A calcium-activated protease possibly involved in myofibrillar protein turnover. Isolation of a low-calcium-requiring form of the protease

Two forms of a calcium-activated neutral protease were isolated and purified from porcine skeletal muscle. The two forms of the protease differ markedly in their requirement for calcium with the low-calcium-requiring form showing one-half maximal activation at 45 μM calcium while the high-calcium-requiring form shows one-half maximal activation at 0.74 mM calcium. Additionally, they chromatograph differently on DEAE-cellulose, exhibit different mobilities in electrophoresis in a nondenaturing buffer, are affected differently by certain divalent cations, and have slightly different pH dependencies. Despite these differences, the purified forms of the calcium-activated protease co-chromatograph in gel permeation chromatography, have identical banding patterns on sodium dodecyl sulfate (SDS)-polyacrylamide gels, cross-react with an antibody directed against the 80 000-dalton subunit of the calcium-activated protease we originally purified from skeletal muscle (Dayton, W.R., Goll, D.E., Zeece, M.G., Robson, R.M. and Reville, W.J. (1976) Biochemistry 15, 2150-2158), and have identical effects on the ultrastructure of myofibrils. The high-calcium-requiring protease purified in this study is very likely identical to the calcium-activated protease we originally purified from skeletal muscle. The properties of the low-calcium-requiring form of the protease suggest that it is the form of the enzyme that is active in vivo.