Intracellular Ca 2+-dependent protease (CALPAIN) and its high-molecular-weight endogenous inhibitor (CALPASTATIN)

Abstract

Ubiquitous distribution of Ca 2+-dependent neutral protease and its specific inhibitor in various tissues and cells is described. A proposal is made to call the protease calpain and the inhibitor calpastatin, both as generic names. Calpain, which implies a Ca 2+-dependent protease belonging to the thiol protease group, has been knoqn to be widely distributed under different names: e.g., phosphorylase kinase activating factor (KAF), mayofibrllar Z-line-removing protease, Ca 2+-activated neutral protease (CANP), recpetor transforming factor (RTF), etc. Some of the common features of these enzymes are discussed with special reference to the effect on them of a common, endogenous inhibitor, calpastatin. The existence of two types of calpain was discovered from rat liver and kidney and human erythrocytes. Calpain I requires only 40 μ m Ca 2+ for 50% activation, while calpain II responds only to a Ca 2+ concentration of m m order. Capain I is the minor component in rat liver, but it is the only type of calpain that can be found in human erythrocytes. Calpastatin was found to exist in the soluble fraction of rat liver, brain, kidney, spleen cells and thymocytes, human liver, erythrocytes and lymphocytes, and monkey and guinea pig erythrocytes. Calpastatin was partially purified by chromatography on DEAE-cellulose and Ultrogen AcA 34. It has a molecular weight of 280,000–300,000 and is extremely heat stable but readily digetible by trypsin. None of the proteases tested, except calpains I and II, were inhibited by calpastatin. Inhibition is not based on sequestering of Ca 2+ from the medium by the inhibitor, while the binding of calpain to calpastatin does need Ca 2+. Calpain frm one tissue can be inhibited by calpastatin not only from the same tissue but also from the other tissues of the same animal or even of a different species. The relative abundance of calpain to calpastatin found in one tissue differs greatly from that found in the other tissue. Teven in the same kind of tissue, the ratio seems to vary depending upon the sex, age and stag eof differentiation, possibly including neoplastic transformation.