The Respiratory Chain of Plant Mitochondria

The oxidation kinetics of the two high potential flavo-proteins, one (Fp(hf)) fluorescent and the other (Fp(ha)) 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 Fp(ha) 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 Fp(ha) about 3-fold, but it has no effect on the oxidation half-time of Fp(hf). 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 Fp(hf) 10-fold to about 2 seconds, while increasing that of Fp(ha) only some 20%. This result implies that the flavoprotein Fp(hf) mediates electron transport to the alternate oxidase from the region of the mitochondrial respiratory chain encompassing Fp(ha), ubiquinone, and the cytochromes b. The oxidation rate of cytochrome b(557) is unaffected by either m-chlorobenzhydroxamic acid or cyanide but is strongly inhibited by antimycin A. This result implies that cytochrome b(557) plays no direct role in the respiratory pathway to the alternate oxidase and is different from cytochrome b(7) found in mitochondria from the spadices of Arum maculatum.