Studies on choline dehydrogenase. II. Components of the choline oxidase system and the site of action of amytal.

Rat liver choline dehydrogenase has been known for many years to be one of the “cytochrome-reducing dehydrogenases” (1). Although it is functionally and structurally linked to the respiratory chain and acts without the mediation of readily dissociable coenzymes (2)) the cytochrome components linking choline dehydrogenase to oxygen have not been established. The purpose of this investigation was a-fold. First, quantitative differences in the titration of the succinic and choline oxidase activities of liver mitochondria with various respiratory chain inhibitors, and other dissimilarities between the two enzyme systems, did not seem to be compatible with the operation of a common respiratory chain (3, 4). It was of interest, therefore, to study the respiratory pigments functioning in the choline oxidase system. Since choline oxidase is the only known enzyme system inhibited by Amytal (3, 5) which does not operate by way of pyridine nucleotide coenzymes (2), it was desirable to localize the site of inhibition by Amytal. The present paper shows that the respiratory pigments operative in choline oxidase are spectroscopically indistinguishable from those functioning in the succinic oxidase chain and the site of Amytal inhibition is identified between flavoprotein and cytochrome b. The implications of these results on the interrelation of choline and succinic oxidases in rat liver are dealt with in the succeeding paper (4).