Ilydroxamic acids, R-CONHOH, are inhibitors specific to the respiratory pathway through the alternate, cyanideinsensitive terminal oxidase of plant mitochondria. The nature of the R group in these compounds affects the concentration at which the hydroxamic acids are effective, but it appears that all hydroxamic acids inhibit if high enough concentrations are used. The benzhydroxamic acids are effective at relatively low concentrations; of these, the most effective are m-chlorobenzhydroxamic acid and m-iodobenzhydroxamic acid. The concentrations required for halfmaximal inhibition of the alternate oxidase pathway in mung bean (Phaseolus aureus) mitochondria are 0.03 mM for m-chlorobenzhydroxamic acid and 0.02 mM for m-iodobenzhydroxamic acid. With skunk cabbage (Symplocarpus foetidus) mitochondria, the required concentrations are 0.16 for m-chlorobenzhydroxamic acid and 0.05 for m-iodobenzhydroxamic acid. At concentrations which inhibit completely the alternate oxidase pathway, these two compounds have no discernible effect on either the respiratory pathway through cytochrome oxidase, or on the energy coupling reactions of these mitochondria. These inhibitors make it possible to isolate the two respiratory pathways and study their mode of action separately. These inhibitors also enhance an electron paramagnetic resonance signal near g = 2 in anaerobic, submitochondrial particles from skunk cabbage, which appears to be specific to the alternate oxidase and thus provides a means for its assay.Mitochondria isolated from a number of plant tissues show incomplete inhibition of respiration by cyanide. Outstanding in this respect are mitochondria isolated from the spadices of aroids; in particular, Arum maculatum (1, 4) and skunk cabbage, Symplocarpusfoetidus (2,12,13,31), which show little, if any, sensitivity to cyanide inhibition. Mitochondria from the hypocotyls of etiolated mung beans (Phaseolus aureus) show partial sensitivity; approximately 70% of the state 3 rate is inhibited by cyanide or antimycin A (16). In contrast, the respiration of mitochondria isolated from potato tubers (Solanum tuberosum) shows nearly complete inhibition by either of these compounds. Bendall and Bonner (2) have critically evaluated the various hypotheses which have been proposed to explain this behavior and conclude
A bstract. The kinetics of oxidation of ubiquinone, flavoprotein, cytochrome c, and the cytochrome b complex in skunk cabbage (Symplocarpus foetidus) mitochondria made anaerobic with succinate have been measured spectrophotometrically and fluorimetrically in the absence of respiratory inhibitor and in the presence of cyanide or antimycin A. No component identifiab,le by these means was oxidized rapidly enough in the presence of one or the other inhibitor to qualify for the role of alternate oxidase. Cycles of oxidation and rereduction of flavoprotein and ubiquinone obtained by injecting 12 jM oxygen into the anaerobic mitochondrial suspension were kinetically indistinguishable in the presence of cyanide or antimycin A, implying that these 2 components are part of a respiratory pathwav between succinate and oxygen which does not involve the cytochromes and does involve a cyanide-insensitive alternate oxidase. The cytochrome b complex shows biphasic oxidation kinetics with half times of 0.018 sec and 0.4 sec in the absence of inhibitor, which increase to 0.2 sec and 1 sec in the presence of cyanide. In the presence of antimycin A, the oxidation of the cytochrome b complex shows an induction period of 1 sec and *a half-time of 3.5 sec. A split respiratory chain with 2 terminal oxidases and a branch point between the cytochromes and flavoprotein and ubiquinone is proposed for these mitochondria.Many plant tissues yield mitochondria whose rate of respiration is partiallv insensitive to inhibition by cvanide (34.42). The most notable of such tissues are the spadices of the flowers of Arum maculatum (1, 2) and the skuinik cabbage. Symplocarpus foetidus (3,28, 299). Mitochondria isolated from these tissues are often totally insensitive to respiratory inhibition by cyanide or antimycin A. The cvtochromes of skunk cabbage mitochondria have been characterized by their spectra (4,16,19)
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