Protection of cytochrome c oxidase against cyanide inhibition by pyruvate and alpha-ketoglutarate: effect of aeration in vitro.

Toxicity of cyanide is related to its inhibitory action on cytochrome c oxidase (COx). The alpha-keto acids pyruvate and alpha-ketoglutarate are known to exert in vivo certain protective effect against CN- toxicity if present when the poison is administered. We characterized in vitro their protection of oxidative phosphorylation and of the activity of COx in rat testis, heart, and liver mitochondria, as well as that of the beef heart enzyme in the pure state. In all cases the keto acids proved to have a protective action, even when KCN was previously added to the incubation mixtures and inhibition had already been established. However, the extent of the protection seemed to depend upon the degree of aeration. In the presence of the alpha-keto acids, O2 succeeded with 77% efficiency in reversing CN- inhibition of [gamma-32P]ATP synthesis under high aeration, whereas only 15% was achieved if aeration was poor. In poisoned heart mitochondria (1 mM KCN) simultaneous estimations of ATP synthesis and COx activity in a closed oxygraph chamber showed a recovery of only 6% of both activities upon the addition of 12 mM pyruvate. Our results suggest that O2 displaced cyanide from the enzyme and the poison was then trapped by the keto acids to form the respective nontoxic cyanohydrin. The combination of both high oxygen concentration and the presence of either pyruvate or alpha-ketoglutarate was necessary to effectively protect COx against cyanide poisoning.