Mycalolide‐B, a novel and specific inhibitor of actomyosin ATPase isolated from marine sponge

Abstract

A toxin isolated from marine sponge, mycalolide‐B, inhibited smooth muscle contractions without changing cytosolic Ca²⁺ levels. It also inhibited Ca²⁺‐induced contraction in permeabilized smooth muscles. In native actomyosin prepared from chicken gizzard, mycalolide‐B inhibited superprecipitation and Mg²⁺‐ATPase activity stimulated by Ca²⁺ without changing myosin light chain phosphorylation. In the permeabilized muscle and native actomyosin preparation thiophosphorylated with ATPγS, mycalolide‐B inhibited ATP‐induced contraction and Mg²⁺‐ATPase activity, respectively, in the absence of Ca²⁺. Mycalolide‐B also inhibited Mg²⁺‐ATPase activity of skeletal muscle native actomyosin. Mycalolide‐B had no effect on calmodulin‐stimulated (Ca²⁺–Mg²⁺)‐ATPase activity of erythrocyte membranes. These results suggest that mycalolide‐B selectively inhibits actin—myosin interaction.