On the cyanide inactivation of succinic dehydrogenase and the relation of succinic dehydrogenase to cytochrome b

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Abstract
Succinic dehydrogenase was slowly but irreversibly inactivated by cyanide. The rate of inactivation depended upon the temp. of incubation and the cyanide concn. The time required for 50% inactivation was inversely proportional to the cyanide concn. The rate of inactivation was independent of the pH of incubation from pH 6.5 to 8, indicating that the reaction involved HCN and not cyanide ion. The enzyme was protected from inactivation by succinate and Na dithionite but not by mal-onate (50 m[image]), fumarate (50 m[image]), pyrophosphate (10 m[image]), cystine (2 m[image]) and cysteine (4 m[image]). The min. concn. of succinate required to give complete protection was 2 m[image] and the min. concn. to give a measurable protection, 0.02 m[image]. Summarized evidence showed that succinic dehydrogenase, apart from being concerned with the specific activation of succinate, was actually its immediate electron acceptor, and only the oxidized form of the enzyme was susceptible to the attack by cyanide. Additional evidence showed that the pyrophosphate inhibition of succinic dehydrogenase was competitive in nature. Matching of the intensity of the alpha-band of reduced cytochrome B of cyanide-treated enzyme prepns. showed no parallel relation with the succinic dehydrogenase activity of these enzyme prepns. After nearly complete inactivation of the dehydrogenase, cytochrome b was still oxidized and reduced by various means. This evidence, and the fact that all cases of cyanide inhibition of haem proteins are reversible, led to the conclusion that succinic dehydrogenase is not identical with cytochrome b. The possible chemical nature of succinic dehydrogenase was discussed.