Abstract
The dependence of superprecipitation and ATPase activity of myosin B on the concentration of ATP was studied in the presence and absence of pyruvate kinase and phosphoenolpyruvate. The minimum concentration of ATP required to maintain the maximum extent of superprecipitation of myosin B decreased on addition of the kinase system. The rate of superprecipitation was proportional to 1.7th power of the ATP concentration. Both the rate of superprecipitation and the ATPase activity increased with increase in the concentration of free Ca++, but were unaffected by free EGTA. ADP and P1 accelerated the rate of superprecipitation of myosin B without enhancing the ATPase activity. In the presence of a sufficient amount of ADP and P1, the extent of superprecipitation was proportional to the concentration of myosin B, but the specific rate of superprecipitation decreased with decrease in the myosin B concentration and superprecipitation did not occur at protein concentrations lower than a critical one.
The rate of superprecipitation of a large amount of myosin B was conspicuously accelerated by the addition of an extremely small amount of previously superprecipitated myosin B. The accelerating effect of superprecipitated myosin B was markedly increased by sonication. The rate of superprecipitation of 0.3mg. per ml. of myosin B induced by ATP was increased 20-30 fold by the addition of 0.3μg. per ml. of superprecipitated and sonicated myosin B. Neither the precipitate dissolved in 0.6M KCl nor the supernatant of superprecipitated myosin B after centrifugation accelerated the rate of superprecipitation. A minute amount of superprecipitated and sonicated myosin B added as an accelerator of the superprecipitation had no effect on the ATPase activity. In the presence of a minute amount of superprecipitated and sonicated myosin B, the specific rate of superprecipitation became independent of the concentration of myosin B but it was proportional to the concentration of the accelerator up to a critical concentration.
On the basis of these results, it was concluded that superprecipitation is a nucleated growth process and a minute amount of previously superprecipitated myosin B serves as a nucleus for the superprecipitation of a large amount of myosin B.
