Endogenous calcium-activated neutral protease in human platelets. Identification of a Mr 70,000 substrate.

Endogenous calcium-activated neutral protease (CAP, calpain), the major proteolytic activity in human platelets, cleaves a number of proteins. Preferential substrates are actin-binding protein, P 235, and a Mr 83,000 component. A fourth substrate completely degraded by CAP in platelet lysate within seconds is a Mr 70,000 protein. This polypeptide has been shown to influence the electrophoretic behaviour of membrane glycoprotein Ib during isoelectric focusing probably by forming a complex. This in vitro effect suggests that cleavage of the Mr 70,000 protein by CAP might modulate glycoprotein Ib receptor function. Proteolysis of known substrates by CAP during thrombin-induced platelet aggregation could not be detected.