The Powerful Stimulatory Action of 6-O-Acetyl-9-methyl-7-bromoeudistomin D on the Contractile Protein System of Rabbit Skeletal Muscle

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6-O-Acetyl-9-methyl-7-bromoeudistomin D (AMBED) (>10^-6 M), a derivative of eudistomin D, markedly enhanced the superprecipitation and the ATPase activity of skeletal muscle myosin B. In both the superprecipitation and ATPase activity of myosin B, the concentration-response curve for Ca²⁺ was shifted upwards by AMBED. AMBED did not affect the Ca²⁺-, K⁺-EDTA- or Mg²⁺-ATPase of myosin or the ATPase of actomyosin reconstituted from actin and myosin. These results suggest that AMBED causes an increase in the actomyosin ATPase activity mediated through the elevation of the maximum response to Ca²⁺, resulting in enhancement of the superprecipitation of skeletal muscle myosin B.