A Comparative Study of Reactive ‐ SH Groups of Cardiac and Skeletal Muscle Myosins

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Abstract
Pig cardiac and rabbit skeletal muscle myosins were treated by N-ethylmaleimide in presence or absence of Mg-ADP. The variations of Ca2+ and K+-ATPase activities and the incorporation of N-[14C]ethylmaleimide into the whole myosin molecule and into its separated subunits (H and L chains) were measured with N-ethylmaleimide treatment for different lengths of time. The Ca2+-ATPase activity of cardiac myosin is activated by N-ethylmaleimide treatment to a lesser extent than that of skeletal myosin. The K+-ATPase activity of both myosins is inhibited in the same quantitative way. The cardiac L chain, L1, contains 1 highly reactive thiol group which is absent from the skeletal L chains. The labeling of 3 SH-groups localized in the heavy subunits of both myosins induced the same degree of inactivation. The difference observed between the degree of inhibition of the Ca2+-ATPase activity for the 2 types of myosin with longer treatments appears to be due to differences in the reactivity of the 4th -SH group labeled on the H chains.