Studies on Myosin from Red and White Skeletal Muscles of the Rabbit

Abstract Myosin obtained from red skeletal muscle of rabbit has a lower adenosine triphosphatase activity than myosin from white skeletal muscle. Structural differences between the two types of myosin are suggested by the higher apparent activation energy of the calcium-activated adenosine triphosphatase reaction catalyzed by red muscle myosin, by the higher rate of inactivation at pH 7.5 to 9.5, and by the previously reported slower rate of tryptic digestion of myosin from red muscle. No evidence has been found for the presence in red muscle, or in myosin prepared from red muscle, of inhibitors or inactivators of myosin adenosine triphosphatase. The two types of myosin also differ with respect to the pattern of activation of adenosine triphosphatase by sulfhydryl reagents. At low ionic strength myosin from red muscles is activated by N-ethylmaleimide and myosin from white muscles is unaffected, whereas at high ionic strength both myosins are activated. Four moles of p-chloromercuribenzoate per 105 g of myosin are needed for maximal activation of fresh myosin from red muscles, but only 2 moles are required per 105 g of myosin from white muscles. This difference disappears in a few days, suggesting a change in the availability of sulfhydryl groups.