Inhibitor studies of diphtherial succinic dehydrogenase.

It had previously been found that the addition of ferrous ion to cultures of the diphtheria bacillus-above that necessary for optimal toxin production-resulted in the disappearance from the culture supernatant of a porphyrin, iron, and diphtheria toxin in the molar ratio of 4:4:1 (Pappenheimer, 1947). On the basis of this data, it was suggested that the toxin was a precursor of the protein moiety of an iron-containing re spiratory enzyme. Further studies suggested that the enzyme involved was succinic dehydrogenase (Pappenheimer and Hendee, 1947). Subsequent work (Pappenheimer and Hendee, 1949) carried out with a preparation partially purified by differential centrifugation, revealed that the enzyme was either identical or closely bound to cytochrome b, that the latter was slowly autooxidizable, and that the presence of cytochrome c and the cytochrome oxidase was in doubt. It was also found that the enzyme would not reduce cytochrome c. Although a direct attack on the question of whether or not the protein moiety of the enzyme arises from diphtheria toxin depends on the solubilization and further purification of the enzyme, it was felt by the authors that valuable data could be obtained by kinetic and inhibitor studies of a partially purified preparation obtained while attempting such a solubilization and purification. Specifically, this paper deals with the determination of the Michaelis constant (K.) of the system, and the enzyme-inhibitor dissociation constants (Ki) for several substances found to be inhibitory.