Inhibition by antimycin A of succinic oxidase in heart-muscle preparations

Abstract

With heart-muscle preparations (Keilin and Hartree, 1947), confirmation was obtained of the general features of the inhibition of succinic oxidase activity by antimycin A, as described by previous workers (Ahmad et al. 1950; Potter and Reif, 1952; Chance, 1952). A general method was described for distinguishing between inhibition as a result of complete loss of activity of a fraction of the enzyme present, leaving the remainder fully active, and inhibition as a result of uniform loss of activity of all the enzyme present. With this general method, it was shown that antimycin A, and 2:3-dimercaptopropanol (BAL) in the presence of air, bring about a uniform loss of activity of all the enzyme present, whereas p-amino-phenylarsenoxide brings about the complete loss of activity of a fraction of the enzyme present, leaving the remainder fully active. By combining the use of antimycin A and p-aminophenylarsenoxide it was shown that antimycin A becomes rapidly redistributed over all the available anti-mycin-inhibitable factor present when antimycin-inhibited and uninhibited samples of heart-muscle preparation are mixed together. Experiments underline the reversible nature of the combination between antimycin A and the factor (Reif and Potter, 1953). When succinic oxidase activity is plotted against decreasing amounts of antimycin A present/mg of heart-muscle preparation, a complex curve is obtained, the major part of which is hyperbolic. It is probable that the decreasing scale of antimycin A is an indirect measure of increasing concentrations of antimycin-inhibitable factor, and that the factor is relatively freely available, within an enzyme-bearing particle, to an adjacent component of the electron-transport system. This interpretation agrees with the fact that antimycin A brings about the uniform loss of activity of all the enzyme present. In preparations partially inactivated in media low in inorganic phosphate, histidine or cytochrome c increased the succinic oxidase activity of the control and simultaneously decreased the degree of inhibition by antimycin A. This effect of histidine and of cytochrome c was attributed to enhancement of electron transport within the enzyme-bearing particles. A sample of heart-muscle preparation, completely inhibited by treatment with BAL in the presence of air, still retained its capacity to combine with antimycin A. The results have been discussed in relation to the possible structure of the electron-transport system in Keilin and Hartree (1947) preparations.