Xestoquinone activates skeletal muscle actomyosin ATPase by modification of the specific sulfhydryl group in the myosin head probably distinct from sulfhydryl groups SH1 and SH2.

Xestoquinone isolated from a sea sponge Xestospongia sapra inhibited both Ca2+ and K(+)-(EDTA) ATPase of skeletal muscle myosin. The inhibition was abolished in the presence of dithiothreitol. Xestoquinone reacted with 2-mercaptoethanol, a sulfhydryl (SH) compound. Unlike N-ethylmaleimide, a well-known SH reagent, modification of 2 mol of SH groups per myosin by xestoquinone caused a marked increase in the actomyosin ATPase activity. Kinetical analysis of stimulatory effects of xestoquinone indicates a decrease in the actin concentrations which gives half of the maximum velocity (Vmax) of actomyosin ATPase reaction without affecting the Vmax, suggesting an increase in the affinity of myosin for actin. N-Ethylmaleimide can still modify both the SH1 and SH2 groups after modification of 2 mol of SH groups by xestoquinone. Xestoquinone modified myosin SH groups which caused changes in the tryptophan fluorescence intensity and circular dichroism. These results suggest that xestoquinone modifies the specific SH groups in myosin distinct from SH1 and SH2, resulting in activation of actomyosin ATPase. It is also suggested that xestoquinone strengthens the interaction between actin and myosin through conformational change in the myosin molecule.