Exp. Eye Res. (1990). 50, 575-582

Intermediary

Metabolism LEO T. CHYLACK,

a The Howe

of the Lens: 1928-l 989” JR.ebcdj-

AND

A Historical

JUDITH

Perspective

FRIENDCd

laboratory of Ophthalmology, b Department of Ophthalmology, c Harvard and d The Brigham and Women’s Hospital, Boston, MA, U.S.A.

Medical

School,

This is a chronological review focusedon glycolysis, the hexose-monophosphate shunt, and the citric acid cycle in the crystalline lens. Key words: intermediary metabolism : glycolysis ; hexose-monophosphate : citric acid cycle. 1. Introduction

It is entirely fitting in a symposium honoring Jin H. Kinoshita Ph.D. to review the work of his predecessors on the biochemical pathways basic to the intermediary metabolism of the crystalline lens. Although early work on the metabolism of the lens adequately outlined the general features of the metabolism of the normal animal lens, it was not until Kinoshita’s work on the sorbitol pathway that a link between metabolism and cataract was established. He and his collaborators, in subsequent studies on inhibitors of aldose reductase, were the first to demonstrate that an enzyme inhibitor could successfully prevent sugar cataract formation in animals. Although the relationship between normal or abnormal metabolism and age-related cataract in humans has been very difficult to define, Dr Kinoshita’s guidance in this area has proven invaluable to many younger investigators. This symposium will illustrate Dr Kinoshita’s impact on diverse aspects of molecular biology, biochemistry, and clinical research on the lens and cataract. This particular review will focus on the anaerobic and aerobic pathways by which the crystalline lens metabolizes glucose and other sugars. Dr Kinoshita’s contributions to our understanding of intermediary metabolism of the lens are numerous and seminal. 2. Intermediary 1920-l

Metabolism

950

In 1928, Kronfeld and Bothman, working at the University Eye Clinic and Physiological Institute in Vienna, Austria, using rabbit lens, were the first to demonstrate that the crystalline lens could convert glucose to lactate. This was only 32 yrs after Eduard Buchner, brother of Hans Buchner, demonstrated ‘life without air ‘, as bubbles formed when sucrose was added to a yeast extract. In 19 33, Kronfeld published a review of metabolism in the normal and cataractous * This work was supported by N.E.I. Grants. No. EY05552, the Brigham Surgical Group Foundation. Fight for Sight. and Research to Prevent Blindness. t For reprints at: The Brigham and Women’s Hospital, 75 Francis St. Boston. MA 02115. U.S.A. 00144835/90/060575+08

SO3.00/0

lens confirming that glycolysis occurs and that the lens does take up oxygen, and he discussed glutathione metabolism and its possible importance in cataract. Also in 1933, Chaikoff and Lachman demonstrated that raising the blood sugar in long-term experimental diabetes resulted in cataract formation in depancreatized dogs. Two yrs later, Mitchell and Dodge (1935) described cataract formation in young rats fed lactose. Nothing more was published on the metabolic properties of the lens until 1936 when H. K. Mueller, working at the University Eye Clinic in Basel, Switzerland, demonstrated that isolated bovine lens extracts incubated at room temperature could produce ‘ alkali-saponifiable ’ phosphate from fructose-diphosphate that had been isolated from other lenses. The following year, 193 7, H. Suellman, also working in the University Eye Clinic in Basel. demonstrated that bovine lens could incorporate inorganic phosphate into various sugars forming ‘ acid-soluble ’ phosphates. These terms ‘ alkali-saponifiable ’ and ‘acid-soluble ’ phosphates are the old terms that were used to describe the high energy phosphate compounds, particularly ATP, although to a lesser extent they also included ADP, AMP and sugar phosphates. It was derived from the biochemical behaviour of the compounds which was known before a clearer definition was available. It was only 1 yr later, in 1938, that Weekers from Liege, working with Suellman in Basel, Switzerland (Weekers and Suellman, 1938). demonstrated that one of the lenticular reactions involving glucose was enzymatic in nature, namely the conversion of glucose to glucose-6phosphate by hexokinase, and that the small amount of hexokinase in the lens limited the rate of glycolysis in the lens. Only one more study on lens metabolism was published before 1950, and that was MuelIer’s study of glucose transport into the lens (1939). In this study using pig, rabbit and bovine lenses, he demonstrated that glucose and ascorbic acid uptake into lenses was inhibited by monoiodoacetic acid. He concluded that uptake of these compounds was a chemically dependent process that required phosphorylation. He discussed the observation that old lenses were known to have less glucose than young lenses, and thought that might reflect reduced uptake 0 1990 Academic Press Limited

576

L. T. CHYLACK

Glucose

7

HK

Hexose Glucose-6-Phosphate . I

+-

AND

J FRIEND

Monophosphate Shunt

G-6-PDH

l-l

Anaerobic Glycolysis

NADPH

6-Phosphogluconate

AR c

NADP

I Sorbitol

6-PGDH

PDH Ribulose-CPhosphate

1 Fructose Lactate

1)

Pyruvate

+,

T

ATP

FIG. 1. Pathways of glucose metabolism. Glycolysis: Glucose is broken down to lactate and pyruvate with some net ATP production (1 glucose results in 4 ATP). Tricarboxylic acid cycle: pyruvate broken down to CO, and water, coupled to electron transport and oxidative phosphorylation resulting in significant net ATP production (1 glucose results in 2 pyruvate and they, in turn, result in 30 ATP). Hexose monophosphate shunt : hexoses are converted to pentoses (e.g. n-ribose) used in nucleic acid synthesis, generation of NAPH from NADP (AR, aldose reductase; ATP. adenosine triphosphate; HK, hexokinase : NADP, nicotinamide adenine dinucleotide phosphate ; NADPH, nicotinamide adenine dinucleotide phosphate, reduced form : PDH, polyol dehydrogenase ; 6-PGDH, 6 phosphogluconate dehydrogenase).

due to: (a) reduced phosphorylation, (b) impaired diffusion through thickened lens capsule, and/or (c) reduced concentration of glucose in aqueous humor in older animals. Thus, these early studies provided a remarkably solid basis on which to begin post-war studies on glycolysis, blood sugars and cataract, and glucose transport.

thrive) and selected the blood glucose level as ‘the most useful measure of the environment of the lens ‘. This was a keen insight, since at that time, it was not known which feature of diabetes caused the cataract. It was obvious that the animals were malnourished and dehydrated, and these factors were thought to be causally related to the cataract. Patterson (1952) was also the first to describe anatomical characteristics of experimental

1950-I

960

During and immediately after World War II, there was little if any work done on the intermediary metabolism of the lens. In the 19 SO’s, when metabolic research on the lens resumed, the focus was on the mechanism of lens opacification in diabetic rabbits and rats. In 1950, Waters demonstrated a reduction in protein synthesis in the lenses of alloxan-diabetic rabbits. Charalampous and Hegsted (19 50) prevented diabetic cataract in the diabetic rat by feeding a high fat diet, but the mechanism of this effect was not elucidated. In 195 1, Patterson detailed the several signs of experimental diabetes (hyperglycemia, polyuria, polyphagia, polydipsia, glycosuria and failure to

diabetic cataract;

the progression

from

early peripheral cortical vacuoles, to fine perinuclear opacities and finally to a dense nuclear cataract. That same year, other scientists elucidated some features of the metabolism of the normal lens. For example, Harris and Gehrsitz (195 1) using paired rabbit lenses in vitro,

measured

the changes

in sodium

and

potassium between lenses incubated in normal medium at 38°C and those subjected to a variety of metabolic insults (cold, decreased calcium, metabolic poisons). Since the metabolic insults all caused a shift in cation concentration toward equilibrium with the extralenticular fluids, they deduced that active processes were involved. Although then not directly related to the lens, Blakeley’s finding (195 1) of polyol dehydrogenase in

INTERMEDIARY

METABOLISM

OF THE

LENS

rat liver, kidney and brain was later to assume great relevance to the mechanism of diabetic cataract formation in the lens. 19 52 provided some major insights into the pathways involved in the intermediary metabolism of the lens: Nordmann and Mandel(1952) in Strasbourg, France, demonstrated all of the phosphorylated intermediates of glycolysis in rabbit, bovine and rat lenses. In particular, they were the first to demonstrate aglycero-phosphate in the lens. They observed a reduction in energy rich compounds with aging, and suggested that these metabolic changes might contribute to cataractogenesis. Their findings supported the inference that sugar metabolism in the lens was enzymatic and followed the Embden Myerhof pathway that had been demonstrated in other tissues. This was conclusively shown that same year by Leinfelder and Christiansen (1952), who demonstrated the heat lability of the glycolytic system in the lens. Although they did not so conclude, they were the first to show that heat lability of hexokinase, the gateway enzyme in lens glycolysis. Inoue (1952) provided another important insight into normal lens metabolism by demonstrating that energy deprivation led to a decrease in lens glutathione. In 1953, Patterson, in his continuing study of the diabetic cataract, clearly demonstrated that the rate of experimental diabetic cataract formation was inversely related to the level of blood glucose, and that a level greater than 250 mg% was needed for any cataract formation to occur. He also demonstrated that phlorizin. which blocks glucose transport into the lens, delayed cataract formation, but none of the other complications of diabetes. He therefore concluded correctly that hyperglycemia. per se, was of primary importance to lens opacification in experimental diabetes. This key experiment reduced much of the speculation about whether: (a) some toxin associated with the ketosis or acidosis, (b) the loss of an essential metabolite in the urine, (c) the destruction of protein, and (d) the failure to absorb glucose in the face of an insulin deficiency, or some other facet of the diabetic condition was primary in the mechanism of lens opacification. Although we now believe that insulin has very little to do with glucose uptake in the lens, Ross’s publication ( 19 5 3) demonstrating that insulin increased glucose and galactose transport into the decapsulated lens, stimulated a great deal of speculation that insulin deficiency per se, rather than hyperglycemia, was responsible for cataract. Ross, however, noted that the increase did not also occur in lens homogenates and concluded that the work supported the concept that insulin acts to increase glucose transport at the cell membrane. He speculated only that these observations suggested that diabetic hyperglycemia is the result of the inability of glucose to enter cells. rather than a failure of intracellular enzymic oxidations. In 19 5 3, we see the first lens-related publication by Ruth Van Heyningen (Van Heyningen and Pirie,

577

1953) ; she and Antoinette Pirie demonstrated the presence of glutathione reductase in the lens. The following year, they (Pirie and Van Heyningen, 19 54) demonstrated that lens extracts with dehydroascorbic acid could oxidize reduced glutathione. In 1954, Patterson (1954a) showed that fasting for 40 hr per week could reduce the rate of cataract formation in the diabetic rat. In another publication ( 19 54b), he was the first to demonstrate that a dietary galactose supplement could accelerate the rate and severity of diabetic cataract formation. He was also the first to show that galactose mimicked the effect of high glucose and that galactose was, in fact, more effective than glucose in production of cataract. A major fmding in 1954 was Kinoshita and Masurat’s demonstration of the ‘ direct oxidative cycle ’ in the bovine cornea1 epithelium. In this cycle, also known as the hexose monophosphate shunt or pentose pathway, glucose-6-phosphate is converted to pentose phosphate (ribulose-S-phosphate) with the production of NADPH and some ATP. The compounds produced are then used in a variety of reactions including glutathione metabolism. Kinoshita had not yet begun his pivotal biochemical study of the lens. Also in 19 54, John Harris and his collaborators (Harris, Hauschildt and Nordquist, 1954) confirmed his earlier work showing that energy was needed for cation balance in the lens, but they incorrectly concluded that oxygen was needed for lens viability. 19 5 5 was a banner year in lens metabolism ; Green and coworkers (Green, Bother and Leopold, 1955a, b, c) demonstrated that rabbit lens extracts could form lactic acid from glucose and fructose diphosphate under anaerobic conditions. They confirmed Weekers and Suellman’s work (193 8 ), showing that the low hexokinase activity limited the glycolytic rate. If glucose-6-phosphate was substituted for glucose, the glycolytic rate increased. They also studied the anaerobic metabolism of triosephosphate, phosphoglycerate and phosphoenolpyruvate. Harris, Hauschildt and Nordquist (1955) studied the kinetics of glucose transport in isolated lenses. They conclusively showed that insulin did not effect the rate of glucose transport by the lens, controverting Ross’s earlier conclusion. Glucose transport was shown to be ‘facilitated’ (i.e. faster than would be predicted by the magnitude of the concentration gradient alone). Although their data demonstrate saturability of the carrier mechanism, they did not infer this. In the terminology of the day, the glucose transport was believed to be ‘metabolically mediated’. Harris et al. (19 5 5) again confirmed that metabolic processes were involved. However, because lens glucose levels did not exceed those of the surrounding medium and glucose movement was proportional to the concentration in the medium, they felt the term ‘metabolically mediated ‘, meaning active transport, was incorrect and suggested use of the term ‘assisted’ or, as it later became known. ‘facilitated’ transport might be preferable.

L. T. CHYLACK

578

Also in 19 5 5. Kinoshita demonstrated for the first time the existence and importance of the hexosemonophosphate shunt and the nearly total inactivity of the citric acid cycle in the lens; the activity of the pentose shunt was shown to be approximately six times that of the citric acid cycle. Extending the earlier work of Van Heyningen and Pirie on glutathione metabolism. he showed that TPNH formed by the pentose shunt could be reoxidized by oxidized glutathione (GSSG) and glutathione reductase. He speculated that a redox cycle involving glutathione was present in the lens and he cited Pirie and Van Heyningen’s earlier work (1954) in implicating dehydroascorbate as the oxidant. This seminal publication indicated that the lens, a tissue virtually devoid of mitochondria and their oxidative systems, was able to reoxidize TPNH. He did not know at the time about aldose reductase, an enzyme later shown to use TPNH and glucose to form sorbitol. At this time, Kinoshita did not speculate about the relevance of his findings in normal lens to the mechanism of diabetic cataract formation. Patterson ( 19 5 5a) continued his study of sugar cataract formation in rats with unilateral carotid ligation ; he hypothesized that a reduction in blood supply, and therefore a reduction in sugar supply to the lens, would reduce the rate and perhaps the severity of cataract formation in the ipsilateral eye. This proved to be true for the galactose cataract, but not for the diabetic cataract. This discrepancy between two models of sugar cataract formation led him to question his belief that hyperglycemia per se was of primary importance in the pathogenesis of the diabetic cataract. Of course, he did not know at the time of the importance of sorbitol nor of polyol dehydrogenase. Polyol dehydrogenase oxidizes sorbitol sufficiently rapidly, so that if the rate of sorbitol formation is slowed (as by decreasing blood flow), there would never be enough osmotic stress to cause a cataract. Polyol dehydrogenase does not oxidize galactitol. In this publication, he speculated that the insulin deficiency and the lens’ ability to use glucose were of primary importance in the mechanism of this cataract. He stated: ‘Furthermore, if diabetic cataracts are the result of the inability of glucose, in the absence of insulin, to supply energy for the lens, the development would be independent of the blood supply. Thus the known facts can be explained by a mechanism that does not involve a direct action of hyperglycemia in the production of cataracts ‘. One can only guess that he reached this conclusion reluctantly, since so much of his earlier work pointed to the primary importance of hyperglycemia. To further support that conclusion, later that same year (1955b), he demonstrated in diabetic rats that a diet of 50 % fructose, 2 5 % casein and 2 5 % fat prevented the cataract formation but did not lower the blood glucose. Thus, he concluded that his results ruled out hyperglycemia as the ‘direct mediator’ in the production of diabetic cataracts.

AND

J. FRIEND

Undoubtedly Patterson’s findings were frustrating not only to himself but to others in the research community interested in the mechanism of diabetic cataract formation. In 1958, Van Heyningen cognizant of the controversy raised by Patterson’s findings sough evidence for the formation of xylulose and phosphorylated metabolites of xylose in the lens. She chose xylose because it was known to be cataractogenic (Mitchell et al.. 19 37) and believed to be metabolized only slightly by the rat (Miller and Lewis, 1932; Blatherwick et al., 1936). She found that xylose was oxidized to xylonic acid by means of a dehydrogenase and DPN in calf lens. Using paper electrophoresis, she separated and identified the metabolites of xylose, and allegedly (pers. commun.) saw, but did not report, xylitol on these chromatograms. She reported that xylose did not interfere with the phosphorylation of glucose by lens dispersions, but did interfere with the uptake of glucose by the intact lens. She did not speculate about the relevance of her findings to sugar cataract formation. Kinsey et al. (19 58) reported that rat lenses incubated in a new rocking culture tube apparatus in high glucose medium accumulated levels of fructose (approximately 220 mg%) in excess of levels of glucose. They did not speculate about the osmotic significance of this finding or its relationship to sorbitol or aldose reductase. That same year, Kinoshita (Kinoshita and Wachtl, 1958) showed that 90% of glucose taken up by the lens is metabolized by the Embden-Meyerhof pathway ; 10 % via the pentose shunt. In any history of sugar cataract research, 1959 must be the most important year. In two seminal publications, Van Heyningen ( 19 59a, b) described aldose reductase and polyol dehydrogenase in the lenses of rats, rabbits, and calf and demonstrated the accumulation in vitro of significant amounts of xylitol in lenses incubated in 3 5 y0 xylose-containing medium She demonstrated the conversion of glucose to sorbitol and galactose to galactitol by the same enzyme with the cofactor TPNH. She concluded with the statement : ‘It is not possible to say whether the accumulation of polyols in the lens is the cause of the (sugar) cataract ‘. The decade brought major advances. Details of intermediary metabolism continued to be elucidated. the mechanisms of glucose and ion transport began to be understood, sugar cataract became a focal point, and the groundwork for Kinoshita’s prize winning explorations on the sorbitol pathway in cataract were presented. 1960-l

970

The decade began with an important observation ; Hers (1960) described the enzyme aldose reductase in the seminal vesicles and placenta of sheep. In 1961, John Kuck, in a most interesting paper, described the formation of fructose from sorbitol in

INTERMEDIARY

METABOLISM

OF

THE

579

LENS

lens dispersions. He showed that intact lenses could not produce as much fructose from sorbitol. illustrating the impermeability of cell membranes to sorbitol. He knew of Van Heyningen’s work, showing that the lens could produce sorbitol from glucose, and referred to the glucose-sorbitol-fructose pathway as the ’ sorbitol pathway ‘, but he did not infer the osmotic significance of the polyol and fructose accumulation. He did raise the possibility of the sorbitol pathway serving as a transhydrogenase converting TPNH to DPNH, and speculated this may have bioenergetic importance, since DPNH, but not TPNH, can serve as a substrate for the cytochrome oxidase system and ultimately a source of ATP. In 1961 in an elegant study, Kinoshita, Kern and Merola (1961) demonstrated that the lens was able to maintain normal wet weight, cation balance, high energy phosphate levels, and amino acid incorporation into proteins in the absence of oxygen as long as sufficient glucose was present. This publication more than any other highlighted the major role of glycolysis in the intermediary metabolism of the lens. In 1962, in three important publications, Kinoshita and coworkers provided evidence that the lens incubated in the presence of elevated sugar could produce sufficient intracellular sugar alcohol to draw water into the cytoplasm (Kinoshita, Merola and Dikmak, 1962a, b: Kinoshita et al., 1962~). This was followed by evidence that such stress could decrease the lens’s ability to accumulate amino acids (Kinoshita, Merola and Hayman, 1965). It is interesting to see, in the second paragraph of this publication, Kinoshita’s characteristic generosity in citing the 1958 publication of Kinsey et al. (1958) as one of the papers responsible for originating the osmotic theory of sugar cataract formation. Clearly, he recognized the importance of their finding of fructose accumulation in the presence of high glucose, even though they did not infer the osmotic significance of this finding. Harris and Gruber (1962) this same year were the first to demonstrate that the lens can create and maintain concentration gradients of potassium and sodium. Becker (1962) reported that the lens could transport rubidium in the same manner as potassium and suggested that they used a common carrier. Kinsey and Reddy (1965) demonstrated that the transport processes for amino acids and rubidium were active and located in the epithelium. Pirie ( 1962) was able to confirm Nordmann and Mandel’s ( 19 52 ) finding of cc-glycerophosphate in the lens, and she speculated that this substance was one of the lens’s endogenous substrates for respiration, cycling between the cytoplasm and the mitochondrion. In 1965, Van Heyningen (1965a) measured the activities in rabbit lens of the glycolytic enzymes ; she emphasized the extremely low level of hexokinase and stressed the regulatory function of this enzyme. In another publication (1965b). she showed that oxygen

was of greater importance to the older than the younger lens. Thus, in the 1960s further progress on transport processes and on glycolysis were reported. The major contribution, however. was enunciation of the role of the sorbitol pathway and osmotic stress in cataract formation. 1970-l

980

In 1970, Varma, Chakrapani and Reddy described an active transport system for myo-inositol in the lens : it was ATP-dependent, ouabain sensitive and influenced by the concentration of sodium in the medium. In 1973 Reddy, Varma and Chakrapani documented degradation of glutathione (GSH) and showed that the rate of this degradation could account for all of the GSH turnover in the lens. They showed that GSH does not diffuse out of the lens, and that the accumulation of GSH in the lens is active and saturable. They measured the ATP consumed by mechanisms involved in GSH synthesis at 11% of total. Between 1975 and 1977, Chylack. Cheng and coworkers (Chylack. 1975 ; Chylack and Schaefer, 1976 : Cheng and Chylack. 1976a. b: Cheng et al. 19 77) studied lens hexokinase and phosphofructokinase of the lens in more detail. They were intrigued by the ‘bottleneck’ or ‘gateway’ function of these enzymes in the control of glycolytic rate in the lens. Not only were the small amounts of these enzymes likely to be of regulatory significance, but also their heat-lability in the absence of substrate could be of importance to the mechanism of hypoglycemic cataract formation. They showed that glucose deficiency is associated with a rapid and nearly complete deactivation of lens hexokinase prior to the deactivation of other glycolytic enzymes, and that this was the primary defect in the experimental hypoglycemic cataract. Barber et al. (1979) demonstrated that the lens has no endogenous substrates (i.e. glycogen); there is a rapid net proteolysis in the lens exposed to glucose-deficient medium. Cheng et al. ( 1977) were able to show that lens phosphofructokinase was inhibited by low concentrations of ATP: in fact he concluded that under normal conditions, there is sufficient ATP present in the cytoplasm to completely inhibit this enzyme. His work suggested that phosphofructokinase might better be considered the ’ gateway ’ or ‘bottleneck ’ enzyme controlling lens metabolism. This conclusion was also reached in 19 78 by Gonzales et al. In 1979, Kawaba, Matsuda and Hayashi argued that lens metabolism is under hormonal control : they found protein kinases in the cytoplasm of rabbit and bovine lenses. These were more active in the epithelium than the cortex and their activities were enhanced by cyclic AMP (CAMP) and cyclic GMP (cGMP).

L T. CHYLACK

In the 19 70’s, transport, glutathione metabolism, glycolysis and the regulatory roles of hexokinase and phosphofructokinase, and the hormonal controls of lens metabolism were explored. 1980-present

In 1980, Orhloff et al. were the first to demonstrate enzymatically inactive, but immunologically reactive, PFK in the lens. The concept of focal inactivation of the active site in an enzyme without degradation of the whole molecule was new at the time and has proven to be true for several enzymes in the lens and other tissues. A major advance in our understanding of the role of GSH in the lens occurred in 1980 when Giblin and Reddy (1980) applied the highly sensitive ‘cycling ’ assay of Nisselbaum and Green (1969) to the measurement of lens pyridine nucleotides, and related their findings to the levels of GSH. Under the conditions of this assay. NAD and NADH (or NADP and NADPH) cycle back and forth between their oxidized and reduced forms. With each cycle a colored compound is formed, but the NAD and NADH (or NADP and NADPH) are not consumed as in other assays. They found very high GSH reductase levels in the epithelium, and levels of NADPH and NADP in the epithelium 25 times higher than in the cortex, and postulated that the cofactors and this enzyme were involved in mechanism(s) which protected the lens against oxidative stress. They have provided evidence that GSH protects against H,O,-derived oxidation, deactivation of Na’, K+-ATPase, oxidation of protein sulfhydryls and removal of xenobiotics. They have also demonstrated that the pentose shunt turns over 100 % of the NADP every 48 set and emphasized that the shunt enzymes, GSH reductase, GSH peroxidase and NADPH, act in concert as an anti-oxidant defense mechanism (Giblin, McReady and Reddy, 1982). When the capacity of this mechanism is exceeded, oxidized glutathione accumulates. In recent years, many investigators have utilized nuclear magnetic resonance spectroscopy (NMR) to study metabolic dynamics in the intact lens. Greiner et al., 198 1) first applied this technique to the measurement of intralenticular pH : in the normal lens it was found to be 6.9, much lower than expected, and under conditions of glucose-deprivation it rose to 7.3. In a recent elegant study, Williams et al. (1988) applied NMR to the study of the relationship between glucose metabolism and lens transparency. They found that ‘the NMR visible phosphorus metabolite profile of the lens does not necessarily correlate with transparency, and that hexosemonophosphate shunt activity provides a sensitive measure of prior or current lenticular stress ‘. It is fitting to end this review of the intermediary metabolism of the crystalline lens with this last citation. for it suggests that we have completed a

AND

J. FRIEND

cycle. From the early work of Patterson, Kinoshita, Nordmann, Green, Harris and many others, arose a firm belief that an understanding of metabolism was crucial to an understanding of the mechanism of cataract formation. Certainly, the work of Van Heyningen, Pirie, Patterson and Kinoshita demonstrated the proof of this in the case of sugar cataract formation in the animal lens. The work of Williams et al. (1988). cited above, suggests that a disordered metabolism. as we understand it today, may not be the fundamental flaw leading to lens opacification in age-related cataract formation. We all await someone with insights as keen as those of Jin H. Kinoshita to guide us in unraveling the mechanism of age-related cataract formation in man. Acknowledgments The senior author would like to acknowledge the generous support and inspirational guidance provided him by Dr Kinoshita during all his years in lens research. It has been a pleasure writing this review for inclusion in this issue honoring Dr Kinoshita. The junior author would also like to express her respect and admiration for Dr Kinoshita.

References Barber, G. W., Rosenberg, S. B.. Mikuni. I., Obazawa, H. and Kinoshita. J. H. (1979). Net proteolysis in glucosedeprived rat lenses incubated in amino acid-free medium. Exp. Eye Res. 29, 663-9. Becker, B. (1962). Accumulation of rubidium-86 by the rabbit lens. Invest. Ophthalmol. 1. 502-6. Blakeley, R. L. (1951). The metabolism and antiketogenic effects of sorbitol. Sorbitol dehydrogenase. Biochem. J. 49, 257-l. Blatherwick, N. R., Bradshaw, P. J., Cullimore, 0. S., Ewing, M. E., Larson, H. W. and Sawyer, S. D. (1936). The metabolism of xylose. J. Biol. Chem. 113, 405-10. Chaikoff, I. L. and Lachman, G. S. (19 33). Occurrence of cataract in experimental pancreatic disease. Proc. Sot. Exp. Biol. Med. N.Y. 31, 2 3 7-41. Charalampous. F. C. and Hegsted. D. M. (1950). Effect of age and diet on development of cataracts in the diabetic rat. Am. I. Physiol. 161, 540-4. Cheng, H. M.. and Chylack, Jr., L. T. ( 19 76a). pH-dependent

temperature sensitivity of rat lens phosphofructokinase. Invest. Ophthalmol. Vis. Sci. 15, 505-9. Cheng. H. M. and Chylack, Jr., L. T. (1976b). Properties of rat lens phosphorfuctokinase. Invest. Ophthalmol. Vis. Sci. 15. 279-87. Cheng, H. M.. Chylack, Jr., L. T., Chien, J. and Baranano, E. C. (1977). Stability of mammalian lens phosphofructokinase. Invest. Ophthalmol. Vis. Sci. 16, 126-34. Chylack, Jr.. L. T. (1975). Mechanism of ‘hypoglycemic ’ cataract formation in the rat lens. I. The role of hexokinase instability. Invest. Ophthalmol. 14. 746-55. Chylack, Jr., L. T. and Schaefer, F. L. (1976). Mechanism of ‘hypoglycemic’ cataract formation in the rat lens. II. Further studies on the role of hexokinase instability. Invest. Ophthalmol. 15, 519-28. Giblin. F. J., McReady, J. P. and Reddy, V. N. (1982). The role of glutathione metabolism in the detoxification of H?Of in rabbit lens. Invest. OphthaImol. Vis. Sci. 22, 330-5. Giblin, F. J. and Reddy. V. N. (1980). Pyridine nucleotides in

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METABOLISM

OF

THE

ocular tissue as determined by the cycling assay. Exp. Eye Res. 31, 601-9. Gonzalez, A. M., Sochor, M., Hothersall, J. S. and McLean, P. ( 19 78). Effect of experimental diabetes on the activity of hexokinase in rat lens: An example of glucose overutilization in diabetes. Biochem. Biophys. Res. Commun. 84.858-64.

Green. H.. Bother, C. A. and Leopold, I. H. (1955a). Anaerobic carbohydrate metabolism of the crystalline lens. I. Glucose and glucose-6-phosphate. Am. I. Ophthalmol. 39, 106-l 3. Green, H., Bother, C. A. and Leopold, I. H. (1955b). Anaerobic carbohydrate metabolism of the crystalline lens. II. Fructose diphosphate. Am. 1. O~h~hulmol, 39, 113-8. Green, H. G.. Bother, C. A. and Leopold, I. H. ( 1955c). Anaerobic carbohydrate metabolism of the crystalline lens. III. Triosephosphate, phosphoglycerate and phosphoenol pyruvate. Am. J. Ophthalmol. 40, 2 3 743. Greiner. J. V.. Kopp. S. J., Sanders, D. R. and Glonek, T. (1981). Organophosphates of the crystalline lens: A nuclear magnetic resonance spectroscopic study. fnvest. Ophthalmol. Vis. Sci. 21. 700-13. Harris. J. E. and Gehrsitz, L. B. (1951). Significance of changes in potassium and sodium content of the lens. Am. 1. Ophthalmol. 34. 13 l-8. Harris, J. E. and Gruber, L. (1962). The electrolyte and water balance of the lens. Exp. Eye Res. 1, 372-84. Harris. J. E., Hauschildt, J. D. and Nordquist, L. T. (1954). Lens metabolism as studied with the reversible cation shift. I. The role of glucose. Am. 1. Ophthalmof. 38, 141-7. Harris, J. E., Hauschildt, J. D. and Nordquist, L. t. (1955). Transport of glucose across lens surfaces. Am. j. Ophthalmol. 39, 161-9. Hers, H. G. (I 960). Le mecanisme de la formation du fructose seminal et du fructose foetal. Biochim. Biophys. Acta 37, 127-X. moue. M. (1952 ). A biochemical study of experimental diabetic cataract caused by alloxan and dithizone. Acta Sot. Ophthalwiol. Jpn. 56, 588-93. Kawaba, T.. Matsuda, H. and Hayashi, S. (19 79). Studies on cyclic AMP and protein kinase of lens. I The states of cyclic AMP and protein kinase in normal rabbit and bovine lenses. Acta Sot. Ophthalmol. Jpn. 83. 2104-11. Kinoshita. J. H. ( 19 5 5). Carbohydrate metabolism of lens. Arch. Ophthulmol. 54, 360-8. Kinoshita, J. H., Kern, H. L. and Merola, L. 0. (1961). Factors affecting the cation transport of calf lens. Biochim. Biophgs. Acta 47, 458-66. Kinoshita, J. H. and Masurat, T. ( 19 54). The direct oxidative carbohydrate cycle in bovine cornea1 epithelium. Arch. Biochrwi. 53. Y-19. Kinoshita. J. H.. Merola, L. 0. and Dikmak, E. (1962a). Osmotic changes in experimental galactose cataract. Erp. Eye Res. 1. 405-10. Kinoshita. J. H., Merola. L. 0. and Dikmak, E. (1962b). The accumulation of dulcitol and water in rabbit lens incubated with galactose. Biochim. Biophys. Acta 62, 176-8. Kinoshita, J. H., Merola. L. 0. and Hayman, S. ( 1965). Osmotic effects on the amino acid-concentrating mechanism in the rabbit lens. I. Biol. Chem. 240. 310-S. Kinoshita, J. H., Merola, L. O., Satoh, K. and Dikmak, E. (1962~). Osmotic changes caused by the accumulation of dulcitol in the lenses of rats fed with galactose. Nature 194,

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Intermediary metabolism of the lens: a historical perspective 1928-1989.

Exp. Eye Res. (1990). 50, 575-582 Intermediary Metabolism LEO T. CHYLACK, a The Howe of the Lens: 1928-l 989” JR.ebcdj- AND A Historical JUDITH...
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