THE INHIBITION OF THE ADENOSINE TRIPHOSPHATASE ACTIVITY OF ACTOMYOSIN BY MAGNESIUM IONS

Abstract

Previous investigations yielded conflicting reports of the action of Mg ions on the ATP-ase activity of myosin and/or actomyosin. In this study, each experiment was set up with 4.0 ml. glycine buffer (pH 8.32 at 37[degree]C), 0.2 ml. actomyosin soln. in 0.6 [image] KC1 (Banga and Szent-Gyorgyi''s method), 1.0 ml. KC1 soln. (final concn. of 0.128 [image] K), 1.0 ml. CaCl2 soln. (final concn. of 0.002 M Ca), 1.0 ml. Na-ATP soln. (Needham''s method), variable amts. of MgCl2 soln., and redistilled H2O to make up vol. to 10.0 ml. The tubes were incubated and the P detd. (Fiske and SubbaRow''s method) in the trichloracetic filtrate. Even though 0.00018 M Mg caused measurable inhibition, as much as 0.4 [image] did not invariably cause complete inhibition. Changing the Mg/Ca ratio from 0-10 depressed ATP-ase activity 75%, but changing the ratio from 10-200 caused relatively little further depression. With the Mg/Ca ratio present in normal rabbit muscle, the ATP-ase activity of actomyosin was about 70% inhibited. Halving or doubling the ratio increased or decreased the activity by less than 10%. Therefore, it did not seem probable that ATP-ase was absolutely inhibited or released from this inhibition through changes in Mg concn. It seemed more likely that the interplay of Mg and Ca ions in muscle served to secure greater stability of enzyme activity, though not maximum activity.