EFFECTS OF MONOVALENT AND DIVALENT CATIONS ON ATPASE ACTIVITY OF MYOSIN

The activation of the ATPase activity of myosin by KCl follows sigmoidal kinetics owing to the simultaneous increase in concentration of two activating ions, K+ and Cl-. Activation by K+ at a constant Cl- concentration follows hyperbolic kinetics but becomes sigmoidal in the presence of a constant concentration of Na+. There appears to be competition between Na+ and K+ for enzyme binding sites since increasing the concentration of K+ overcomes the inhibitory effect of a given concentration of Na+. Tryptic digestion of myosin to Subfragment-1 does not alter the sigmoidal activation by KCl nor that by K+ in the presence of Na+. The dependence of activity on the concentration of ATP follows Michaelis-Menten kinetics under all conditions tested. In the presence of K+ or NH4+, the ATPase activity of myosin is inhibited by divalent cations. Both the extent of the inhibition and the concentration of divalent cation required for half maximal inhibition vary with the nature of the divalent cation. The concentration of divalent cation required for half maximal inhibition appears to depend primarily on the ionic radius, the larger the ion the higher the concentration required to inhibit. The extent of inhibition of K+-activated ATPase by divalent cations is inversely related to the extent of activation by divalent cations in the presence of Na+, a non-activating cation. Divalent cations which activate in the presence of Na+ produce only a partial inhibition in the presence of K+. At high concentrations of divalent cations, ATPase activity is the same regardless of the monovalent cation present. These results suggest that the inhibition of K+-activated ATPase by divalent cations involves the conversion of myosin from a monovalent cation-activated enzyme to a divalent cation-activated enzyme. © 1969.