Pathways of Oxidation in Cell-Free Potato Fractions

In the previous paper (11), the oxidation of reduced diphosphopyridine nucleotide (DPNH) by isolated potato tuber mitochondria was examined. It was also shown that the supernatant fraction rapidly attacks DPNH by a reaction that does not involve molecular oxygen. More recently, it has been shown that the inclusion of cysteine in the isolating medium greatly enhances the ability of potato mitochondria to oxidize organic acids (10, 23). Accordingly, it was of interest to know whether the presence of eysteine has any effect on the reaction between the supernatant fraction and DPNH. The present study indicates that the cysteine-containing fraction does not catalyze the anaerobic DPNH attack, but it can, on the other hand, oxidize DPNH very rapidly by a reaction that requires oxvgen. Thus, the soluble fraction can catalyze either an aerobic or an anaerobic reaction with DPNH, depending on whether or not cysteine is included in the grinding medium. This paper describes the properties of the oxidase system, and evidence that ascorbic acid is involved in the reaction is presented.