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The Respiratory Chain of Plant Mitochondria

XVI. Interaction of Cytochrome b₅₆₂ with the Respiratory Chain of Coupled and Uncoupled Mung Bean Mitochondria: Evidence for Its Exclusion from the Main Sequence of the Chain ¹

^a Johnson Research Foundation, University of Pennsylvania, Philadelphia, Pennsylvania 19174

Cytochromes b₅₅₃, b₅₅₇, and b₅₆₂ of mung bean (Phaseolus aureus) mitochondria become partially reduced with endogenous substrate on addition of antimycin A to the aerobic mitochondrial suspension. Addition of ATP causes partial reoxidation of the three cytochromes. This partial oxidation by ATP is inhibited by oligomycin and reversed by uncoupler. Ubiquinone does not appear to act as electron acceptor for the oxidation reaction, but a nonfluorescent flavoprotein, or possibly ironsulfur protein, component does appear to act as acceptor. This is consistent with reverse electron transport driven by ATP across the first site of energy conservation of the respiratory chain. Endogenous pyridine nucleotide and the fluorescent flavoprotein with E_m7.2 = –155mv (midpoint potential at pH 7.2, referred to normal hydrogen electrode) in uncoupled mitochondria become reduced in anaerobiosis attained by oxidation of succinate in the absence of respiratory inhibitors of the cytochrome chain, provided that Pi and ATP are present. Under these same conditions, cytochrome b₅₅₇ is completely reduced but cytochrome b₅₆₂ remains nearly completely oxidized. There is no equilibration across the first site of energy conservation between the carriers on the low potential side and cytochrome b₅₆₂ with E_m7.2 = –77mv on the high potential side. It is concluded that cytochrome b₅₆₂ is not a part of the main sequence of electron transport carriers of the mitochondrial respiratory chain of plants; it can participate in redox reactions with the respiratory chain in coupled mitochondria but not in uncoupled mitochondria unless antimycin A is present.

¹ This research was supported by National Science Foundation Grant GB-23063 and United States Public Health Service Grant GM-12202 and was carried out in part during the tenure of United States Public Health Service Career Development Award K3-GM-7311.