Distribution of calpain I, calpain II, and calpastatin in bovine lens.

Two types of Ca2+-requiring cysteine proteinase (calpain, EC 3.4.22.17), which required for full activation 100 microM Ca2+ (calpain I) and 1 mM Ca2+ (calpain II) were found to exist in the cytosolic fraction of bovine lens. Since calpain may play an important role on the degradation of lens proteins during the aging process of the lens, we attempted to study the distribution of calpain I, calpain II, and calpastatin (an endogenous specific inhibitor of calpain) in bovine lens. It was found that both the capsule-epithelium and cortex fiber cells contained calpains I and II and calpastatin, although the content of calpain I was much lower than that of calpain II. Calpains I and II and calpastatin activities were not detected in the nuclear region at all.