Autolysis of the millimolar Ca2+-requiring form of the Ca2+-dependent proteinase from chicken skeletal muscle

The millimolar Ca2+-requiring form of the Ca2+-dependent proteinase from chicken breast skeletal muscle contains two subunit polypeptides of 80 and 28 kDa, just as the analogous forms of this proteinase from other tissues do. Incubation with Ca2+ at pH 7.5 causes rapid autolysis of the 80-kDa polypeptide to 77 kDa and of the 28-kDa polypeptide to 18 kDa. Autolysis of the 28-kDa polypeptide is slightly faster than autolysis of the 80-kDa polypeptide and is 90-95% complete after 10 s at 0 degrees C. Autolysis for 15 s at 0 degrees C converts the proteinase from a form requiring 250-300 microM Ca2+ to one requiring 9-10 microM Ca2+ for half-maximal activity, without changing its specific activity. The autolyzed proteinase has a slightly lower pH optimum (7.7 vs. 8.1) than the unautolyzed proteinase. The autolyzed proteinase is not detected in tissue extracts made immediately after death; therefore, the millimolar Ca2+-requiring proteinase is largely, if not entirely, in the unautolyzed form in situ.