Oxidative stress in lens in vivo: inhibitory effect of caffeine. A preliminary report

PURPOSE

Experiments have been conducted to study the hypothesis that caffeine would inhibit reactive oxygen species induced oxidative stress in the lens in vivo, with implications of attenuating or preventing cataract formation.

METHODS

Oxidative stress was directly induced by administering 24% galactose diet to young adult rats. The treated group was fed a diet containing 24% galactose + 1% caffeine. Oxidative stress inflicted to the lens was assessed by measurement of glutathione (GSH) depletion and observing the status of lens clarity.

RESULTS

Caffeine administration was found to minimize the loss of GSH. This was also associated with a better maintenance of lens transparency as compared to the untreated galactosemic group.

CONCLUSIONS

The studies demonstrate that caffeine could be helpful in inhibiting oxidative stress in the lens with the consequence of attenuating cataract formation.

Electron impact mass spectroscopic studies on mouse retinal fatty acids: effect of diabetes

AIM

The primary objective of these investigations was to determine the fatty acid composition of the mouse retina as affected by diabetes. Additionally, in order to ascertain if there is any accumulation of lipids in the diabetic retina as occurs in many diabetic tissues, its total fatty acid content was also determined.

METHODS

Lipids in the retina of normal and diabetic mice were trans-methylated with methanolic HCl. The esters so prepared were analyzed for fatty acids by gas chromatography-mass spectrometry, qualitatively as well as quantitatively.

RESULTS

The major fatty acids in the retina were palmitic (PA), oleic (OA), stearic (SA), arachidonic (AA) and docosahexaenoic (DHA) acids. The content of all these fatty acids increased significantly in the diabetic retina, reflecting lipidosis. The major increases (approximately 3 times the normal) were found in PA, OA, SA and AA. The increase in DHA, however, was much less (approximately 1.4 times). The relative percentages of fatty acids were also affected. While the relative percentages of PA and OA increased in the diabetic retina, there were insignificant changes in the percentages of SA and AA. Interestingly, the relative percentage of DHA underwent a significant decrease, about 50% of the normal.

CONCLUSIONS

The results show that there is excessive accumulation of lipids in the diabetic retina. This is consistent with the known increased mobilization of lipids from the adipose tissue and their accumulation in other tissues under diabetic conditions. On a percentage profile basis, while the relative percentage of most of the fatty acids increased, the DHA percentage significantly decreased. This could be explained by its dilution by the fatty acids coming from the fat depots which lack DHA. Physiologically, the excessive accumulation of fatty acids in the diabetic retina correlates with the lipofuscinosis and neural dysfunction associated with this disease.

Possibility of topical antioxidant treatment of cataracts: corneal penetration of pyruvate in humans

AIM

It was our aim to study the penetration of sodium pyruvate into the aqueous humor of humans after its topical application as an eye drop.

METHODS

Two hours prior to surgery, one drop of 5% sodium pyruvate prepared in artificial tears was instilled in the eye to be operated upon for cataract extraction. The drops were given 4 times at 10-min intervals. Eye drops containing tropicamide, phenylephrine, flurbiprofen and tobramycin were also given preoperatively. At the start of surgery, an aqueous sample was withdrawn, kept refrigerated and analyzed for pyruvate within 2 h. The cataracts were then extracted either by routine extracapsular extraction or by phacoemulsification, both followed by intraocular lens implantation. The pyruvate level was determined colorimetrically by reacting it with 2,4-dinitrophenyl hydrazine.

RESULTS

The level of pyruvate in the aqueous sample of patients that did not receive pyruvate eye drops was only 0.145 +/- 0.06 mM. In the group given pyruvate, it increased to approximately 0.35-0.525 mM.

CONCLUSION

The results demonstrate that topical administration of pyruvate is effective in elevating its concentration in the aqueous humor to a level adequate to offer protection against oxidative stress to the lens and other intraocular tissues. Hence, it should be feasible to carry out clinical trials with this compound aimed at treating diseases such as cataracts and diabetic retinopathy induced by generation of reactive oxygen species and consequent oxidative stress.

Oxidative damage to lens in culture: reversibility by pyruvate and ethyl pyruvate

It is generally believed that prophylactic intake of antioxidants is beneficial in delaying the onset of some aging manifestations such as cataract. However, whether such a supplementation will also be effective if the pathophysiological process has already set in remains a largely open question. We examined this possibility with lens changes leading to cataract formation, since cataract genesis is intimately related to a continued generation of reactive oxygen species (ROS) in the aqueous humor. Since the formation of cataract is a well-defined progressive disease, starting with an early refractive change and leading to gradual enhancement of opacification, we hypothesized that even a belated start with an appropriate anti-oxidant could halt the pathological process and delay cataract maturation and vision impairment. Using lens cultures, we tested this hypothesis with pyruvate, known to be an effective and highly potent ROS scavenger. Adding pyruvate to the culture medium after lenses had sustained a 50% damage was significantly effective in preventing progress. This was apparent by better maintenance of the active rubidium transport activity in these lenses compared to controls without pyruvate treatment. Glutathione levels were also higher in the pyruvate group.

Combination of glycemic and oxidative stress in lens: implications in augmentation of cataract formation in diabetes

It is well known that the incidence of cataract is higher in diabetics as compared to non-diabetics. Its rate of maturation is also faster in the diabetics. The precise mechanism of this acceleration is not clearly understood. It is hypothesized that this could be a result of the combination of the metabolic and oxidative stress induced by glycemia itself with the age-associated increase in ambient generation of oxyradical species. In the current studies, we have investigated this possibility using the galactose cataract model. Galactosemia was induced by feeding rats a 50% galactose diet. The increased susceptibility of the glycemic lenses to physiological damage by reactive oxygen species (ROS) was studied by incubating them in Tyrode in the absence and presence of menadione. The resulting physiological damage to the lens was assessed initially in terms of its ability to maintain Na+-K+ ATPase dependent active transport of potassium ions, as represented by the uptake of rubidium ions. Subsequently, the level of ATP, indexing the general metabolic status, and the level of glutathione (GSH), indexing the status of antioxidant reserve, were also determined. The uptake of rubidium in the normal lenses incubated in the presence of the quinone was depressed to more than 50% of the controls run in the basal medium. A similar depression existed in the galactosemic lenses in comparison to the normal lenses. However, in the presence of menadione, the inhibition of the uptake was accentuated further in the case of galactosemic lenses, the uptake here being only 20% of the normal controls. Similarly, the galactosemic lenses were also more susceptible to menadione dependent decrease in ATP and GSH.

Attenuation and delay of diabetic cataracts by antioxidants: effectiveness of pyruvate after onset of cataract

Cataract is one of the most significant vision-impairing complications of diabetes. The present study examined the feasibility of inhibiting cataract formation by treatment with pyruvate, a metabolite known to effectively scavenge reactive species of oxygen and inhibit protein glycation, both known to be involved in the genesis of diabetic cataracts. In addition, pyruvate stimulates tissue metabolism, which is depressed with the onset of cataract formation. The objective of our experiments was to determine if this compound could be effective in offsetting the progress of cataract, specifically if administered after the diabetes-induced lens changes have begun, as opposed to the previous reports wherein it has been reported to delay cataract formation if administered prophylactically with the immediate onset of diabetes. Diabetes was induced by intraperitoneal administration of streptozotocin to mice. Lens transparency was assessed by slit lamp examination and its photography. ATP was determined enzymatically by reacting it with luciferin-luciferase mixture and measuring the fluorescence intensity. The findings described herein are in accordance with this possibility. The incidence of cataract in the group of diabetic animals, where treatment with pyruvate was initiated after the initial lens changes set in, was significantly lower at all times of observation in comparison to the untreated diabetic group. In addition, the severity of opacities in the pyruvate-treated group, when present, was much minor, the transparency of these cases being close to that in the control animals. The ophthalmic findings are supported biochemically by ATP levels, which were significantly higher in the pyruvate group in comparison to the untreated group. The present findings emphasize the clinical usefulness of initiating treatment with anti-oxidants and metabolic agonists even when the lens changes are detected at the time of the diabetes diagnosis. The latter usually comes much later than the onset of visual aberrations. Prophylaxis is not an absolute requirement.

Prevention of cataract by pyruvate in experimentally diabetic mice

Previous studies have demonstrated that administration of pyruvate prevents cataract formation in diabetic rats. It is known that the induction of cataractous process in this case is initiated by aldose reductase (AR) catalyzed synthesis and accumulation of excessive sorbitol in the lens fibres and epithelium and their consequent osmotic hydration. Synthesis of this and other polyols is competitively inhibited by pyruvate. The objective of the present investigations was hence to determine whether pyruvate would have a similar protective effect in species where cataract formation is relatively independent of sorbitol synthesis such as in humans where the lens AR activity is extremely low, especially with glucose as a substrate. The Km of AR for glucose is known to be very high. The possible protective effect of pyruvate in the low AR models was conceived on the basis of our previous findings suggesting that it can also exert substantial antiglycating as well as antioxidant effects. The present studies have hence been conducted with mice, a species known to be low in lens AR, similar to that in humans. As stipulated, pyruvate administration has indeed been found to offer a significant protection against development of diabetic cataract in this model also. The effect correlated with the inhibition of protein glycation as well as of oxidative stress. The latter was apparent by the prevention of the loss of glutathione known to be associated with diabetes. Although there was a small but noticeable increment in the sorbitol content of the diabetic lenses, this was osmotically insignificant. Even this increase was prevented by pyruvate. The magnitude of the elevation in the contents of glycated proteins and the depression in the level of glutathione were, on the contrary, highly pronounced, suggesting a more prominent role of the latter factors. In addition, the possibility of a direct metabolic support it could offer to the tissue is also imminent by its effect on the maintenance of ATP, as shown earlier. The present studies are therefore considered more relevant to the pathogenesis of cataract in human diabetics and its possible prevention by endogenous compounds with antiglycating and antioxidant properties. Inhibition of cataract formation by pyruvate in an animal model with low lens AR, similar to that in humans, has been shown for the first time.

Morphogenetic and apoptotic changes in diabetic cataract: prevention by pyruvate

Studies have been conducted to ascertain the preventive effect of pyruvate against diabetes induced damage to DNA and associated morphogenetic changes in the mouse lens. Such changes were characterized by DNA nicks as well as by gross morphological changes in the nuclei, evident respectively by TUNEL and Hoechst staining procedures. Morphogenetic changes were also apparent by abnormal diferentiation of the germinal epithelial cells and errors in their migratory pathway. These changes were prevented by simultaneous administration of pyruvate to the diabetic animals. The preventive effect of this agent is attributable to its property of scavenging oxy-radicals generated by high levels of the sugars.

Effect of alpha-ketoglutarate against selenite cataract formation

We have previously shown that pyruvate protects against reactive oxygen species (ROS) induced damage to lens in vitro. It has also a significant effect against cataract development. Its effectiveness has been ascribed to the presence of alpha-keto-carboxylate group in the molecule, acting as a scavenger of ROS. Hence, it was felt desirable to determine if other alpha-keto-acids could have similar effects. These studies have hence been conducted with alpha-ketoglutarate (alpha-KG), a compound with greater stability and without any known significant effect on the glycolysis. Its effectiveness has been assessed by monitoring cataract development in rat pups given sodium selenite. A large percentage of such animals (about 80%) developed nuclear opacity 7-8 days after its administration. In animals treated with alpha-ketoglutarate, the incidence of cataracts was only 23%. The agent therefore has a very substantial anticataractogenic effect, as apparent by direct slit lamp examination followed by photography, as well as by examination of the isolated lenses through transillumination. The significance of the ophthalmologic findings was apparent also by better physiological maintenance of the tissue, reflected by higher levels of ATP and GSH. In view of these in vivo beneficial effects, studies are in progress to identify the biochemical and metabolic sites of its action. Whether the effectiveness is related only to its action as a ROS scavenger or it could be contributed also by some metabolic effects independent of ROS remains to be determined.

Cataracts in experimentally diabetic mouse: morphological and apoptotic changes

AIM

The objective of these investigations was to extend our earlier study on the induction of cataracts in diabetic mice, a low aldose reductase (AR) animal model at morphological level. Previous studies were done primarily at biochemical level.

METHODS

Diabetes was induced by intraperitoneal administration of streptozotocin. The lenses isolated after the establishment of diabetes were then subjected to histologic and electron microscopic studies.

RESULTS

Morphological alterations were characterized by shrinkage, elongation and lobulization of the nuclei of the epithelial cells. This was associated with chromatin condensation and its margination. Similar structural aberrations were also observed in a significant number of the subepithelial fibre cells representing defect in fibre maturation. More interestingly, unlike that in other common animal models of diabetic cataract, such abnormally nucleated cells were also found to be prevalent in the posterior subcapsular region, a finding common in human diabetics also.

CONCLUSION

The present studies further affirm the suitability of the mouse model for a study of cataractogenesis induced by diabetes. Because of the findings reported herein, as well as the known biochemical similarity between the lenses of the mice and humans in respect of AR deficiency, contrary to the rat model where it is very high, use of this species is considered more useful towards understanding the basic aetiology as well as for evaluating the efficacy of various referred nutritional and metabolic antioxidants against such cataracts.

Ophthalmoscopic and morphogenetic changes in rat lens induced by galactose: attenuation by pyruvate

BACKGROUND

Investigations have been conducted on the potential of pyruvate, a normal tissue metabolite, in the prevention of cataract formation.

METHODS

Cataract was induced by maintaining young rats on a diet containing 30% galactose. The progress of cataract was monitored by visual inspection and ophthalmoscopic and slit-lamp examinations. The protective effect of pyruvate was assessed by incorporating it in the galactose diet and drinking water. The progress of cataract was substantially thwarted by this dietary regimen.

RESULTS

Substantial morphogenetic changes in the lenses of the galactosaemic animals, which by themselves can offer obstruction to light penetration through the lens and scattering, were significantly attenuated. These changes were ascertained by histological detection of errors in cellular differentiation and their migration in unwanted areas.

CONCLUSIONS

As per previous studies, the pyruvate effect is attributed to its direct effect on the biochemistry of lens related to the inhibition of oxidative stress, as well as to its effect on tissue physiology related to the lifelong process of organogenesis, characteristic of this tissue.

Protective effect of ascorbate against oxidative stress in the mouse lens

The purpose of this study was to determine if high ascorbate of the human aqueous protects the lens against oxidative stress. Previous studies with the rat lens have been inconclusive because of its fortification with aldose reductase (AR), an important antioxidant. The human lens is deficient in this activity. These studies were hence done with the mouse lens, a species deficient in this enzyme. The reactive oxygen species (ROS)-induced physiological damage to the tissue was assessed in organ culture, by measuring its ability to actively transport 86Rb(+) ions, in the absence and presence of ascorbate. In addition, the status of tissue metabolism and its antioxidant reserve were assessed by quantitating ATP and glutathione (GSH). As expected, ROS decreased the membrane transport activity as well as the levels of ATP and GSH. Ascorbate minimized these toxic effects substantially. The presence of high ascorbate, therefore, appears highly beneficial in protecting the lens against oxidative damage and cataract formation, despite a deficiency of AR. The findings therefore appear to be significant from the point of view of using this nutrient for delaying the onset of cataract development in human beings, therapeutically as well as nutritionally.

Oxidative damage to mouse lens in culture. Protective effect of pyruvate

Studies have been conducted to examine the feasibility of preventing oxyradical-dependent oxidative stress to mouse lens in culture, using pyruvate as an antioxidant. The extent of oxidative damage to the tissue was assessed by measurement of the status of Na(+)-K(+) ATPase dependent active transport of rubidium 86Rb(+). The tissue levels of adenosine triphosphate (ATP), glutathione (GSH), malonaldehyde (MDA) and catalase were also determined. While the measurement of 86Rb(+) uptake provides an assessment of the integrity of the primary active transport system, measurement of the other components reflects the status of intracellular oxidative stress. ATP measurement also reflected on the overall status of metabolic integrity. Incubation of the lens with xanthine (XA)/xanthine oxidase (XO) system had an adverse effect on all these parameters. Incorporation of pyruvate was strikingly protective. The protective effect of pyruvate is apparently due to its ability to scavenge ROS generated in the medium with the possibility of its action on tissue metabolism as well. The findings are hence considered useful for further studies on the prevention of oxidative stress to tissues by exogenous supplementation with pyruvate, specially the human lens where the biochemistry of its antioxidant mechanisms is similar to the mouse lens, contrary to the rat lens.

Establishment of mouse as an animal model for study of diabetic cataracts: biochemical studies

AIM

The primary aim of this study was to understand the pathogenesis of diabetic cataracts at biochemical level in an animal model where lens aldose reductase (AR) activity is low, similar to that in the human lens.

METHODS

Mouse, which is known to have low lens AR, was selected for these studies. Diabetes was induced by intraperitoneal administration of streptozotocin. Biochemical changes in the lens were monitored in freshly isolated lenses with standard chromatographic, enzymatic and culture experiments described in the section on methods.

RESULTS

The present studies provide evidence of significant biochemical changes associated with such cataract formation despite very low levels of aldose reductase. The level of glycated proteins increased to 9 mg/100 mg of total water-soluble lens protein in the diabetic lenses, as compared with the normal lenses where it was only about 1.3 mg/100 mg of total protein. Glutathione (GSH), the major antioxidant in the lens, decreased from 2.35 micro mol/g in the normal lenses to about 1.17 micro mol/g in the diabetic lenses. Malonadehyde, a product of lipid peroxidation, increased from 50 micro mol/100 g in the normal to 70 micro mol/100 g in the diabetic lens. The level of adenosine triphosphate (ATP), an indicator of the overall metabolic status of the tissue, also decreased from 962 +/- 154 nmol/g in the normal to 487 +/- 130 nmol/g in the diabetic lenses. The function of the Na+-K+ ATPase was also adversely affected in diabetes, as indicated by the ability of the lens to accumulate (86)rubidium ions against its concentration gradient. The transport activity, expressed as CL/CM, was 24 in the normal lens, whereas it was only 12 in the diabetic lens. The level of sorbitol in the diabetic lens was only in the micromolar region. Hence, it was considered osmotically insignificant.

CONCLUSION

Overall, the results suggest that induction of cataracts in diabetes can be related to multiple biochemical effects such as oxidative stress and glycation. Sorbitol accumulation in low aldose reductase situations, being minor, could, however, act synergistically with other factors.

Establishment of the mouse as a model animal for the study of diabetic cataracts

The purpose of the present study was to investigate the suitability of using the mouse, a species known to have a low lens aldose reductase activity, as a model animal for studying the pathogenesis of diabetic cataract. Earlier studies with diabetic rats whose cataract development is much faster can only partially explain the etiology of cataracts in humans where lens aldose reductase is substantially low. CD-1 mice were injected intraperitoneally with streptozotocin according to Rossini's method. Blood glucose levels were estimated after 7 days, and animals having blood glucose between 300 and 400 mg/dl were selected for further experiments. Development of lenticular opacity was followed by examining the animals every 3-4 weeks by direct ophthalmoscopy, slitlamp examination and Scheimpflug photography. Additionally, the animals were sacrificed at appropriate intervals, eyes enucleated and subjected to morphological studies. The presence of refractive changes and early cataract in the diabetic mice was initially ascertained by the distorted appearance of the grid pattern when seen through the isolated lenses. Early cataracts were visible on slitlamp examination and by ophthalmoscopy as early as 3-4 weeks after the establishment of diabetes. Advanced opacity was clearly documentable by photography after 5-6 months. Similar to that in other species, a single layer of anterior epithelial cells abutting the anterior capsule was seen in the histological sections of normal mouse lenses. On the contrary, the epithelium in the diabetic lens was multilayered, and numerous nucleated cells were visible in the superficial anterior cortex. These studies therefore suggest that further studies with mice may throw additional light on the contribution of diabetes in the pathogenesis of cataracts in low lens aldose reductase models.

Green tea (Camellia sinensis) protects against selenite-induced oxidative stress in experimental cataractogenesis

Cataract is the leading cause of blindness worldwide. It is a multifactorial disease primarily associated with oxidative stress produced by free radicals. The protection offered by various antioxidants in cataract development is well established. Polyphenolic compounds present in green tea (Camellia sinensis) are reported to possess antioxidant property in various pathological conditions. The present study was undertaken to evaluate the anticataract potential of green tea leaf (GTL) extract in the development of lens opacification. Enucleated rat lenses were randomly divided into normal, control and treated groups and incubated for 24 h at 37 degrees C. Oxidative stress was induced by sodium selenite in the culture medium of the two groups (except the normal group). The medium of the treated group was additionally supplemented with GTL extract. After incubation, lenses were subjected to glutathione and malondialdehyde estimation. Enzyme activity of superoxide dismutase, catalase and glutathione peroxidase was also measured in different sets of the experiment. In vivo cataract was induced in 9-day-old rat pups of both control and treated groups by a single subcutaneous injection of sodium selenite. The treated pups were injected GTL extract intraperitoneally prior to selenite challenge and continued for 2 consecutive days thereafter. Cataract incidence was evaluated on 16th postnatal day by slit lamp examination. There was positive modulation of biochemical parameters in the organ culture study. Green tea was also found to reduce the incidence of selenite cataract in vivo. The results suggest that green tea possesses significant anticataract potential and acts primarily by preserving the antioxidant defense system.

Fructose induced deactivation of antioxidant enzymes: preventive effect of pyruvate

Glycation initiated changes in tissue proteins, which are triggered by the Schiff base formation between the sugar carbonyl and the protein -NH2, have been suggested to play an important role in the development of diabetes-related pathological changes such as the formation of cataracts. While the initial reaction takes place by the interaction of >C=O of the parent sugars with the -NH2 of proteins, reactive oxygen species (ROS) dependent generation of more reactive dicarbonyl derivatives from the oxidation of sugars also plays a significant role in these changes, altering the structural as well as functional properties of proteins. The purpose of this study was to examine whether the activities of glyceraldehyde-3-phosphate dehydrogenase (GAPDH), catalase and superoxide dismutase (SOD) could be affected by the high levels of fructose prevalent in diabetic lenses. Incubation of the enzymes with this sugar led to a significant loss of their activities. GAPDH was inactivated within a day. This was followed by the inactivation of catalase (3-4 days) and SOD (6 days). The loss of the activities was prevented significantly by incorporation of pyruvate in the incubation mixture. The protective effect is ascribable to its ability to competitively inhibit glycation as well as to its ROS scavenging activity. Hence, it could play a significant role in the maintenance of lens physiology and cataract prevention.

Diabetes-induced biochemical changes in rat lens: attenuation of cataractogenesis by pyruvate

AIM

Studies have been conducted to determine the effect of pyruvate administration on the biochemistry of rat lens and the status of its transparency as affected by diabetic conditions.

METHODS

Sprague-Dawley rats were rendered diabetic by intravenous (i.v.) injection of streptozotocin (55 mg/kg body weight (b.w.)) and treated with sodium pyruvate (2%) in drinking water. The levels of glucose, fructose, sorbitol, ATP, GSH, MDA as well as glycated proteins in the lenses were determined at various intervals after the onset of diabetes and the values compared with untreated diabetic controls. The progress of cataract formation and associated histological changes in the tissue were also monitored.

RESULTS

Studies show that the pyruvate treatment decreased the extent of several biochemical changes known to be associated with cataract formation, such as the elevation in the levels of glycated proteins, sorbitol, lipid peroxidation (MDA) and inhibition of the cation pump. The progress of cataract was also significantly delayed.

CONCLUSION

Exogenous administration of this compound hence was found to exert an overall protective effect against cataract formation induced by the diabetic conditions.

Fructose-mediated damage to lens alpha-crystallin: prevention by pyruvate

Post-translational modifications in lens crystallins due to glycation and oxidation have been suggested to play a significant role in the development of cataracts associated with aging and diabetes. We have previously shown that alpha-keto acids, like pyruvate, can protect the lens against oxidation. We hypothesize that they can also prevent the glycation of proteins competitively by forming a Schiff base between their free keto groups and the free -NH(2) groups of protein as well as subsequently inhibit the oxidative conversion of the initial glycation product to advanced glycation end products (AGE). The purpose of this study was to investigate these possibilities using purified crystallins. The crystallins isolated from bovine lenses were incubated with fructose in the absence and presence of pyruvate. The post-incubation mixtures were analyzed for fructose binding to the crystallins, AGE formation, and the generation of high molecular weight (HMW) proteins. In parallel experiments, the keto acid was replaced by catalase, superoxide dismutase (SOD), or diethylene triaminepentaacetic acid (DTPA). This was done to ascertain oxidative mode of pyruvate effects. Interestingly, the glycation and consequent formation of AGE from alpha-crystallin was more pronounced than from beta-, and gamma-crystallins. The changes in the crystallins brought about by incubation with fructose were prevented by pyruvate. Catalase, SOD, and DTPA were also effective. The results suggest that pyruvate prevents against fructose-mediated changes by inhibiting the initial glycation reaction as well as the conversion of the initial glycated product to AGE. Hence it is effective in early as well as late phases of the reactions associated with the formation of HMW crystallin aggregates.

Attenuation of sugar cataract by ethyl pyruvate

Studies describe an attenuation of sugar cataract formation by topical administration of ethyl pyruvate. Cataract formation was induced by feeding young rats a 30% galactose diet. Mature cataracts appeared in about thirty days. Instillation of the eye drops containing 5% ethyl pyruvate decelerated the process significantly. Biochemically, the effect was reflected by lowering in the contents of dulcitol and glycated proteins. The ATP levels were also higher in comparison to the placebo treated group. The effects are hence attributable to the effect of pyruvate in inhibiting dulcitol synthesis and protein glycation, in addition to its antioxidant properties and metabolic support. The use of esterified pyruvate instead of the unesterified pyruvate was preferred because of its greater penetration through the cornea and consequently a higher concentration attained in the aqueous humor.

Non-enzymatic glycation of lens proteins and haemoglobin-inhibition by pyruvate: an in-vivo study

AIM

Previous studies have demonstrated that pyruvate can prevent protein glycation and oxidative stress under in-vitro conditions. The aim of this study was to examine the in-vivo effectiveness of this metabolite against glycation of lens crystallins and haemoglobin in galactosemic rats.

METHODS

Sprague-Dawley rats were maintained on a 30% or 50% galactose-containing diet in the absence or presence of 2% or 5% pyruvate in food and water, respectively. The animals were killed subsequently and the extent of glycation of lens crystallins and haemoglobin was determined using an affinity column chromatograpic technique.

RESULTS

Maintenance of rats on the high galactose diet resulted in a significant increase in glycation of both the proteins. The increase was faster and more substantial in the animals maintained on the 50% galactose diet than that in the animals fed a 30% galactose diet. The increase in the latter was also very significant. Supplementation with pyruvate inhibited the process.

CONCLUSION

The inhibition is attributable to a competitive binding of pyruvate to the protein NH2 groups as well as to the antioxidant effect of the compound. The studies therefore suggest that this and other alpha-keto-acids may be physiologically useful in minimizing glycation and oxidative stress induced tissue pathology by the hyperglycaemic conditions, such as diabetes and galactosemia. The results are also considered pharmacologically significant.

Attenuation of galactose-induced cataract by pyruvate

Data in the present paper demonstrate a significant inhibition in the progress of sugar cataract formation by systemic administration of pyruvate. The formation of the cataract was induced by feeding young rats a diet containing 30% galactose. All animals fed this diet developed nuclear lens opacity by the end of 30 days. This was delayed if the diet and water contained, in addition, 2% sodium pyruvate. The incidence of cataract in the latter group was 0% at day 30 and only 25% at day 55. Physiologically, the inhibition was associated with the prevention of lens membrane damage as reflected by its ability to maintain transport of rubidium ions against a concentration gradient; decreased tissue hydration as indexed by the lens wet weight; inhibition of protein glycation, and higher levels of ATP. Since pyruvate, being a normal tissue metabolite, is likely to be non-toxic, the findings are considered useful for further pharmacological studies with this and other similar metabolites, relevant to protection against various secondary complications of diabetes and galactosemia.

Corneal damage by half mustard (2-chloroethyl ethyl sulfide, CEES) in vitro preventive studies: a histologic and electron microscopic evaluation

The effect of half-mustard (2-chloroethyl ethyl sulfide, CEES) on the morphology and ultrastructure of the cornea has been studied in vitro. Extensive necrotic changes were observed histologically as well as electron microscopically. The outer layer of corneal epithelium was observed to undergo vacuolization and globulization prior to its denudation. The epithelium becomes separated from the Bowman's membrane. These necrotic changes are prevented from taking place in the presence of a mixture of taurine, pyruvic acid, alpha-keto glutaric acid and pantothenic acid suggesting the use of this mixture in the prevention of mustard damage.

Fructose induced deactivation of glucose-6-phosphate dehydrogenase activity and its prevention by pyruvate: implications in cataract prevention

Glucose-6-phosphate dehydrogenase (G6PDH) is an important lens enzyme diverting about 14% of the tissue glucose to the hexose monophosphate shunt pathway. The main function of such a pronounced activity of the enzyme is to support reductive biosyntheses, as well as to maintain a reducing environment in the tissue so as to prevent oxy-radical induced damage and consequent cataract formation. Sugars are one of the well-known cataractogenic agents. Several reports suggest that the cataractogenic effect of the sugars in diabetes as well as in normal aging is initiated by the glycation of the proteins including the enzymes and subsequent formation of more complex and biologically inactive or harmful structures. In a diabetic lens the concentration of fructose exceeds significantly the concentration of glucose, suggesting that the contribution of fructosylation may be greater than that of glucosylation. These studies were undertaken to examine further the possibility that in addition to glycation, generation of oxygen free radicals by fructose and consequent oxidative modifications in certain enzymes may be an important participant in the cataractogenic process. This hypothesis was tested by using G6PDH. The enzyme was incubated with various levels of fructose (0-20mM) and its activity determined as a function of time. This led to a significant loss of its activity, which was prevented by superoxide dismutase, catalase, mannitol and myoinositol. Most interestingly, pyruvate at levels between 0.2 and 1.0 mM also offered substantial protection. Hence, the results, while elucidating further the mechanism of enzyme deactivation by sugars such as fructose, also demonstrate the possibility of therapeutic prevention of cataracts by pyruvate and other such keto acids, in diabetes and other disabilities involving oxygen free radicals in the pathogenetic process.

Half mustard (CEES) induced damage to rabbit cornea: attenuating effect of taurine-pyruvate-alpha-ketoglutarate-pantothenate mixture

Studies have been conducted on the corneal damage by half mustard (2-chloroethyl-ethyl sulfide, CEES) and its possible prevention by a mixture of taurine, alpha-ketoglutarate, pyruvate and pantothenate. CEES has been found to damage the membrane permeability function of the corneal epithelium as evidenced by increased flux of the rubidium ion from the epithelial to the endothelial side. The cornea also loses its transparency. These damaging effects are preventable by the above mixture labeled as VM. It is conceived that use of such a mixed formulation may provide a pharmacological means of prophylactic and post-exposure treatment against the tissue damage caused by exposure to the mustards.

Oxidative stress to rat lens in vitro: protection by taurine

The concentration of taurine is high in the lens. However, its function therein remains unknown. Studies from other tissues suggest that in addition to several other modes of action, it acts as an antioxidant. We therefore hypothesize that taurine may be a part of the antioxidant defense mechanisms involved in protecting the lens against oxidative stress and consequent cataract formation. In these studies, the protective effect of taurine was examined using lens culture system with menadione as an oxidant. Inclusion of this compound in the incubation medium was found to have several adverse effects on the lens, such as a decrease in its ability to accumulate rubidium against a concentration gradient and fall in the levels of glutathione, ATP and an increase in water insoluble proteins. All these deleterious effects were attenuated significantly by addition of physiological amounts of taurine to the menadione-containing medium.

Delayed manifestation of ultra violet radiation induced erythema in guinea pigs by sodium pyruvate--a free radical scavenger

Sodium pyruvate, a free radical scavenger was evaluated for anti-inflammatory activity using UV radiation induced dermal erythema on guinea pig and compared with that of standard naproxen. Oral as well as topical pyruvate exhibited significant activity against UV induced dermal erythema model and the activity was comparable to that of naproxen. In the other pharmacodynamic studies, such as the studies on rat blood pressure, isolated guinea pig ileum and rat uterus, it showed no effect on any of these. In conclusion, sodium pyruvate showed a significant protection in the UV induced dermal erythema in guinea pigs. It also showed good absorption in UV-B range and this property can be utilised to develop the sodium pyruvate as a sunscreening agent.

Prevention of intracellular oxidative stress to lens by pyruvate and its ester

Pyruvate is a well-known scavenger of hydrogen peroxide (H2O2). In addition, it scavenges superoxide radical (O2.-). However, evidence on its intracellular antioxidant function is meager at present. Hence, we have examined the effectiveness of this metabolite and its ethyl ester against intracellular oxidative damage to the lens under organ culture. Menadione, a redoxcycling quinone, was used to generate the reactive oxygen species (ROS). It was found to inhibit lens metabolism as evidenced by a decrease of ATP. Additionally, tissue oxidation was apparent by loss of glutathione (GSH), and increase in the level of oxidized glutathione (GSSG), coupled with increase of the urea soluble proteins (water insoluble). The overall physiological damage was apparent by the inhibition of the Na+-K+-ATPase dependent cation pump, as evidenced by a decreased rubidium transport. These deleterious effects were attenuated by pyruvate and ethyl-pyruvate. The later was found to be more effective.

Prevention of lens protein glycation by taurine

Modifications in lens protein structure and function due to nonenzymic glycosylation and oxidation have been suggested to play a significant role in the pathogenesis of sugar and senile cataracts. The glycation reaction involves an initial Schiff base formation between the protein NH2 groups and the carbonyl group of a reducing sugar. The Schiff base then undergoes several structural modifications, via some oxidative reactions involving oxygen free radicals. Hence certain endogenous tissue components that may inhibit the formation of protein-sugar adduct formation may have a sparing effect against the cataractogenic effects of sugars and reactive oxygen. The eye lens is endowed with significant concentration of taurine, a sulfonated amino acid, and its precursor hypotaurine. It is hypothesized that taurine and hypotaurine may have this purported function of protecting the lens proteins against glycation and subsequent denaturation, in addition to their other functions. The results presented herein suggest that these compounds are indeed capable of protecting glycation competitively by forming Schiff bases with sugar carbonyls, and thereby preventing the glycation of lens proteins per se. In addition, they appear to prevent oxidative damage by scavenging hydroxyl radicals. This was apparent by their preventive effect against the formation of the thiobarbituric acid reactive material generated from deoxy-ribose, when the later was exposed to hydroxyl radicals generated by the action of xanthine oxidase on hypoxanthine in presence of iron.

Formation of advanced glycation end (AGE) products in diabetes: prevention by pyruvate and alpha-keto glutarate

Glycation of proteins and their subsequent structural and functional modifications have been ascribed to play a prominent role in the pathogenesis of several secondary complications of diabetes, such as cataract and retinopathy. In addition, it plays a role in the generalized ageing process as well. Investigations have been conducted to explore the possibility of preventing the above process by use of pyruvate and alpha-keto glutarate as representatives of physiologically compatible keto acids. The results demonstrate that both these compounds are effective in preventing the initial glycation reaction as well as the formation of AGE products. Both these compounds also inhibit the generation of high molecular weight aggregates associated with cataract formation. Mechanistically, the preventive effects appear to be due to (1) competitive inhibition of glycation by the keto acids and (2) the antioxidant (radical scavenging) properties of these compounds. The results are hence considered useful from the point of view of developing these and other keto acid derivatives as pharmacological agents useful in preventing glycation related protein changes and consequent tissue pathological manifestations.

Nitrite-induced photo-oxidation of thiol and its implications in smog toxicity to the eye: prevention by ascorbate

Studies have been conducted on nitrite-induced oxidation of corneal thiols and reduced glutathione (GSH). Oxidation of GSH in the presence of nitrite (NaNO2) was minimal in the dark. Exposure of GSH to UV (365 nm) in the presence of nitrite substantially accelerated this oxidation; only < 10% of the original GSH remained at the end of 20 minutes. A similar Thiol depletion was observed in the case of corneal epithelial extracts irradiated with UV in the presence of the nitrite. Nitrite is therefore considered to be a potent phototoxicant with possible pathophysiological implications to the external eye tissues. Ascorbate was found to be effective in preventing thiol oxidation, suggesting the possibility of preventing nitrogen oxide-based smog irritation to the eye by this physiologically compatible antioxidant.

Studies on L-threose as substrate for aldose reductase: a possible role in preventing protein glycation

L-threose is a product of ascorbate oxidation and degradation. By virtue of its free aldehyde group it can form Schiff-bases with tissue proteins, altering their normal function. In this study, we have examined the possibility of its detoxification to L-threitol by aldose reductase in the lens. The rat lens enzyme present in fresh homogenate as well as after 100 fold purification was found to utilize L-threose with a km of 7.1 x 10(-4) M. The specificity of the reaction was affirmed by its inhibition with sorbinil and quercetin, the well known aldose reductase inhibitors. Further studies on the role of this enzyme in preventing toxicity due to degradation products of ascorbate are in progress.

Oxyradical scavenging effects of dehydroascorbate

Dehydroascorbate (DHA) has been shown to possess vitamin C like activities as well as to protect the lens against oxidative stress and cataract formation. The results presented here suggest that some of the beneficial effects of DHA can be attributed to its property of undergoing peroxidative decarboxylation and of O2-. scavenging. Incubation of 1-14C-DHA with peroxide at physiological pH has been found to liberate 14CO2 in quantitative yields, with recovery of 79-94%. The recovery increases with the increase in the amount of DHA used. Its O2-. scavenging activity was apparent by inhibition of O2-. dependent reduction of ferricytochrome c and nitroblue tetrazolium. The authenticity of the latter mechanism was proved by inhibition of the O2-. dependent reactions also by superoxide dismutase.

Levels of superoxide dismutase mRNA in rat lens: effect of aging

Superoxide dismutase, the enzyme catalyzing the dismutation of O2.- to H2O2 is known to be present in various ocular and nonocular tissues. In this communication we have determined the gene expression of this enzyme in rat lenses. The investigations have been conducted as a function of age using RNase protection assay. These in vitro assays for the corresponding mRNA suggested that the transcription of the gene is age variant, increasing as a function of age. The levels were significantly lower in the young lenses in comparison to the older lenses.

Prevention of oxidative damage to rat lens by pyruvate in vitro: possible attenuation in vivo

Studies have been conducted to assess the possible preventive effect of pyruvate against lens protein oxidation and consequent denaturation and insolubilization. Rat lens organ culture system was used for these studies. The content of water insoluble proteins (urea soluble) increased if the lenses were cultured in medium containing hydrogen peroxide. Incorporation of pyruvate in the medium prevented such insolubilization. The insolubilization was associated primarily with loss of gamma crystallin fraction of the soluble proteins. PAGE analysis demonstrated that insolubilization is related to -S-S- bond formation which was preventable by pyruvate. Since pyruvate is a normal tissue metabolite the findings are considered pathophysiologically significant against cataract formation. This was apparent by the prevention of selenite cataract in vivo by intraperitoneal administration of pyruvate.

Oxidative denaturation of lens protein: prevention by pyruvate

The denaturation of lens proteins as apparent by the generation of protein carbonyl in the presence of active oxygen and the prevention of such denaturation by pyruvate were studied. Active oxygen was generated by the action of xanthine oxidase on xanthine under aerobic conditions. Rat lens protein when incubated with xanthine and xanthine oxidase produced significant amounts of the carbonyl derivative. The formation of such carbonyl was substantially inhibited by pyruvate. In addition, the keto acid also was found to stimulate the utilization of glucose through HMP shunt, a mechanism known to transport reducing equivalents from glucose to peroxide. The results suggest that pyruvate exerts a beneficial effect in attenuating the age-related protein modifications and consequent physiological impairments. These studies are also considered useful from the therapeutic point of view.

Inhibition of polyol formation in rat lens by verapamil

Accumulation of sorbitol and xylitol in rat lenses incubated in medium-199 with and without verapamil has been studied. This antihypertensive drug, known to attenuate hypertension by its calcium channel blocking effect, is also known to inhibit cataract formation in diabetes. The present studies have demonstrated that verapamil's effect against cataract could also be partially related to its aldose reductase inhibitory activity, in addition to the Ca++ channel blocking activity. The accumulation of sorbitol in the lenses incubated with high glucose in the presence of 400 microM verapamil was only 2.3 mmoles/Kg wet weight against 11.3 mmoles/Kg in its absence. The level of xylitol attained in the presence of 10 mM xylose was 25.7 +/- 2.4 mmoles/Kg. It decreased to 4.8 +/- 1.2 mmoles/Kg in presence of 400 microM verapamil. Hence, verapamil is significantly effective in inhibiting lens aldose reductase dependent polyol synthesis, an action simultaneous with its effect on calcium penetration.

H2O2 determination in rat lens: chemiluminescent versus radioisotopic methods

Hydrogen peroxide levels have been determined in rat lenses by using two methods, a chemiluminescent and a radioisotopic method. The average content was found to be 155 +/- 20 and 127 +/- 18 nmol/g wet weight of the tissue, respectively, by the two methods. The reaction of H2O2 with dichlorophenol-indophenol in the presence of peroxidase was also studied. However, this was found to be less suitable. The results of the chemiluminescent determinations are similar to the results with the radioisotopic methods demonstrating the feasibility of determining H2O2 by chemiluminescence measurement as well.

Peroxide damage to rat lens in vitro: protective effect of dehydroascorbate

The possible protective effect of dehydroascorbate against peroxide damage to rat lens under in vitro organ culture has been studied by measuring the levels of ATP, GSH and the uptakes of rubidium and alpha-aminoisobutyric acid. All these parameters were adversely affected by the presence of 0.5 mM hydrogen peroxide in the culture medium. Dehydroascorbate (1 mM) protected the lens against such effects. The protective effect is tentatively attributable to the utilization of peroxide in peroxidative decarboxylation of the dehydroascorbate, in a manner similar to that of other alpha-ketoacids.

Prevention of cataracts by nutritional and metabolic antioxidants

Among aging disabilities, the one associated with the progressive decline of vision is functionally most disadvantageous. Cataracts are one of the more common causes of such visual disability. Several predisposing factors have been identified in the genesis of this disease. While it is perhaps a multifactorial process, significant developments have taken place in recent years suggesting that oxygen radicals are involved in the development of this aging manifestation. Antioxidant enzymes, such as catalase and superoxide dismutase, have been demonstrated to protect the lens cell membrane from oxidative stress as reflected by the prevention of the Na(+)-K(+)-ATPase-dependent pump deterioration due to oxyradical-dependent oxidation of its proteins and lipids. From the nutritional point of view, antioxidants such as ascorbate and vitamin E also offer significant protection to the lens against damage due to oxidative stress. Evidence regarding the protective effect of these nutrients has been based on lens organ culture studies in the presence of active oxygen, generated photochemically as well as enzymatically. The experiment involving photochemical environs simulate the status of the eye during the photopic vision. In vivo, the effectiveness of ascorbate against cataracts has been tested in rat pups developing cataracts under the oxidative influence of sodium selenite. Certain antioxidants produced metabolically also may be useful in protecting against cataracts. Pyruvate produced in glucose metabolism seems to be an important antioxidant. The efficacy of this compound has been tested within in vitro organ culture as well as in vivo, the latter experiments being done with selenite-treated rats. There is a hope that these and other nutritional and metabolic antioxidants may one day be useful in delaying or even preventing cataract formation in human beings.

Studies on Emory mouse cataracts: oxidative factors

Emory mouse cataracts were analyzed for amino acids, protein carbonyls and fatty acids. The tissue membrane integrity was assessed by studying chromium-51 efflux. An effect of vitamin E-free diet on cataract progression was also studied. Chromium leakage was faster from the cataractous lenses, indicating a generalized membrane damage. This was also apparent from the loss of amino acids. The damage involves oxidation of proteins, as well as of lipids. Protein oxidation was apparent by a hydrazone formation with 2,4-dinitrophenyl hydrazine. The lipid oxidation was apparent from a decrease in oleic acid and appearance of the corresponding ketoacids. Lipid oxidation was also apparent by an attenuating effect of vitamin E.

Effect of metabolic inhibitors and anaerobiosis on rat lens

The effect of the inhibitors of aerobic metabolism on the transport of rubidium and ATP content in rat lens has been studied under organ culture system. The inhibitors used were malonate, monofluoroacetate, cyanide, antimycin A, 2-heptyl-4-hydroxyquinoline-N-oxide (HQNO), 2,4-dinitrophenol (DNP) and dicumarol. All these compounds inhibited the transport of rubidium, reflecting a decrease in the ability of the lens to transport potassium. The levels of ATP also decreased. The results were similar to those obtained under anaerobic conditions where the lenses were incubated in a nitrogen atmosphere without the inhibitors. The degree of inhibition observed by the metabolic inhibitors as well as by anaerobiosis indicate that the rat lens depends substantially on aerobic oxidation for its energy needs, unlike bovine and rabbit lenses studied before.

Transient disappearance of a symptomatic macular hyperfluorescent lesion following vitrectomy: a case report

We have presented a case of symptomatic macular fluorescence of a presumed vascular etiology that transiently disappeared post vitrectomy. Laser photocoagulation to the involved area resulted in a good visual outcome. We propose that the transient improvement in the fluorescein leakage represented a temporary alteration of cellular metabolism, or vascular perfusion, attributable to the perfusion solution. We suggest that additional studies are indicated to further explore this phenomenon and its potential clinical applicability.

Radio-isotopic determination of hydrogen peroxide in aqueous humor and urine

The concentrations of hydrogen peroxide in the aqueous humor and urine of several animal species and humans have been determined. The determinations are based on peroxide-dependent decarboxylation of I-[14C]-alpha-ketoglutaric acid and measurement of the resulting 14CO2 by quantitating the radioactive disintegration. The levels of H2O2 in most animals varied between 5.0 and 41 microM for aqueous, and 115 and 187 microM for urine. The levels of peroxide in the urine of steer, cat and baboon were lower and fell out of the above range. In the aqueous of humans with cataracts, the levels ranged from 33 to 324 microM, the overall average being 189 +/- 88 microM. The source of such high levels in the aqueous of cataract patients is currently being studied.

Hydrogen peroxide in the eye lens: radioisotopic determination

Lenses from normal rabbits, mice, rats, cattle, guinea pigs, lambs, chicken, cats, baboons, blue acara (fish) and dogs were examined for the presence of H2O2. No previous reports exist on the presence and levels of H2O2 in normal eye lenses. Freshly isolated lenses of these animals were extracted with trichloroacetic acid and the extract neutralized with Tris. H2O2 was assayed in these extracts by reacting them with 1-14C-alpha-ketoglutarate and measuring the 14CO2 produced by peroxide-dependent decarboxylation. Peroxide of the order of 10(-4)M was detected in most of the lenses except in baboons wherein it exists between 10(-4) and 10(-5)M. Culture experiments with rat lenses demonstrated that GSH may make a major contribution to the formation of H2O2 in the intact lens in vivo.

Prevention of selenite cataract by vitamin C

Studies have been conducted to determine the efficacy of vitamin C in the prevention of cataracts induced by selenite. Administration of the latter to rat pups results in the development of advanced cataracts within 5 days. Treatment with ascorbate had a significant preventive effect. The observations indicate that selenite cataract is due to an oxidative stress to the lens. In addition, the findings are in conformity with our view that ascorbate functions as an anticataractogenic substance.

Hydrogen peroxide in human blood

Blood and plasma of humans and rats were analyzed for hydrogen peroxide. The samples were analyzed after deproteinization with trichloroacetic acid, immediately after they were withdrawn from human volunteers or rats. A radio-isotopic technique based on peroxide-dependent decarboxylation of 1-14C-alpha-ketoacids and consequent liberation of 14CO2 was used. The results demonstrate the presence of micromolar levels of H2O2, both, in the plasma as well as in the whole blood. The values in the whole blood were substantially greater than the plasma. This was true for rats as well as humans. The presence of such significant quantities of H2O2 in the blood have been demonstrated for the first time. The investigation, therefore, opens a newer avenue of research on diseases purported to be related to the generation of oxygen radicals in vivo.

Scientific basis for medical therapy of cataracts by antioxidants

Cataract is one of the major causes of age-dependent visual impairment and blindness. The geographic distribution of cataract is known to be associated with the intensity and duration of sunlight--especially of the ultraviolet frequency--at particular places. Exposure of animals and humans to oxygen has also been known to result in cataract formation. Studies described in this communication indicate that the ocular lens is physiologically damaged when exposed to an environment of active species of oxygen, commonly referred to as oxyradicals. Several photochemical and nonphotochemical models have been described. The results suggest that an intraocular generation of active oxygen may constitute a significant risk factor in the overall pathogenesis of senile cataracts. The cataractogenic effect of oxyradicals, however, can be thwarted by nutritional and metabolic antioxidants such as ascorbate, vitamin E, and pyruvate. These agents, therefore, may be useful for prophylaxis or therapy against cataracts.