title: Reduction of 2,4,6-trinitrobenzenesulfonate by glutathione reductase and the effect of NADP+ on the electron transfer.
abstract: Glutathione reductase has been found to catalyze an NAD(P)H-dependent electron transfer to 2,4,6-trinitrobenzenesulfonate (TNBS). In the presence of oxygen TNBS is not consumed in the reaction, but is rapidly reoxidized with concomitant production of hydrogen peroxide. Cytochrome c can replace oxygen as the final electron acceptor, indicating that a one-electron transfer takes place. The rate is slightly higher in the absence than in the presence of oxygen, ruling out superoxide anion as an obligatory intermediate in cytochrome c reduction. In the absence of oxygen (or cytochrome c), TNBS limits the reaction and accepts a total of four electrons. The TNBS-dependent NADPH (or NADH) oxidation is markedly stimulated by NADP+, and to a smaller extent also by NAD+. The TNBS-dependent reactions are inhibited by excess of NADPH but not by NADH. The kinetics of these reactions are consistent with a branching reaction mechanism in which a pathway including a ternary complex between the two-electron reduced enzyme and NADP+ has the highest turnover. NADPH-dependent reductions of ferricyanide or 2,6-dichloroindophenol catalyzed by glutathione reductase are also markedly influenced by NADP+. Evidently NADP+ facilitates a shift of the catalyzed reaction from the normal two-electron reduction of glutathione disulfide to a more unspecific one-electron reduction of other acceptors. Spectral as well as kinetic data suggest that the rate of radical formation limits the reactions with the artificial electron acceptors and that NADP+ promotes this rate-limiting step.
mesh_terms: D003574:Cytochrome c Group / Q000378:metabolism; D004047:Diethyl Pyrocarbonate / Q000494:pharmacology; D004579:Electron Transport / Q000187:drug effects*; D005980:Glutathione Reductase / Q000378:metabolism*; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007461:Iodoacetates / Q000494:pharmacology; D019807:Iodoacetic Acid; D007700:Kinetics; D009243:NAD / Q000378:metabolism; D009249:NADP / Q000378:metabolism*; D009578:Nitrobenzenes / Q000378:metabolism*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000378:metabolism; D013379:Substrate Specificity; D014302:Trinitrobenzenesulfonic Acid / Q000378:metabolism*
chemical_list: D003574:Cytochrome c Group; D007461:Iodoacetates; D009578:Nitrobenzenes; D009243:NAD; D009249:NADP; D014302:Trinitrobenzenesulfonic Acid; D005980:Glutathione Reductase; D004047:Diethyl Pyrocarbonate; D010100:Oxygen; D019807:Iodoacetic Acid