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

1. At low ionic strength, when turbidity and viscosity measurements indicated dissociation of acto-heavy-meromyosin, its adenosine triphosphatase was strongly activated by Mg(2+) and Ca(2+). 2. The characteristics of the adenosine triphosphatase of dissociated acto-heavy-meromyosin in the presence of Mg(2+) were similar to those reported for myofibrils and actomyosin. 3. In the presence of Ca(2+) the adenosine-triphosphatase activity was much less sensitive to ionic strength than was the case with Mg(2+). 4. At low ionic strength Mg(2+) was more effective in maintaining the dissociation of acto-heavy-meromyosin in the presence of ATP than was Ca(2+). This difference was not apparent when ATP was replaced by ITP. 5. Although the recovery of viscosity was complete on reassociation of acto-heavy-meromyosin the turbidity did not return to the original value. 6. The general implications of Mg(2+) activation of acto-heavy-meromyosin when classical interpretation indicates dissociation of the complex are discussed.