Effect of calmodulin on sarcoplasmic reticulum Ca(2+)-ATPase isolated from cardiac muscle

A Ca(2+)-dependent ATPase, purified from cardiac microsomal membranes by solubilization and chromatography, is identified as cardiac sarcoplasmic reticulum ATPase on the basis of its electrophoretic mobility and its trypsin digestion pattern. The ATPase (both in membranous and purified form) is stimulated by calmodulin, while the skeletal muscle ATPase is not. Rapid kinetic experiments demonstrate that the calmodulin stimulation is already present within the first enzyme cycle following the addition of ATP, and consists of an increased turnover of the phosphorylated enzyme intermediate. The calmodulin effect does not involve the phosphorylation of any protein other than the ATPase. Following the incubation of ATPase with [gamma-32P]ATP, even in conditions of calmodulin stimulation, radioactive phosphorus is found only on the ATPase electrophoretic band, corresponding to the phosphorylated enzyme intermediate. These observations, together with the results obtained for [125I]calmodulin binding to the ATPase, suggest that the stimulation in turnover produced by calmodulin on the ATPase is due to a direct effect on the enzyme. This may provide an independent regulation of the cardiac sarcoplasmic reticulum Ca(2+)-ATPase, in addition to the known regulation mediated by other accessory proteins.