Pathways of Oxidation in Cell-free Potato Fractions. II. Properties of the Soluble Pyridine Nucleotide Oxidase System.

Abstract

The soluble fraction prepared from a sucrose-phosphate-cysteine homogenate of potato tuber tissue catalyzes the rapid oxidation of DPNH and TPNH. The reaction requires molecular oxygen and can be inhibited by cyanide and diethyldithiocarbamate; it is also blocked by certain -SH group inhibitors. Dialysis of the fraction removes all activity. The dialyzed fraction can oxidize DPNH in the presence of added ascorbate or a suitable hydrogen acceptor, such as pyruvate, 2,6-dichlorophenol-indophenol, ferricyanide, or menadione. The ascorbate-activated reaction is qualitatively similar to that catalyzed by the undialyzed fraction. It is concluded that the original DPNH (TPNH) oxidase reaction involves both ascorbic acid and a step that is sensitive to -SH inhibitors. The soluble fraction prepared in the absence of cysteine also attacks DPNH rapidly, but this reaction does not require oxygen and is less sensitive to the inhibitors tested. It is suggested that the presence of cysteine in the grinding medium not only permits the soluble DPNH oxidase to operate, but also prevents the formation of components required for the anaerobic DPNH attack.