Mycalolide-B is a marine sponge-derived compound that functions as a selective inhibitor of actomyosin ATPase. It effectively hinders ATP-induced contraction and Mg2+-ATPase activity in the absence of Ca2+.

Mycalolide-B belongs to macrolide antibiotics and consists of tris-oxazole with strong cytotoxic activity (IC 50 : 10-50 nM for growth of L1210 murine leukemia cells). Mycalolide-B, a toxin isolated from marine sponge, inhibits smooth muscle contractions without changing cytosolic Ca 2+ levels. Mycalolide-B also inhibits Ca 2+ -induced contraction in permeabilized smooth muscles. Mycalolide-B inhibits superprecipitation and Mg 2+ -ATPase activity stimulated by Ca 2+ without changing myosin light chain phosphorylation. Mycalolide-B inhibits ATP-induced contraction and Mg 2+ -ATPase activity in the absence of Ca 2+[1].