Relation between the inhibitory action of native tropomyosin and the dual effect of magnesium on the ATPase activity of actomyosin

Abstract

The effects of various concentrations of Mg 2+ on the ATPase activities of native tropomyosin-free synthetic actomyosin, myosin B, and synthetic actomyosin to which native tropomyosin was added, and on the superprecipitation of myosin B, were investigated in the presence of 6.2 × 10 −8 m Ca 2+ and 1.7 × 10 −5 m Ca 2+. At 6.2 × 10 −8 m Ca 2+, ATPase activity of synthetic actomyosin was enhanced by 5 × 10 −6 to 1 × 10 −4 m Mg 2+. Even with supramaximal amounts of Mg 2+, the ATPase activity was not inhibited below the original level attained without added Mg 2+. At 1.7 × 10 −5 m Ca 2+, the ATPase was markedly activated by Mg 2+ in concentrations lower than 10 −5 m. Further addition of Mg 2+ decreased the ATPase activity toward, but not below, the original level. Unlike synthetic actomyosin, Mg 2+-activated ATPase activity of myosin B and of synthetic actomyosin to which native tropomyosin was added falls below the original level at high Mg 2+ concentrations if Ca 2+ level is very low. This inhibitory effect of Mg 2+ is abolished by high concentrations of Ca 2+. On the basis of these and related findings, we suggest mechanisms for the inhibitory action of native tropomyosin and for the dual effect of Mg 2+.