Effects of N-ethylmaleimide on the ATPase activities of cardiac myosin from thyrotoxic rabbits.

The ATPase activities of cardiac myosin from thyrotoxic and euthyroid rabbits have been compared. The Ca²⁺-ATPase activity of myosin from thyrotoxic animals was elevated by 200%, while the K⁺(EDTA)-ATPase activity was the same as in euthyroid animals. Modification by N-ethyl-maleimide of the most rapidly reacting class of sulfhydryls (SH₁) in myosin from euthyroid animals increased Ca²⁺-ATPase activity about 177% over the unreacted value. Modification of the SH₁ groups in myosin from thyrotoxic animals had no effect on CA²⁺-ATPase activity. We conclude that thyroxin may increase cardiac myosin ATPase activity by a conformational change in the same region as the SH₁ thiols.