Calcium-dependent proteinases and specific inhibitors: calpain and calpastatin

There are two forms of Ca2+-dependent cysteine proteinase: low Ca2+ (microM concentration)-requiring calpain I and high Ca2+ (mM concentration)-requiring calpain II. We have recently succeeded in establishing an efficient method for isolating calpains I and II from several tissues and also a method for separating the heavy (80 kDa) and light (30 kDa) subunits from a calpain molecule. These methods enabled us to obtain a variety of calpains I and II and their subunit proteins each in a highly pure state so that these proteins can be compared physically, chemically and immunologically. Calpastatin, a calpain-specific endogenous inhibitor protein, was also purified and characterized. Although the intercellular localization of calpains I and II is being elucidated by use of the respective monospecific antibodies, the true biological functions of calpain and calpastatin still remain rather obscure.