinase. The lens has virtually no glycogen reserves and essentially no gluconeogenic potential for restoring intracellular glucose levels to normal. In the absence of glucose, no ATP is produced and both stabilizing substrates of hexokinase (i.e., glucose and ATP) are sufficiently depleted to destabilize lens hexokinase. Within 8-10 hours of glucose deprivation, 80% of lens hexokinase activity is lost irreversibly. This metabolic stress is sufficient to cause increase in lens weight and opacification in a pattern remarkably similar to that which occurs in humans (Fig. 2). There are many similarities and no significant dissimilarities between rat lens and human lens hexokinase8 and it is assumed that the same phenomena occur in rat and human lens. Most likely, intermittent hypoglycemic attacks are responsible for the lamellar nature of the cataract. Maintenance of normal blood sugar and prevention of repeated attacks of severe hypoglycemia should reduce or eliminate the risk of cataract formation. These reports 1~,18 represented the first published examples of how a deficiency of a key glycolytic enzyme can lead to cataract formation.

Formation and Prevention of " S u g a r Cataracts" In contrast to the effects of glucose deficiency, glucose, galactose or xylose excess can lead to cataract formation in experimental animals, in incubated lenses and also presumably in humans. During the past 15 years, the importance of the sorbitol pathway in "sugar cataract" formation has been conclusively established by van Heyningen, Kinoshita and others. 11,4°,41,42,44,51,71-7~ The pathway is shown in Fig. 1. The hexose monophosphate (or pentose shunt) is an alternate pathway for the metabolism of glucose6-phosphate. The reducing equivalents (NADPH) produced as G-6-P is oxidized to

30

Surv Ophthalmol

23 (1) July-August,

1978

CHYLACK,

FIG.2. Light transmission microscopy of “hypoglycemic” rat lens. The lenses were incubated at 37.5V for 20 hrs. in isotonic media supplemented with various concentrations of glucose: (1) 12mM; (2) 2mM; (3) 1.5mM; (4) 1 mM; (5) OmM; and (6) OmM/48 hrs. The extent of disorganization of lens fibers is directly related to the degree of hypoglycemia.

and its accumulation within the cell renders 6-phosphogluconic acid and ribulose-5the cytoplasmic milieu hypertonic. To phosphate are utilized by many reaction pathways in the lens, not the least important neutralize this, extracellular water moves into of which is the sorbitol pathway. Without the cell and the lens swells. Sugar alcohols per se are nontoxic to the cell; the adverse effect elevated hexose levels and sufficient NADPH, aldose reductase could not reduce of their accumulation is due to the resultant glucose or galactose to its respective sugar hypertonicity. Fiber swelling can be opposed alcohol. by the Na+ -K+ cation pump but its capacity Accumulation of the sugar is found, for ex- is limited, and after this capacity is exceeded, ample, in the case of galactosemic cataract. membrane damage occurs. Na+ moves into Absence or deficiency of galactokinase or the cell; K+ and several other soluble galactose- l-phosphate uridyl transferase molecules move out of the cell. The net effect leads to accumulation of galactose. If the is a hypertonicity which is counteracted by galactose level is high enough, as after a lac- even more water entering the cell. The loss of tose or galactose load, galactose is converted glutathione, soluble protein, amino acids and by the enzyme aldose reductase to its sugar other intracellular components constitutes an alcohol, galactitol (du1cito1).‘1~72 Membranes irreversible stress46 (fig. 3). If the galactose are impermeable to this sugar alcoho1’4*52*‘7 load is decreased before membrane damage

CURRENT

NORMAL

RESEARCH

LENS

PRE-VACUOLE STAGE

FIG. 3. A diagrammatic (Reprinted from Kinoshita

INITIAL

VACUOLAR STAGE

LATE

VACUOLAR STAGE

NUCLEAR

CATARACT

STAGE

presentation of mechanisms involved in galactose cataractogenesis. JH”” with permission of the author and Investigative Ophrhalmo/ogv.)

Treatment of Diabetic Cataracts occurs, then the cataracts are reversible. This same sequence of formation and reversibility The renewed interest in sugar metabolism applies for glucose and xylose, albeit the rate in the lens is derived largely from the unof cataract formation reflects the rate at derstanding of the sorbitol pathway and the which the hexose is converted to its sugar promise it holds for treatment of senile alcohol. cataracts in diabetic patients. The number of Sugar cataracts can be prevented by incor- patients with galactosemia, xylosemia and porating an aldose reductase inhibitor in the uncontrolled juvenile-onset diabetes mellitus lens incubation medium with the high sugar is exceedingly small, probably too small to level. This blocks the conversion of the sugar encourage pharmaceutical companies to in(glucose, xylose, galactose) to its sugar vest large amounts of capital in programs to alcohol.” Sugar cataracts were the first to be develop safe, highly effective aldose reductase prevented. Several inhibitors have been inhibitors. However, there is reason to believe developed (tetramethylene glutaric acid, that diabetes mellitus increases the incidence AY22,284 and others), but the most promis- and possibly accelerates the maturation of ing group of inhibitors in terms of the senile cataracts in diabetics. The best strength of inhibition is the flavonoids18,46,74, evidence for the increased incidence in e.g., quercetin, quercitrin, and myricitrin diabetics derives from the Framingham Eye (Fig. 4). These compounds are ubiquitously Study, a subdivision of the Framingham distributed within the plant kingdom and Heart study.32 The blood sugars in men and their biological role has not yet been defined. women aged 52-64 with cataracts were 107 They are quite nontoxic and as such are and 88 mg% respectively, compared to blood promising as useful aldose reductase inhibitors. Most of these compounds have to be OH injected into the vitreous to have an effect on the lens, although some are effective orally in delaying or preventing the early onset of galactosemic cataract in rats. At present, several laboratories are engaged in an intensive search for more potent aldose reductase CjH ij OH inhibitors. Additional evidence confirming the importance of the sorbitol pathway to sugar cataract formation is found in mice with congenital hyperglycemia.75 In spite of blood glucose levels persistently greater than normal, these mice do not develop cataracts. Their resistance is due to the low activity of aldose reductase in the lens. Even with exFIG. 4. Chemical structure of two of the most tremely high hexose levels, only insignificant potent aldose reductase inhibitors, i.e., Quercitrin amounts of polyol are formed, amounts insuf- (top) and Quercitryl-2” acetate (bottom). These inhibitors have been shown to arrest the developing ficient to cause intracellular hypertonicity experimental diabetic cataracts. and lens swelling.

HopQ&”

32

SW Ophthalmol SCHEME

1

23 (1) July-August,

1978 SCHEME

CHYLACK, CHENG II

FIG. 5. Cataractogenic mechanisms: Scheme I: Syncataractogenesis, in which the formation of an opacity is due to the influence of two subliminal factors that do not affect transparency when acting alone; Scheme II: Co-cataractogenesis, in

0 sugars of X9 and 79 mg% in patients without cataract. These differences were significant at or below .05. In this report, other statistically signi~cant associations with cataract were: education, systemic blood pressure, height, vital capacity, serum phospholipids and hand strength. Of these, only diabetes mellitus has been suggested as a risk factor for senile cataract2 That the sorbitol pathway is involved in the evolution of the human diabetic cataract is suggested by the following. (1) Changes in refractive errors occur with fluctuations in lens sorbitol (and also lens volume). The refractive power of the lens is a function of its shape, volume and refractive indextZ4 (2) Reversible lens opacities occur in diabetics.“’ These are similar to the reversible sugar cataracts in experimental animals. (3) Varma has shown that polyol formation in viuo in the rat lens is accelerated in diabetes.‘$ While there is no evidence establishing the accumulation of sorbitol as the sole or most important cause of senile cataracts in diabetics, there is reason to believe that it may be a contributing factor. If sorbitol formation were blocked with an aldose reductase inhibitor, presumably, the osmotic stress, a cataractogenic stress, would be eliminated. This might lead to slower cataract maturation or complete cessation of cataract formation in diabetics. The experimental basis for the above is found in several articles in both the European and American literature referred to in an excellent recent review article.30 Many examples are cited in which two subcataractogenic stresses are combined to produce a cataract (Fig. 5). Also, it is possible for a subcataractogenic stress to enhance a known cataractogenic stress. Conversely, by eliminating one of several subcataractogenic stresses (i.e., sorbitol accumulation within the diabetic lens), it

which a subliminal factor potentiates the action of a direct cataractogenic factor. (Reprinted from Koch HR et al’? with permission of the authors and Metabolic Ophthalmology) might be possible to decelerate or stop the development of cataracts. Aldose reductase inhibition offers the prospect of eliminating one of the known cataractogenic stresses. The challenge is now to find an inhibitor of sufficient potency and safety to be effective in humans and to devise a means of measuring the effect of this enzyme inhibitor on cataract maturation. Not only is the change likely to be subtle and apparent only after years of observation, but very large numbers of treated and control patients will be necessary to establish statistically significant results. A controlled randomized clinical trial will be In addition to photographic necessary. documentation of cataractous changes, it might be possible to monitor the effects of diabetes and aldose reductase inhibitors on the refractive state and/or lens volume of the precataractous eye. Some promising results have already been obtained with this approach.

Conclusion The past fifty years have seen many miniscule advances in our understanding of the metabolism of the normal and cataractous lens. While not always clearly interrelated or even relevant, the cumulative result of these research efforts has been to provide us with what may be a reasonable nonsurgical approach to the treatment of cataract.

References 1. Becker B: Accumulation rubidium-86 by the rabbit lens. Invest Ophthalmol 1:502-506, 1962

2. Caird FI, Pirie A, Ramsell TG: Diabetes & the Eye. Oxford

and Edinburgh, Blackwell Scientific Publications, 1968, pp 122-126 3. Cheng H-M, Chylack LT Jr: Properties of rat lens phosphofru~tokinase. Invest Oph~almol 15:279-287, 1976 4. Cheng H-M, Chylack LT Jr: pH-dependent

temperature

sensitivity

of

rat

lens

CURRENT

RESEARCH

33

phosphofructokinase. 15:505-509,

invest

Ophthaimoi

1976

H-M, Chylack LT Jr, Chien J, Barafiano EC: Stability of mammalian lens phosphofructokinase. Invest Ophthalmol Vis

23. Giles KM, Harris JE: Radioelectrophoretic patterns of aqueous and plasma. Am

5. Cheng

Sci 16:126-134, 1977 6. Cheng H-M, Chylack LT Jr: Factors

the

affecting in rat lens.

rate

of lactate production Ophthal Res (in press) 7. Chylack LT Jr, Kinoshita JH, Kasabian R: Nature and distribution of two distinct forms of hexokinase within the mamalian lens. Exp Eye Res 8. Chylack Eye Res 9. Chylack

10:250-262,

1970

LT Jr: Human 15:225-233,

LT hexokinase. IO. Chylack LT hexokinases Res 6:93-106,

lens hexokinase.

Exp

1973

Jr: Thermo-instability

of rat lens

Exp Eye Res 17:109-l

17, 197

Jr: Soluble, insoluble and latent in the mamalian lens. Ophthal 1974

Ophthalmol 24. Granstrom

1969 12. Chylack

LT Jr: Mechanism of “hypoglycemic” cataract formation in the rat lens. I. The role of hexokinase instability. invest

Ophthalmol 14:746-755, 1975 13. Chylack LT Jr, Schaefer F: Mechanism

of “hypoglycemic cataract formation in the rat lens. Ii. Invest Ophthalmol 15: 510-528, 1976 14. Colas MC, Peres R, Malangeau P: Excretion rtnale comparte des polyols lineaires et du meso-inositol. Ann Pharm Franc 17:260-269, 1959 15. Coulter

JB, Knebel RL: Availability of insulin lens via aqueous humor. invest Ophthalmol Visual Science (Suppl ARVO):l3, 1977 16. Dvornik D. Simard-Duquesne N, Krami M, et al: Polyol accumulation in galactosemic and diabetic rate: Control by an aldose reductase inhibitor. Science 182:1146-l 148, 1974 17. Epstein DL Reversible unilateral lens in a diabetic patient. Arch opacities to

the

1933 25. Green

H, Bother CA, Leopold IH: Anaerobic carbohydrate metabolism of crystalline lens.

Am J Ophthalmol 39:106-l 18, 1955 26. Green H, Solomon SA: Hexokinase of rabbit lens. Arch Ophthalmol 61:616-625, 1959 27. Grunt JA, Howard RO: Eye findings in children with ketotic hypoglycemia. Can J Ophthalmol 7:151-156, 1972 28. Harris JE, Gruber L, Talman E, Hoskinson

G: The influence of oxygen on the photodynamic action of methylene blue on cation transport in the rabbit lens. Am J Ophthalmol 48:528-534, 29. Harris, JE, Hauscheldt

Transport

1959

JD, Nordquist LT: of glucose across the lens surfaces.

Am J Ophthalmol 39:161-169, 1955 30. Hockwin 0, Koch HR: Combined noxious influence, in Bellows JG (ed): Cataract and Abnormalities of the Lens. New York, Grune and

Stratton, 1975, pp 243-254 31. Hull D: Cataracts associated with metabolic disorders in infancy. Proc Roy Sot Med 62:694-696, 1969 32. Kahn HA, Liebowitz HM, Ganley JP, et al: The Framingham eye study. II. Association of ophthalmic pathology with single variables previously measured in the Framingham Heart Study. Am J Epidemiol 106:33-41, 1977 33. Katzen HM, Schimke RT: Multiple forms of hexokinase in the rat: Tissue distribution, age dependency, and properties. Proc Nat Acad Sci 54:1218-1225, 1965 34. Kern HL: Accumulation of amino acids by calf lens. Invest Ophthaimol 1:368-376, 1962 35. Kern HL: The transport of sugars into the lens, in Harris JE (ed): Symposium on the Lens. St. Louis, CV Mosby, 1965, pp 304-314 36. Kinsey VE, Reddy VN: Studies on the crystalline lens. X. Transport of amino acids. Invest Ophthalmol 2:229-236, 1963 37. Kinoshita JH, Kern HL, Merola LO: Factors

Ophthalmol 94:461-463, 1976 18. Farkas TF, Patterson JW: Insulin and the lens. Am J Ophthalmol 44:341-346, 1957 19. Fraser GR, Friedman AI: The Causes of Blindness in Childhood. Baltimore, Johns

38.

Hopkins Press, 1967 20. Frohman CE, Kinsey VE: Studies on the crystalline lens: V. Distribution of various phosphate-containing compounds and its significance. Arch Ophthalmol 48: 12, 1952 21. Gabilan JC, Chaussain JL: L’association hypoglycemic idiopathique et cataracte chez I’enfant. Arch Franc Pediat 26:633, 1969 22. Giles KM, Harris JE: The accumulation of C” from uniformly labeled glucose by the normal and diabetic rabbit lens. Am J Ophthalmol

Acta 62:176-178, 1962 4 I. Kinoshita JH, Futterman

48:508-5 17, 1959

J

1958

KD: Refraktionsveranderungen mellitus. Acta Ophtl (Kbb) ll:l.

bei diabetes

I I. Chylack

LT Jr, Kinoshita JH: A biochemical evaluation of a cataract induced in a highglucose medium. invest Ophthalmol8:401-412,

46:196-204,

affecting

the cation

transport

of calf

Biochim Biophys Acta 47:458-466,

lens.

1961

Kinoshita metabolism

JF: Pathways of glucose in the lens, in Harris JE (ed): Symposium on the Lens. St. Louis, CV Mosby, 1965, pp 243-252 39. Kinoshita JH, Merola LO, Dikmak E: Osmotic changes in experimental galactose cataracts. Exp Eye Res 1:405-410, 1962 40. Kinoshita JH, Merola LO, Dikmak E: The accumulation of dulcitol and water in rabbit lens incubated with galactose. Biochim Biophys

LO: Factors

affecting

S, Satoh K, Merola the formation of sugar

Surv Ophthalmol

34

23 (1) July-August,

1978

CWYLACK, CHENG

alcohols in ocular lens. Biochim Biophys Acta 74:340-350,

61.

1963

Kinoshita JH, Merola LO, Satoh K, Dikmak E.: Osmotic changes caused by the accumulation of ducitol in the lenses of rats fed with galactose. Nature 194:1085-1087, 1962 43, Kinoshita JH, Merola LO: Hydration of the lens during the development of galactose cataract. Invest Ophthalmol 3:577-584, 1964 44. Kinoshita JH: Cataracts in galactosemia.

Bibliotheca

42.

Invest Ophthalmol

4:786-799,

62. 63.

64.

1965

Kinoshita JH, Merola LO, Tung B: Changes in cation permeability in the galactoseexposed rabbit lens. Exp Eye Res 7:80-90, 1968 46. Kinoshita JH, Dvornik D, Kraml M, Gabbay KH: The effect of an aldose reductase inhibitor on the galactose-exposed rabbit lens. 45.

Bioehim Biophys Acta 158:472-475, 1968 47. Koch HR, Hockwin 0, Ohrloff C: Metabolic disorders of the lens. Metabol Ophthalmol IS-61, 1976 48. Kronfeld P, Bothman L: Zur Frage de Linsenatmung. L Augenheilkd 65:41-62, 1928 49. Kuck JF Jr: Glucose metabolism and fructose synthesis in the diabetic rat lens. Invest Ophthalmol 1:390-395, 1962 50. Kuck JF Jr: Sugar and sugar alcohol levels in the aging rat lens. Invest Ophthalmol 2:607611, 1963 51. Kuck JFR Jr: Sorbitol pathway metabolites in the diabetic rabbit lens. Invest OphthaImol 5:65-74, 1966 52. LeFevre PG, Davis R: Active transport into

53.

66.

67.

68.

69.

70.

71. 72.

73.

L951

74.

22:361-369, 54.

65.

the human erythrocyte: Evidence from comparative kinetics and competition among monosaccharides. J Gen Physiol 34:s 15-524, Levari R, Kirnblueth W, Wertheimer E: The effect of insulin on the uptake of monosaccharides by the rat lens. J Endocrinol

75.

1961

Lou MF, Kinoshita JH: Control of lens giycolysis. Biochim Biophys Acts 141:547-559,

76.

1967 55.

McKenna AJ: Neonatal hypoglycemia: Some ophthalmic observations. Can J OphthalmoI 1:56-59,

56.

1966

Merin S, Crawford JS: Hypoglycemia and infantile cataract. Arch Ophthalmol 86:495-498, 1971

Nordmann J, Mandel P: Le metabolisme des glucides dans le cristallin I. La glycolyse anaerobie. Ann Ocul 185:929, 1952 58. Nordmann J: Biologie due Cristallin, Rapport present& a la Societi: Francaise d’ Ophthalmologic. Paris, Masson, 1954 59. Pirie A, van Heyningen R: Metabolism of the lens in Biochemistry of the Eye. Oxford, Blackwell Scientific Publications, pp36, 1956 60. Pirie A: Metabolism of glycerophosphate in 57.

the lens. Exp Eye Res 1:427-435, 1962 Pirie A: The biochemistry of the Ophthalmol

49:287-377,

eye.

1957

Ross EJ: Insulin and the permeability of cell membranes to glucose. Nature 171:125, 1953 Scheie HC, Rubenstein RA, Albert DM: Congenital glaucoma and other ocular abnormalities with idiopathic infantile hypoglycemia. J Ped Ophthalmol l:45, 1964 Sullman H: Carbohydrate metabolism of the lens: I. Formation of phosphate esters in the lens. Arch Angenh 110:303-320, 1937 Trayhurn P, van Heyningen R: Aerobic metabolism in the bovine lens. Esp Eye Res 12:315-327, 1971 Trayhurn P, van Heyningen R: The metabolism of amino acids in the bovine lens. Biothem J 136:67-75, 1973 Trayhurn P: Ph.D. Thesis,

Oxford, University of Oxford, 1972 Trayhurn P, van Heyningen R: The role of respiration in the energy metabolism of the bovine lens. Biochem J 129:507-509, 1972 van Heyningen R: Some glycolytic enzymes and intermediates in the rabbit lens. Exp Eye Res 4:298-301, 1965 van Heyningen R, Linklater J: The metabolism of the bovine lens in air and nitrogen, Exp Eye Res 20:393-396, 1975 van Heyningen R: Metabolism of xylose by the lens. Biochem J 73:197-207, 1959 van Heyningen R: Formation of polyols by the lens of the rat with “sugar” cataract. Nature 184:194-195, 1959 Varma SD, Kinoshita JH: Sorbitol pathway in diabetic galactosemic rat lens. Biochim Biophys Acts 338:632-640, 1974 Varma SD, Mikuni I, Kinoshita JH: Flavonoids as inhibitors of lens aldose reductase. Science 188:1215-1216, 1975 Varma SD, Kinoshita JH: The absence of cataracts in mice with congenital hyperglycemia. Esp Eye Res 19:557-582, 1974 Weekers R, Sullman H: Beziehungen Zwischen Phosphatumsatz und Michsaurebildung in der Linse. Arch fnternat der Med Exper 13:483-497,

Wick AN, permeability of 166:421, 1951 78. Wilson WA: hypoglycemia. 77.

67:355-368.

1938

Drury DR: Insulin and cells to sorbitol. Am J PhysioI Ocular

Trans 1969

findines Am

in ketotic

Ophthalmol

Sot

This work was supported by U.S.P.H.S. Grants EY 01276 and EY 00089. Requests for reprints should be addressed to Leo T. Chylack, Jr., M.D., Howe Laboratory of Ophthalmology, 243 Charles Street, Boston, Mass. 02114.