The Inhibition of the Adenosine Triphosphatase Activity of Actomyosin by Magnesium Ions

In 1939 Engelhardt and Ljubimova (2) suggested that myosin (the contractile substance of muscle) and adenosine triphosphatase are probably identical, and that 'the calcium ion is a specific activator of this enzyme. Working with a myosin suspension made from rabbit muscle (by the method of Greenstein and Edsall (4) or of Weber and Meyer (11)) D. M. Needham (9) tested the activating powers of both magnesium and calcium ions and found that both activate the enzyme at pH 7.4 or pH 9.0 but that the calcium ion is much more effective than the magnesium ion. Bailey (1), continuing and extending Needham's work, also found that the calcium ion activates adenosine triphosphatase more than the magnesium ion does, but the magnesium ion activates adenosine diphosphatase more strongly than the calcium ion. Bailey further found that the enzymatic activity of myosin preparations is greater in a glycine buffer than in a bicarbonate-carbonate buffer. However, in either buffer the optimum activity is at pH 9.0. The activation of adenosine triphosphate breakdown by magnesium decreases with the progressive purification of myosin. In other words, as the myosin becomes purer it loses its adenosine diphosphatase activity, and it is this latter activity which the magnesium ion affects most. Bailey finds that even in preparations freed from adenosine diphosphatase the magnesium ion still activates adenosine triphosphate breakdown. However, Lohman (6) found that only adenosine triphosphatase was activated by magnesium ions, while Ljubimova and Pevsner (5) reported that, working with myosin which had been freed from adenosine diphosphatase, magnesium ions actually inhibit adenosine triphosphatase activity. Banga and Szent-GySrgyi (10) conducted their investigation on the influence of salts on the phosphatase activity with definite advantages over their predecessors. In the first place, pure myosin crystallized by Szent-Gy6rgyi (10) was available to them; in the second place, Straub (10) had already discovered actin, which forms a complex with myosin and which was probably present as a contaminant in the myosin preparations of the previous investigators. Banga and Szent-Gy5rgyi found that the magnesium ion inhibits the adenosine triphosphatase activity of myosin but, except in very high concentrations (0.1 •), activates this activity of the actomyosin (a combination of actin and myosin). Still, in the presence of 0.01 ~ Ca ion, the Mg ion (0.001 M to 0.100 M) inhibits the action of both myosin and actomyosin. The action of Mg ions on actomyosin depends upon the concentration of K ions in the medium. Mg enhances the enzymatic activity in the presence of 0.01 ~ K + but inhibits it in the presence of 0.1 ~t K +. Banga and 411