A Second Type of the Inhibitory Effect of Magnesium on Superprecipitation and on ATPase of Myosin B, and Its Reversal by p-Chloromercuribenzoate and by Adenosine Diphosphate

Abstract

Two different types are distinguishable in the inhibition effect of MgATP on superprecipitation of myosin B. The first type of inhibition is removable by adding calcium, and it is understood as the inhibition involving relaxing protein. The second type of inhibition is removed by adding p-Chloromercuribenzoate (CMB) or adenosine diphosphate (ADP). However, if added after addition of ATP, CRIB or ADP is no longer effective in removing the second type of inhibition. It is there-fore suggested that the second type of inhibition involves a MgATP-induced conformational change of myosin A which protects myosin A from the effect of CMB or ADP. From the differences observed between the effect of CMB and that of ADP, and from a kinetic study of the competition between ADP and MgATP, it is further suggested that if added before the ATP addition, CMB or ADP prevents the MgATP-induced conformational change from occurring; free ADP does so by competing with MgATP for the binding sites on myosin A whereas CNIB does so, perhaps by inducing another conformational change of myosin A which opposes the MgATP-induced one. Based on the observation that the first type and the second type of inhibitions are marked in the rate and in the extent of superprecipitation, respectively, a possible interrelation between the two different types of inhibition is discussed.