Cardiac myosin adenosinetriphosphatase of rat and mouse. Distinctive enzymatic properties compared with rabbit and dog cardiac myosin.

Cardiac myosin obtained from rats and mice (smaller animals) had a higher adenosinetriphosphatase (ATPase) activity in the presence of calcium ions (Ca) than did cardiac myosin from rabbits and dogs (larger animals). Structural differences between the two types of cardiac myosin were suggested by the lower apparent activation energy of the Caactivated ATPase reaction catalyzed by cardiac myosin from the smaller animals and by the lower rate of inactivation of ATPase at pH 9.0. No evidence for activators of myosin ATPase was found in heart muscle or cardiac myosin from rats and mice. The cardiac myosin from these animals was also distinctive with respect to the pattern of activation and inhibition of ATPase by salts and sulfhydryl reagents. ATPase activity of cardiac myosin from the smaller animals was less sensitive to the inhibitory effect of KC1 in the presence of Ca and was not activated by iV-ethylmaleimide, suggesting a difference in the myosin molecule at or near the active site. When sodium dodecyl sulfate-polyacrylamide gel electrophoresis was performed, no difference was observed in the molecular weight or the proportion of the light chains between the two types of cardiac myosin. ATPase activity of skeletal myosin was approximately the same in all animals and showed a pattern similar to that of the cardiac myosins from rabbits and dogs. The structural difference in the cardiac myosin from rats and mice suggested by these experiments appears to account for the enhanced myocardial contractility found in these animals.