A protein factor inhibiting the magnesium-activated adenosine triphosphatase of desensitized actomyosin.

1. The preparation and properties of a myofibrillar protein factor which inhibits the Mg(2+)-activated adenosine triphosphatase of desensitized actomyosin is described. 2. This factor had negligible effect on the Mg(2+)-activated adenosine triphosphatase of natural actomyosin and on the Ca(2+)-activated adenosine triphosphatases of desensitized actomyosin and myosin. 3. The Mg(2+)-activated inosine triphosphatase activity of desensitized actomyosin was not affected by the factor. 4. The inhibitory effect was sensitive to ionic strength. In addition to their ionic effects Mg(2+) and Ca(2+) appeared to have a specific action in reducing the effect of the inhibitor. 5. F-actin reduced the inhibition whereas Bailey-type tropo-myosin had little effect. 6. As far as can be judged from the reported experiments this factor is different from any of the previously described myofibrillar components.