The adenosine-triphosphatase activity of dissociated acto-heavy-meromyosin

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Abstract
At low ionic strength, when turbidity and viscosity measurements indicated dissociation of acto-heavy-meromyosin, its adenosine tri-phosphatase was strongly activated by Mg2+ and Ca2+. The characteristics of the adenosine triphosphatase of dissociated acto-heavy mero-myosin in the presence of Mg2+ were similar to those reported for myofibrils and actomyosin. In the presence of Ca2+, the adenosine-triphosphatase activity was much less sensitive to ionic strength than was the case with Mg2+. At low ionic strength, Mg2+ was more effective in maintaining the dissociation of acto-heavy-meromyosin in the presence of adenosine triphosphate (ATP) than was Ca2+. This difference was not apparent when ATP was replaced by inosine triphosphate (ITP). Although the recovery of viscosity was complete on reassociation of acto-heavy-meromyosin the turbidity did not return to the original value. The general implications of Mg2+ activation of acto-heavy-meromyosin when classical interpretation indicates dissociation of the complex are discussed.