Abstract
Calpain II ( EC 3.4.22.17), a calcium-dependent neutral protease, was purified approximately 7000-fold from the soluble fraction of rat lens. The estimated molecular weight of rat lens calpain II was 120 000, and the enzyme was composed of 80 000 and 28 000 MW subunits. Calpain II required 400 μ m calcium, a reducing agent, and pH = 7·5 for maximal activity. The enzyme could not be activated by magnesium, and was inhibited by leupeptin and iodoacetate, but not by phenylmethylsulfonyl fluoride. Purified calpain II degraded rat α-, β H -, and β L -crystallins, insoluble proteins, and intrinsic membrane proteins. γ-Crystallin was not degraded. The proteolysis caused by purified calpain II was similar to proteolysis occurring during the formation of several experimental cataracts in rodents; this suggested that the enzyme may play a role in cataract formation.
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