Role of calcium in crystallin precipitation after proteolysis by Calpain

Purpose. To determine the role ot calciurr in the in vjro precipitation of rat lens crystallins after proteotysis by m-calpain. Calpains are calcium-activated, neutral proteases believed to be important in many rodent models of cataract. Methods. Lens, total soluble proteins from 10-14 day-old rats were proteolyzed b> either endogenous lens m-calpain or addition of purified m-calpain In vitro light scattering at A measured precipitation of crystallins over ten days in five groups: no Ca2', continuous Ga2', Ca2' for the fifst 24 hrs only, Ca2' at 4 days, and substitute Mg2' for Ca2> after the first 24 hi. Refills. Preliminary data indicated that after proteolysis by calpain, lens crystallins precipitated most rapidly i calcium was present continuously. Removal of Ca! with EGTA after 24 hours reduced, but did not prevent the rate of precipitation of crystallins. Adding Ca2' again at 4 days, accelerated the rate of precipitation. Replacing Ca2' with Mg!< at 24 hr did not accelerate precipitation. Conclusions. Once crystallins were proteolyzed by calpain, precipitation occurred in the absence of calcium. However, Ca²⁺ accelerated precipitation of crystallii fragments. Factors besides Ca²⁺, such as exposure of hydrophobic residues on β-crystallins fragments, dimer rearrangement, or syneresis after toss it N-terminal extensions may contribute to precipitation of crystallins. These data suggest that even temporary increases In lens calcium may lead to calpain-induced opacity in rodent models.