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.