Two endogenous protein kinase activities, cAMP-dependent and calmodulin-Ca'^-dependent, are associated with isolated cardiac sarcoplasmic reticulum (SR) vesicles. Both klnases phosphorylate an endogenous substrate of -22,000 daltons (phospholamban). The phosphorylation of phospholamban by either the intrinsic or by exogenous cAMP-dependent protein kinase is found to be Ca 1+independent between 0.06 and 100 pM free Ca >+ . Calmodulin-dependent phosphorylation, on the other hand, does not require

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... cAMP and is absolutely dependent on the presence of free Ca 1+ over a concentration range that corresponds to physiological levels (10~T to 10"* M). Phosphorylation of SR vesicles by both kinasee is additive and the extent of saturation of the cAMP-gpeciflc sites has no effect on the degree of stimulation by cahnodulin or its Ca l+ -dependence. Trifluoperarine, an inhibitor of calmodulin, inhibitg cahnodulin-dependent phosphorylation without affecting cAMP-dependent phosphorylation, indicating the presence of two types of kinases. This is made further evident by the selectivity of each kinase for exogenous substrates. Whereas cAMP-dependent protein kinase appears to phosphorylate histone QA (a basic protein) preferentially, cabnodulin-dependent protein kinase prefers phosvitin (an acidic protein). Ore Res 49: [1356][1357][1358][1359][1360][1361][1362] 1981