Purification of an endogenous 68,000-dalton inhibitor of Ca2+-activated neutral protease from chicken skeletal muscle.

An endogenous inhibitor of Ca2+-activated neutral protease has been purified from chicken skeletal muscle. The inhibitor, which was isolated by acid treatment and four subsequent chromatographic procedures, is a protein composed of a single polypeptide having a molecular weight of 68,000. It contains large amounts of glycine, proline and glutamic acid but the contents of aromatic amino acids are low. The inhibitor appeared to act on Ca2+-activated neutral protease in a stoichiometric manner, suggesting a complex formation. The inhibitory action was not caused by sequestering of Ca2+. The inhibitor did not inhibit other well-known thiol proteases, but was slightly effective for trypsin.