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The Ca2+-ATPase activity of cardiac myosin is increased in thyrotoxic animals. However, the physiological significance of this observation is uncertain since, in living muscle, Mg-ATP is hydrolyzed by myosin under the stimulating influence of actin. In this study, we have compared the actin-activated ATPase activity of myosin from euthyroid (myosin-N) and thyrotoxic (myosin-T) rabbits and the derivatives of myosin-N and myosin-T formed by blocking the most rapidly reacting class of thiols (SH1) with N-ethylmaleimide (NEM). Also, we have studied the activity of these myosins in the presence of a complex of troponin and tropomyosin that confers calcium sensitivity on the system. Vmax for the actin-activated ATPase of myosin-T was about 168% greater than for myosin N. The apparent dissociation constant for actin, Kapp, for myosin-T was about 42% of the normal value. After NEM modification, Vmax and Kapp for NEM-modified myosin-T and myosin-N decreased, becoming essentially the same for both myosins. In the presence of troponin-tropomyosin complex, the actin-activated ATPase of myosin-T exhibited calcium sensitivity that could be reduced by thiol modification. These results suggest that the SH1 thiols or the region near these groups are important to the actin-activated ATPase of myosin-N and are essential to the increased activity of myosin-T. Also, they suggest that the changes in the enzymatic properties of myosin induced by thyroxine may be responsible for altering the contractile properties of the heart.

Actin-activated adenosine triphosphatase activity of native and N-ethylmaleimide-modified cardiac myosin from normal and thyrotoxic rabbits.