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

Post-translational modifications in lens crystallins due to glycation and o xidation have been suggested to play a significant role in the development of cataracts associated with aging and diabetes. We have previously shown t hat alpha-keto acids, like pyruvate, can protect the lens against oxidation . We hypothesize that they can also prevent the glycation of proteins compe titively by forming a Schiff base between their free keto groups and the fr ee -NH2 groups of protein as well as subsequently inhibit the oxidative con version of the initial glycation product to advanced glycation end products (AGE). The purpose of this study was to investigate these possibilities us ing purified crystallins. The crystallins isolated from bovine lenses were incubated with fructose in the absence and presence of pyruvate. The post-i ncubation mixtures were analyzed for fructose binding to the crystallins, A GE formation, and the generation of high molecular weight (HMW) proteins. I n parallel experiments, the keto acid was replaced by catalase, superoxide dismutase (SOD), or diethylene triaminepentaacetic acid (DTPA). This was do ne to ascertain oxidative mode of pyruvate effects. Interestingly, the glyc ation and consequent formation of AGE from alpha-crystallin was more pronou nced than from beta-, and gamma-crystallins. The changes in the crystallins brought about by incubation with fructose were prevented by pyruvate. Cata lase, SOD, and DTPA were also effective. The results suggest that pyruvate prevents against fructose-mediated changes by inhibiting the initial glycat ion reaction as well as the conversion of the initial glycated product to A GE. Hence it is effective in early as well as late phases of the reactions associated with the formation of HMW crystallin aggregates. (C) 2000 Elsevi er Science B.V. All rights reserved.