The protein inhibitor of calcium-dependent proteases: purification from bovine heart and possible mechanisms of regulation.

A bovine heart protein which specifically inhibits calcium-dependent proteases has been purified to near homogeneity. The purified inhibitor had a Stokes radius of 6.8 nm estimated by gel filtration and a molecular weight of 145,000 estimated by sodium dodecyl sulfate-gel electrophoresis. There is evidence that it may be a glycoprotein. The inhibitor could be phosphorylated by bovine heart cyclic AMP-dependent protein kinase, and its inhibitory effect on Peak II (high-calcium-requiring) protease was modestly increased. However, no other phosphorylating or dephosphorylating conditions significantly influenced its activity. The inhibitor was not hydrolyzed by calcium-dependent proteases, but it was very sensitive to proteolytic inactivation by trypsin or proteases present in a lysosomal fraction from rat heart. Thus, proteolysis may represent a mechanism for decreasing the activity of the inhibitor in different physiologic or pathologic conditions.