Effect of phosphorylation of porcine cardiac troponin I by 3':5'-cyclic AMP-dependent protein kinase on the actomyosin ATPase activity.

1. Porcine cardiac native tropomyosin was phosphorylated by bovine cardiac 3':5'-cyclic AMP-dependent protein kinase. Most of the phosphate incorporation was observed in troponin I, the maximum of which was 0.7 mol of Pi per mol of troponin I. 2. In the presence of phosphorylated native tropomyosin, actomyosin ATPase activity was 15-40% lower than that in the presence of the unphosphorylated preparation at all calcium ion concentrations (1.5 x 10(-8) M-2.4 x 10(-5) M). Half-maximum activation of ATPase was obtained with a concentration of 7 x 10(-7) M Ca2+ (unphosphorylated) and 1.3 x 10(-6) M Ca2+ (phosphorylated), respectively. Maximum ATPase activity was reached with 3 x 10(-6) M Ca2+ (unphosphorylated) and 1.0 x 10(-5) M Ca2+ (phosphorylated). 3. Porcine cardiac troponin I isolated by affinity chromatography inhibited ATPase activity of desensitized actomyosin in the presence of tropomyosin. There was little difference between phosphorylated troponin I and a control preparation with regard to the inhibitory effect of ATPase activity. 4. Troponin C from rabbit skeletal muscle neutralized the inhibitory effect of troponin I. The minimum amount of troponin C required for complete neutralization was approximately equimolar to troponin I. The inhibitory effect of phosphorylated troponin I was neutralized by troponin C less effectively than that of unphosphorylated preparation.