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 b oth Ca2+ and K+-(EDTA) ATPase of skeletal muscle myosin. The inhibitio n was abolished in the presence of dithiothreitol. Xestoquinone reacte d with 2-mercaptoethanol, a sulfhydryl (SH) compound. Unlike N-ethylma leimide, a well-known SK reagent, modification of 2 mol of SH groups p er myosin by xestoquinone caused a marked increase in the actomyosin A TPase activity. Kinetical analysis of stimulatory effects of xestoquin one indicates a decrease in the actin concentrations which gives half of the maximum velocity (V-max) of actomyosin ATPase reaction without affecting the V-max, suggesting an increase in the affinity of myosin for actin. N-Ethylmaleimide can still modify both the SH1 and SH2 grou ps after modification of 2 mol of SH groups by xestoquinone. Xestoquin one modified myosin SH groups which caused changes in the tryptophan f luorescence intensity and circular dichroism. These results suggest th at xestoquinone modifies the specific SH groups in myosin distinct fro m SH1 and SH2, resulting in activation of actomyosin ATPase. It is als o suggested that xestoquinone strengthens the interaction between acti n and myosin through conformational change in the myosin molecule.