Lens crystallins are structural proteins that comprise over 90% of the dry mass of the lens. In cataracts, the crystallins are found to be heavily modified, particularly via oxidation. This oxidative damage is thought to be a critical factor in the etiology of lens opacification. This laboratory is working toward elucidation of the actual functions of crystallins in the normal lens. We are studying how normal lens function is affected by modification of crystallin structure (e.g., by oxidation) or by change in the composition of crystallins through the loss by mutation of a particular crystallin. In the guinea pig model we are studying, a mutation in the zeta-crystallin gene causes cataract. We have now shown that zeta-crystallin is an enzyme/crystallin which specifically binds NADPH and which is capable of functioning catalytically while the mutant form of zeta is enzymatically inactive. Zeta-crystallin, which catalyzes the reduction of a class of quinones, appears to be distinct from previously described quinone reductases. This system provides a unique opportunity to investigate the effects on the lens of a defined structural change in a major lens protein. Use of this system also allows us to address fundamental questions concerning the function(s) of enzyme/crystallins. Two major potential mechanisms of oxidative damage to the lens are photochemical processes and Fenton reactions involving metal ions and hydrogen peroxide (H202). In collaboration with Dr. D. Balasubramanian, we have quantitated the capacities of various photosensitizers present in the lens to produce singlet oxygen and superoxide anion. In addition, we have evaluated the effects of H202 and copper on lens crystallins.
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