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Previous studies from our laboratory have demonstrated that pyruvate, an endogenous alpha-keto acid metabolite, has a protective effect against oxidative stress induced damage to the ocular tissues including the lens, in which in addition to exerting its protective effect against tissue damage caused by oxyradicals generated under organ culture, it is also found effective in preventing actual cataract formation in vivo in animal models undergoing direct oxidative stress as well as in diabetes. In the latter studies, pyruvate was administered mixed with diet and drinking water. However, with the view of the desirability of treating eye diseases by topical administration of the pharmacological agents, the present studies were conducted to determine the penetrability of pyruvate through the cornea to the aqueous humor and the lens following its topical administration as its ester, ethyl pyruvate (EP). These experiments were done in CD-1 mice. After instillation of the drops in the conjunctival cul-de-sac, aqueous humor samples were aspirated at the desired times and analyzed for pyruvate. In a separate group of animals, analyses were done also in the lens. Analyses were done spectrophotometrically by monitoring the decrease in absorption of NADH due to the reduction of pyruvate to lactate by lactate dehydrogenase. The levels of pyruvate were found to be significantly elevated in both the aqueous humor as well as the lens, the peak concentrations being 4.7 and 3.6 mM, respectively. Such levels have been previously shown to be effective in exerting its antioxidant effects. The results are therefore considered pharmacological significant from the point of view of its potential use for topical treatment of cataracts induced by oxidative stress and diabetes.

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.

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.

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.

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.