One-electron reduction of quinones by the neuronal nitric-oxide synthase reductase domain.

Abstract

Quinones including antitumor quinones undergo facile reduction and oxidation. One-electron reduction of a quinone gives the semiquinone radicals while two-electron reduction gives the hydroquinonel. In 1969-70, Iyanagi and Yamazaki2,3 reported the mechanism of quinone reduction by flavin enzymes, and the reduction of quinones and oxygen by flavin enzymes falls into three mechanistic categories: one-electron, two-electron and mixed-type reactions. NAD(P)H:quinone oxidoreductase(QR), also known as DT-diaphorase contains one FAD as prosthetic group, catalyzes the obligatory two-electron reduction of quinone to hydroquinone. On the other hand, microsomal NADPHcytochrome P450 reductase (P450 reductase) and NADH-cytochome b5 reductase, and mitochondrial NADH-ubiquinone oxidoreductase, and ferredoxin: NADP+ reductase catalyze typical one electron reduction of bivalent quinones. Xanthine oxidase/ dehydrogenase catalyzes both reactions of one-electron and two-electron. These results have contributed greatly to the study on the formation of free-radicals in biological systems, especially in the quinone-mediated cyotoxicity.