Interactions of uncoupling agent and of antimycin A with cytochrome b in yeast and heart muscle preparation under anaerobic conditions

Abstract

When added to an anaerobic suspension of aerobically grown yeast, of mitochondria isolated from it, or of Keilin-Hartree heart muscle preparation, uncoupling agents elicited a considerable oxidation of cytochrome b. For a maximum effect, about three orders of magnitude higher concentrations were required than those effective in uncoupling oxidative phosphorylation, but an extrapolation to zero of the curve relating oxidation of cytochrome b to uncoupler concentration indicates that a slight effect may also occur at uncoupling concentrations. Ubiquinone was also partly oxidized, while cytochromes a·a 3 and c + c 1 remained unaffected. An unknown component of mitochondria may accept electrons from cytochrome b under these conditions. The effect was rather specific for uncoupling agents and was not displayed by a number of compounds devoid of uncoupling capacity. Antimycin A added to intact yeast cells or to isolated yeast mitochondria under anaerobic conditions also induced the anaerobic oxidation of cytochrome b. Under aerobic conditions, however, antimycin A prevented the cytochrome b oxidation via the respiratory chain and caused an additional slight reduction of cytochrome b to a form whose absorption maximum in the α-region was shifted to 565 nm. In the heart muscle preparation, antimycin A did not induce cytochrome b oxidation under anaerobic conditions but did cause an additional reduction by succinate of cytochrome b to the form absorbing at 565 nm. In normal ubiquinone-containing heart muscle particles, uncoupling agents did not induce the anaerobic oxidation of that part of cytochrome b which wits reducible by succinate in the presence of antimycin A. However, in ubiquinone-depleted particles, this part of cytochrome b became accessible to the uncoupler-induced anaerobic oxidation. The antimycin A block preventing oxidation of cytochrome b by air via the respiratory chain was relieved by some uncouplers at very high concentrations.