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

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Pathways of electron transport have been studied in mitochondria isolated from hypocotyls of etiolated mung bean seedlings and skunk cabbage spadices that show cyanide-resistant respiratory activity. The (27) confirmed the high oxygen affinity in tissues of the Arum maculatum spadix.Hypothesis 3. The simplest explanation of cyanide-insensitive respiration is that there is incomplete inhibition of cytochrome c oxidase. Such a situation could arise if the mitochondria contained a large excess of cytochrome oxidase relative to a rate-limiting step in the respiratory chain. The excess oxidase hypothesis has been discussed in relation to plants