One- and two-electron reduction of quinones by glutathione reductase.

Yeast glutathione reductase (E.C. 1.6.4.2) catalyzes the oxidation of NADPH by p-quinones and ferricyanide with a maximal turnover number (TNmax) of 4-5 s-1.NADP+ stimulates the reaction and the TNmax/Km value of acceptors is reached at NADP+/NADPH greater than or equal to 100. TNmax is increased up to 30-33 s-1. The stimulatory effect of NADP+ may be associated with its complexation with the NADPH-binding site in the reduced enzyme (Kd = 40-60 microM). It is suggested that NADP+ shifts the electron density towards FAD in the two-electron-reduced enzyme and, evidently, changes its one-electron-reduction potentials, while quinones oxidize an equilibrium form of glutathione reductase containing reduced FAD. In the absence of NADP+ the reduction of quinones by glutathione reductase proceeds mainly in a two-electron manner. At NADP+/NADPH = 100 a one-electron reduction makes up 44% of the total process. At pH 6.0-7.0 the reduced forms of naphthoquinones undergo cyclic redox conversions. A hyperbolic dependence exists of the log TN/Km of quinones on their one-electron-reduction potentials.