The inhibitory action of relaxing-factor preparations on the myofibrillar adenosine triphosphatase

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Abstract
The bulk of the myofibrillar adenosine-tri-phosphatase-inhibitory activity was found to be associated with the heavier fractions sedimented from rabbit-muscle extracts. In such extracts highest specific adenosine-triphosphatase-inhibitory activity was associated with the granule fraction of highest oxidative activity. Granules isolated from pigeon-breast muscle had a considerably lower specific adenosine-triphosphatase-inhibitory activity than those from rabbit muscle and the more active fractions were sedimented less readily. Oxalate, pyrophosphate and citrate potentiated the adenosine-triphosphatase-inhibitory activity of granules; succinate, malonate, hypophosphite and sulphite had no such action. The loss of inhibitory activity which occurred on aging at 0[degree] and in the presence of low concentrations of calcium chloride could be reversed by oxalate. Myofibrillar inosine-triphosphatase and uridine-triphosphatase activities were not inhibited by granular preparations which strongly inhibited adenosine-triphosphatase activity under otherwise similar conditions. Synthetic and natural actomyosin adenosine-triphosphatase activities were inhibited by the granular preparations. A hypothesis for the mechanism of inhibition is suggested on the basis of these findings and earlier studies on the myofibrillar adenosine triphosphatase.