Abstract
The ATPase activities of cardiac myosin from thyrotoxic and euthyroid rabbits have been compared. The Ca 2+-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 (SH 1) in myosin from euthyroid animals increased Ca 2+-ATPase activity about 177% over the unreacted value. Modification of the SH 1 groups in myosin from thyrotoxic animals had no effect on CA 2+-ATPase activity. We conclude that thyroxin may increase cardiac myosin ATPase activity by a conformational change in the same region as the SH 1 thiols.
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