Regulatory properties of myocardial myosin.

The ATPase activity of purified myocardial myosin was activated by either K(+) or Ca(++); the addition of one in the presence of the other caused inhibition. According to Hill-plot analyses the K(+)-saturation curves were sigmoidal (n = 2.92), while the Ca(++)-saturation curves were hyperbolic (n = 1.25). Ca(++)-saturation curves in the presence of K(+) were inhibitory with sigmoidicity (n = 4.11), while K(+)-saturation curves in the presence of Ca(++) followed the Michaelis-Menten inhibition kinetics (n = 1.11). Substrate saturation curves were hyperbolic for both Ca(++) and K(+) systems. There was no enzymatic activity when Na(+) was used as the activating metal; furthermore, Na(+) inhibited in the presence of either K(+) or Ca(++). Both Na(+) curves of inhibition followed the Michaelis-Menten relationship.