Effects of Ca2+ and Mg2+ on the actin-activated ATP hydrolysis by phosphorylated heavy meromyosin from arterial smooth muscle.

Actin-activated ATP hydrolysis by phosphorylated arterial myosin is Ca2+-dependent at Mg2+ concentrations which allow myosin to bind 2 mol of Ca2+ per mol (S. Chacko and A. Rosenfeld (1982) Proc. Natl. Acad. Sci. U. S. A. 79, 292-296). The possible effects of myosin filament formation and conformational change involving the rod portion of the myosin molecule on actin-myosin interaction and actin-activated ATP hydrolysis was eliminated by utilizing soluble heavy meromyosin (HMM). HMM was prepared from phosphorylated arterial myosin by chymotryptic digestion and purified by gel filtration on Sepharose CL-6B. Actin-activated ATPase activity of the phosphorylated HMM was measured either at constant Mg2+ and variable Ca2+ or at constant Ca2+ and variable Mg2+ concentrations. At constant (0.05 M) ionic strength and pCa 5, the actin-activated ATPase activity increased until the free Mg2+ reached between 0.5-1 mM; this was followed by a decrease in activity to a very low level at 7 mM free Mg2+. Removal of Ca2+ at 1 mM free Mg2+ lowered the actin-activated ATPase activity (40-60% inhibition). These experiments demonstrated that Mg2+ and Ca2+ had a direct effect on actin-activated ATP hydrolysis. This effect was not due to myosin filament formation and was independent of the conformational change involving the rod portion of the myosin molecule.