The Ca(2+)-activated adenosine triphosphatase of heavy meromyosin is maximally stimulated by lower relative molar concentrations of phenylmercuric acetate than are required with myosin. 2. Stimulation of the Ca(2+)-activated adenosine triphosphatase of both heavy meromyosin and myosin by thiol reagents is markedly affected by ionic strength, the effects being greater with the former than with the latter. In particular, N-ethylmaleimide strongly inhibits the Ca(2+)-activated adenosine triphosphatase of heavy meromyosin at ionic strength below about 0.2. 3. The precise behaviour of the thiol reagents at low ionic strength is slightly modified by the age of the heavy meromyosin and myosin preparations. 4. Stimulation of the Mg(2+)-activated adenosine triphosphatase of heavy meromyosin by thiol reagents is relatively insensitive to ionic strength. 5. The adenosine triphosphatases of heavy meromyosin and myosin activated by potassium chloride in the absence of bivalent activators are inhibited by thiol reagents over the range of ionic strength at which stimulation occurs in the presence of calcium chloride as activator. 6. The modifying effects of potassium chloride and sodium chloride are qualitatively different when heavy-meromyosin adenosine triphosphatase is stimulated with phenylmercuric acetate. No such difference is observed when the enzyme is stimulated with N-ethylmaleimide.
