Effects of Magnesium and Calcium on the Superprecipitation of Myosin B.

The effects of magnesium and calcium on the adenosine triphosphate-induced contraction of glycerinated muscle fibers were recently reexamined. Magnesium and calcium were both shown to have a dual effect on the contractility of glycerinated fibers (1). By using myosin B suspensions in place of glycerinated fibers, we compared the dual effect of magnesium on the contractile phenomenon called “superprecipitation” with the effect of magnesium on hydrolysis of ATP (2, 3). A close association between superprecipitation and ATP hydrolysis was then observed in the presence of magnesium. This was especially evident when measurement of the change in turbidity of myosin B suspensions was employed to follow the superprecipitation phenomenon. Here a rapid hydrolysis of ATP was found to be associated with superprecipitating-myosin B rather than superprecipitated myosin B (3). In this paper, the dual effect of calcium (in the presence of magnesium) on both superprecipitation and ATPase activity of myosin B suspensions will be reported. In connection with the calcium effect, a relationship between superprecipitation and inosine triphosphate hydrolysis and an effect of p-chloromercuribenzoate on ATP-induced superprecipitation will also be reported.