The Respiratory Chain of Plant Mitochondria: VII. Kinetics of Flavoprotein Oxidation in Skunk Cabbage Mitochondria

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The oxidation kinetics of the two high potential flavo-proteins, one (Fphf) fluorescent and the other (Fpha) nonfluorescent, in mitochondria from skunk cabbage (Symplocarpus foetidus) spadices have been measured by combined spectrophotometry and fluorimetry. In the absence of respiratory inhibitors, both flavoproteins are oxidized at nearly the same rate with half-times between 120 and 160 milliseconds at 24 C. When slight differences in rate are observed, it is Fpha which consistently has the shorter half-time. The presence of 0.3 millimolar KCN has no perceptible effect on the oxidation rate of either component. Antimycin A (2 nanomoles per milligram of protein) increases the oxidation half-time of Fpha about 3-fold, but it has no effect on the oxidation half-time of Fphf. In contrast to these two inhibitors, m-chlorobenzhydroxamic acid—an inhibitor specific to the cyanide insensitive, alternate oxidase pathway in these mitochondria—increases the oxidation half-time of Fphf 10-fold to about 2 seconds, while increasing that of Fpha only some 20%. This result implies that the flavoprotein Fphf mediates electron transport to the alternate oxidase from the region of the mitochondrial respiratory chain encompassing Fpha, ubiquinone, and the cytochromes b. The oxidation rate of cytochrome b557 is unaffected by either m-chlorobenzhydroxamic acid or cyanide but is strongly inhibited by antimycin A. This result implies that cytochrome b557 plays no direct role in the respiratory pathway to the alternate oxidase and is different from cytochrome b7 found in mitochondria from the spadices of Arum maculatum.

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