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The effect of ouabain (10−8 to 10−3 M) on the hydrolysis of ATP by beef cardiac myosin B was measured at various KC1 concentrations and at various interaction times at 25°C. Using an ionic strength of 0.24 M KC1 and a 10-minute contact time, 10−7 and 10−5 M ouabain approximately doubled the ATPase activity of freshly prepared natural actomyosin. After storage of the myosin B sample in 0.6 M KC1 at 2°C, ouabain had a small inhibitory effect in 0.24 M KC1 on the ATPase activity. At an ionic strength of 0.36 M KC1, only the inhibitory effect was observed. This effect was dependent on contact time and on calcium ion concentration. Previous treatment of the myosin B with EDTA removed the activating effect observed in 0.24 M KC1 at short contact times. Addition of 0.1 mM MgCl2 restored the activating effect of ouabain. In the EDTA-treated myosin B in 0.6 M KC1 at high ouabain concentrations in the presence of 1 mM CaCl2, a small activation was observed. Addition of 0.1 mM MgCl2 under the same experimental conditions produced a small deactivation. The effect of ouabain on the ATPase of myosin B in solution appears to be dependent on protein preparation, age of the protein preparation (which may affect aggregation), ionic strength, ouabain concentration, contact time, and divalent ions present. All effects induced by ouabain on the ATPase activity of myosin B may be related to the conformation of the myosin B in solution. However, no simple relationship was observed with any individual variable such as preparation, age, contact time, or ouabain concentration.

Effect of Ouabain on the ATPase of Cardiac Myosin B at High Ionic Strength