Restoration of enzymatic activity in frog lens rho-crystallin

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From frog lens cDNA library, the clone (R7FF) encoding rho-crystallin was isolated. Expression vector pMR derived from R7FF was employed for the expression of mature-type rho-crystallin-II in E.coli.(HB101). N- and C-termini of the recombinant rho-crystallin were identical to those of rho-crystallin-II purified from frog lens. Among aldo-keto reductases such as prostaglandin F synthase, aldoes reductase, and aldehyde reductase, 55-threonine is specific for rho-cystallin and tyrosine is conserved among all of other aldo-keto reductases. Therefore, 55-tyr-rho-crystallin mutant was prepared, the mutant showed a aldo-keto reductase activity. It was demonstrated that the restoration of enzymatic activity in rho-crystallin was achieved by the single point mutation of 55-threonine to tyrosine. When 9,10-phenanthrenequinone was used as substrate at pH 6.5, the specific activity of the mutant was determined to be 0.63 units/mg protein at 30ﾟC.Reduction of glucose was also observed in the mutant enzyme. The activity of the mutant enzyme corresponded to that of aldose reductase which contribute to formation of diabetic complications. During the molecular evolution of rho-crystallin, point mutation of 55-tyrosine to threonine may occurred to avoid aldose reductase like activity.