Purification and immunological characterisation of two calcium-activated neutral proteinases from rabbit skeletal muscle.

Two Ca2+-activated neutral proteinases have been prepared to a high degree of purity from rabbit skeletal muscle. One, calpain I, is optimally activated by 100 microM Ca2+ and the other, calpain II, by 1 to 2 mM Ca2+. Both enzymes have two subunits of molecular weight 80 000 and 28 000. Antibodies have been raised against the native forms of both enzyme. It was found that the antibody to native calpain I reacted only with calpain I and not with calpain II, and similarly the antibody to native calpain II reacted only to calpain II. This suggested that the epitopes in the two enzymes are located in regions that are structurally different. However, immunoblotting of the denatured calpains after SDS-polyacrylamide-gel electrophoresis revealed cross-reaction between the two subunits for both enzymes. Therefore, although the denatured enzymes have common antigenic sites it would appear that these are not exposed equally in the native proteins.