Sensitivity of Actomyosin ATPase to Calcium and Strontium Ions. Effect of Hybrid Troponins1

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Abstract
Hybrid or reconstituted troponins were prepared from troponin components of rabbit skeletal muscle and porcine cardiac muscle and their effect on the actomyosin ATPase activity was measured at various concentrations of Ca 2+ or Sr 2+ . The Ca 2+ concentration required for half-maximum activation of actomyosin ATPase with troponin containing cardiac troponin I was slightly higher than that with troponin containing skeletal troponin I. The Sr 2+ concentration required for half-maximum activation of actomyosin ATPase with troponin containing skeletal troponin C was higher than that with troponin containing cardiac troponin C. Reconstituted cardiac troponin was phosphorylated by cyclic AMP-dependent protein kinase. The Ca 2+ sensitivity of actomyosin ATPase with cardiac troponin decreased upon phosphorylation of troponin I; maximum ATPase activity was depressed and the Ca 2+ concentration at half-maximum activation increased. On the other hand, phosphorylation of troponin I did not change Sr 2+ sensitivity. The inhibitory effect of cardiac troponin I on the actomyosin ATPase activity was neutralized by increasing the amount of brain calmodulin at high Ca 2+ and Sr 2+ concentrations but not at low concentrations. ATPase activity of actomyosin with a mixture of troponin I and calmodulin was assayed at various concentrations of Ca 2+ or Sr 2+ . The Ca 2+ or Sr 2+ sensitivity of actomyosin ATPase containing skeletal troponin I was approximately the same as that of actomyosin ATPase containing cardiac troponin I. Phosphorylation of cardiac troponin I did not change the Ca 2+ sensitivity of the ATPase. The Ca 2+ or Sr 2+ concentration required for half-maximum activation of actomyosin ATPase with troponin I-T-calmodulin was higher than that of actomyosin ATPase with the mixture of troponin I and calmodulin. Maximum ATPase activity was lower than that with the mixture of troponin I and calmodulin.