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Articles

The Respiratory Chain of Plant Mitochondria

XVII. Flavoprotein-Cytochrome b₅₆₂ Interaction in Antimycin-treated Skunk Cabbage Mitochondria ¹

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

During the transition from the aerobic steady state with succinate as substrate to anaerobiosis, in suspensions of skunk cabbage (Symplocarpus foetidus) mitochondria treated with antimycin A, cytochrome b₅₆₂ becomes reoxidized to the extent of about 20%, synchronously with the reduction of cytochrome c₅₄₉. This reoxidation occurs in both the absence and presence of m-chlorobenzhydroxamic acid, a specific inhibitor for the alternate terminal oxidase of plant mitochondria. A flavoprotein component, amounting to 13% to 15% of the total nonfluorescent mitochondrial flavoprotein, undergoes reduction synchronously with the oxidation of cytochrome b₅₆₂ during the aerobic to anaerobic transition with succinate as substrate in the presence of both antimycin A and m-chlorobenzhydroxamic acid. This flavoprotein component remains reduced in the presence of cyanide. The half-time for reduction of the flavoprotein component and cytochrome c₅₄₉ and for oxidation of cytochrome b₅₆₂ during the aerobic to anaerobic transition with succinate as substrate in the presence of both antimycin A and m-chlorobenzhydroxamic acid is 2 seconds. The half-times for oxidation of cytochrome c₅₄₉ and the flavoprotein component are 2.1 and 170 milliseconds, respectively, during the anaerobic to aerobic transition induced by addition of 14 µM O₂ to the mitochondrial suspensions. The half-time for reduction of cytochrome b₅₆₂ under these conditions is 150 milliseconds, synchronous with the flavoprotein component. The synchrony of the flavoprotein oxidation and of the cytochrome b₅₆₂ reduction at a rate much slower than that of cytochrome c₅₄₉ oxidation implies that, in antimycin-treated plant mitochondria, the state of the cytochrome b₅₆₂/antimycin complex is regulated by the redox state of this flavoprotein component, rather than by cytochrome c₅₄₉. It is tentatively suggested that these two components are not part of the main sequence of the respiratory chain, but may be part of a multienzyme complex active in the hydroxylation reactions required for ubiquinone biosynthesis in the inner mitochondrial membrane.

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.