Further Evidence for the Pool Function of Ubiquinone as Derived from the Inhibition of the Electron Transport by Antimycin

Abstract

The role of ubiquinone (Q) in the respiratory chain is studied by analyzing the inhibition by antimycin of the electron transport in uncoupled submitochondrial particles. As a result, the antimycin inhibition curve of substrate oxidation is quantitatively explained. The participation of Q in the electron transport is required for the occurrence of nonlinear antimycin inhibition curves as demonstrated with external hydroquinones of different redox potentials as hydrogen donors. The respiration is non‐linearly inhibited with a hydroquinone of sufficiently low redox potential to reduce the mitochondrial Q (menadiol) whereas a linear inhibition curve is found with hydroquinones of higher redox potentials (durohydroquinone, dimethoxy‐dimethylbenzohydroquinone). If Q is extracted, the inhibition curve also with menadiol becomes linear. This demonstrates the relation of the nonlinearity of the inhibition curve to the function of Q as a distributor of hydrogen between parallel cytochrome chains. The electron transport through the cytochrome segment is inhibited linearly with increasing amounts of antimycin as demonstrated by the linear titration of three different functions: (a) the oxidation rate of Q red on addition of oxygen to anaerobic submitochondrial particles, (b) the repiratory activity with added hydroquinones, (c) the acceptor activity as defined by the ratio of the respiration rate to the redox state of Q (Kröger, Klingenberg and Schweidler, 1973). The ratio of donor to acceptor activity to Q (Vred/Vox) is the main parameter in controlling the shape of the antimycin titration curve. This curve becomes more nonlinear with decreasing this ratio. Also the redox state of Q is nonlinearly dependent on the amount of antimycin and controlled by the donor/acceptor activity ratio. A theoretical hyperbolic relation of the rate of substrate oxidation to the amount of antimycin is derived, involving the ratio donor to acceptor activity and the linear inhibition by antimycin of the electron transport through the cytochrome segment. With independently measured donor/acceptor activity ratios, antimycin titration curves are calculated which give a good fit to the experimental titrations over the range from highly nonlinear to almost linear curves. The observed nonlinearity is thus recognized to result from a hyperbolic relationship. Also the calculated titration curves for the redox state of Q are in agreement with the experiments. The deviation of the experiment from the calculated curves near equimolarity between the amount of antimycin and inhibition sites can be accounted for by the dissociation of antimycin from the site of inhibition.