Release of the inhibitory action of the natural ATPase inhibitor protein on the mitochondrial ATPase

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Abstract
The rate of ATP hydrolysis by submitochondrial particles prepared from bovine heart mitochondria in the presence of Mg2+ and ATP increases from a value of 0.4 .mu.mol min-1 mg-1 to 6-7 .mu.mol min-1 mg-1 upon incubation for 5-6 h at 38.degree. C. The increase in activity does not occur in particles that have been passed through a Sephadex column. The activation is prevented and partially reversed by ATP. This indicates that the increase in hydrolytic activity is due to abolition of the inhibitory action of the natural ATPase inhibitor protein of Pullman and Monroy (1963). At maximal activation .apprx. 50% of the inhibitor protein of the starting preparation remains in the particles as inferred from direct assay of inhibitor protein content and by its interaction with 125I-labeled antibodies directed against the inhibitor protein. The extent of the activation, which presumably is an index of the equilibrium between active and inactive enzymes, is strictly dependent on salts. The rate of the activation depends on the concentration of salts and is favored by alkaline pH. From results of experiments on the effect of temperature on the rate of activation of the ATPase, it was calculated that the activation energy, 1H.dbldag. and 1S.dbldag. of the process were 53.34 kJ/mol, 50.83 kJ/mol and -158.99 J mol-1 K-1, respectively. In its native inhibiting state, the interaction of the inhibitor protein with the enzyme involves electrostatic interactions. Abolition of the inhibitory action of the protein on ATPase activity is not compulsorily linked to release of the protein into the water space.

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