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Articles

The Respiratory Chain of Plant Mitochondria

XII. Some Aspects of the Energy-linked Reverse Electron Transport from the Cytochromes c to the Cytochromes b in Mung Bean Mitochondria

¹ Johnson Research Foundation, University of Pennsylvania, Philadelphia, Pennsylvania 19104

The cytochromes c of mung bean (Phaseolus aureus) mitochondria become reduced when sulfide, a cytochrome oxidase inhibitor free from uncoupling side effects, is added to the aerobic mitochondrial suspension in the absence of added substrate. The cytochromes b remain largely oxidized. Subsequent addition of ATP results in partial oxidation of the cytochromes c and partial reduction of the cytochromes b due to ATP-driven reverse electron transport through the second site of energy conservation, or coupling site, of the respiratory chain. Cytochrome a is also oxidized under these conditions, but there is no concomitant reduction of the flavoprotein components, of ubiquinone, or of endogenous pyridine nucleotide. The reaction is abolished by oligomycin. The reducing equivalents transported from the cytochromes c and a in ATP-driven reverse electron transport are about 2-fold greater than those which appear in the cytochromes b. It is suggested that the equivalents not accounted for are present in a coupling site enzyme at the second site of energy conservation which interacts with the respiratory chain carriers by means of the dithiol-disulfide couple; this couple would not show absorbance changes with redox state over the wavelength range examined. With succinate present, reverse electron transport can be demonstrated at both coupling sites in both the aerobic steady state and in anaerobiosis. ATP-driven reverse electron transport in anaerobiosis maintains cytochrome a 30% oxidized while endogenous pyridine nucleotide is 50% reduced.

When mung bean mitochondria, oxidizing succinate in the presence of sulfide through the alternate, cyanide- and sulfide-insensitive terminal oxidase, become anaerobic, cytochrome b₅₅₇, which has remained largely oxidized, becomes slowly reduced. The slow reduction is observed in coupled, energized mitochondria and in uncoupled mitochondria; the time course parallels the reduction of cytochrome a₃ under the same conditions. It appears that sulfide-liganded, oxidized cytochrome a₃ may be in close enough proximity to cytochrome b₅₅₇ in the membrane to inhibit the reduction of the latter.