Purification and characterization of 210,000-dalton inhibitor of calcium-activated neutral protease from rabbit skeletal muscle and its relation to 50,000-dalton inhibitor.

An endogenous inhibitor of calcium-activated neutral protease (CANP), which was isolated from rabbit skeletal muscle with chemically drastic pretreatments, comprised major (high-molecular-weight form, HMW-inhibitor) and minor (low-molecular-weight form, LMW-inhibitor) components. HMW-inhibitor was purified to homogeneity using FPLC and preparative electrophoresis. The purified inhibitor appeared as a single protein with a molecular weight of 110,000 on SDS-polyacrylamide gel electrophoresis, and a molecular weight of 210,000 on gel filtration. It was therefore presumed that the inhibitor is a dimer protein under native conditions. It contained large amounts of glutamic acid, alanine, and proline, and small amounts of aromatic amino acids, showing an amino acid composition similar to that of LMW-inhibitor. HMW-inhibitor inhibited CANPs with both low (m-type) and high (mu-type) Ca2+-sensitivity but had no effect on any other proteases examined. It was demonstrated that the inhibition was due to the formation of a stoichiometric complex between rabbit mCANP and inhibitor subunit in the ratio of five to one. These results suggest that HMW-inhibitor might have several reactive sites per molecule and that LMW-inhibitor subunit might be a proteolytic fragment of HMW-inhibitor containing an active site.