Isolated bovine myocardial sarcolemma and sarcoplasmic reticulum vesicles contain tightly bound calcium-dependent protease inhibitor.

Bovine myocardial sarcolemma and sarcoplasmic reticulum vesicle preparations contained calcium-dependent protease inhibitor protein. No inhibitor was detected in mitochondrial membranes. The membrane-bound inhibitor co-purified with the marker enzymes for sarcolemma and sarcoplasmic reticulum, Na+,K+-ATPase and Ca2+,K+-ATPase respectively, on isopycnic ultracentrifugation through linear sucrose density gradients. Sarcolemma and sarcoplasmic reticulum vesicles contained about 1 mg of inhibitor per g of membrane protein. However, about one-half of the inhibitor in sarcoplasmic reticulum vesicles was not tightly associated with the membrane. The membrane-bound inhibitor may function to modulate calcium-dependent proteolytic cleavage of sarcolemmal or sarcoplasmic reticulum-associated proteins.