Eye lens crystallins were first thought to play only a structural role in lens transparency. The recent unexpected findings that some of the taxon-specific crystallins which have been isolated from eye lenses have siimilar amino acid sequences to some glycolytic enzymes and possess endogenous enzymatic activity raised some interesting questions : If there is no metabolism in a lens. what is the physiological meaning of these metabolic enzyme/crystallins in vivo? What is the evolutionary meaning of glycolytic enzyme employment as the lens structural proteins? In this review, we summarize recent progress on the characterization of enzyme/crystallins conducted in this laboratory. It includes duck and caiman ε-crystallinrlactate dehydrogenase, duck and chicken δ-crystallin/argininosuccinate lyase, octopus S-crystallin/glutathione S-transferase, and octopus Ω-crystallin/aldehyde dehydrogenase . We compare the kinetic and chemical mechanisms of these enzyme/crysta llins with their cytosolic enzyme counterparts. Based on the chemical reactions catalyzed by these enzyme/cryatallins. we posit relationships between the possible physiological significance of these enzymatic reaction in vivo and stress. The possibilities of applying these enzyme/crystallins as natural mutants in molecular enzymology research are also discussed.
Lactate dehydrogenase, Argininos uccinate lyase, Glutathione transferase, Aldehyde dehydrogenase, Stress proteins
