ONE-ELECTRON-TRANSFER REACTIONS IN BIOCHEMICAL SYSTEMS .3. ONE-ELECTRON REDUCTION OF QUINONES BY MICROSOMAL FLAVIN ENZYMES

The microsomal flavin enzymes cytochrome b5 reductase (NADH: cytochrome b5 oxidoreductase, EC 1.6.2.2) and NADPH-cytochrome c reductase (NADPH: cytochrome c oxidoreductase) catalyze one-electron reduction of quinones. Semiquinones thus formed are detached from the enzymes and undergo dismutation. In the presence of suitable electron acceptors, such as cytochromes and molecular oxygen, electron transfer occurs from semiquinone to electron acceptor. In the presence of 2-methyl-1,4-naphthoquinone, for instance, NADPH-cytochrome c reductase catalyzes the reduction of cytochrome b5 and molecular oxygen. Reduction of cytochrome c is catalyzed by cytochrome b5 reductase in the presence of p-benzoquinone. Electron transfer from reduced cytochrome b5 reductase to monodehydroascorbate occurs at a considerable rate. These one-electron-transfer reactions catalyzed by the microsomal flavin enzymes are investigated quantitatively with electron spin resonance spectroscopy. © 1969.