A MODEL STUDY ON THE EFFECT OF AN AMINO GROUP ON THE ANTIOXIDANT ACTIVITY OF GLUTATHIONE-PEROXIDASE

In order to investigate mechanistic roles of the amino nitrogens located at the active center of glutathione peroxidase (GPX), a selenium-containing antioxidant enzyme, kinetic analyses and characterization of the intermediates of the model reaction (H2O2 + 2PhSH - 2H2O + PhSSPh) catalyzed by di-2-(N-cyclohexyl,N-(methylamino)methyl)phenyl diselenide (1) have been performed. The rate equation in methanol at 25-degrees-C by changing the initial concentrations of H2O2 and catalyst 1 suggests that the model catalyst (1) behaves precisely like GPX in the reduction catalytic cycle. On the basis of the Se-77 NMR experiments in a 1:1 mixture of CD3OD and CDCl3 under rigorous nitrogen atmosphere, three intermediates, selenenyl sulfide 4, selenolate 5', and selenenic acid 6, are characterized. Three specific roles of proximate nitrogen atoms on the GPX-like activity are proposed. (1) Both theory (MO calculation) and experiments (Se-77 NMR) suggest that the proximate nitrogen base activates the selenol intermediate (5) into the corresponding selenolate anion (5', Se-77 NMR observed at delta 22 ppm), which should play a key role in accelerating the catalytic cycle. (2) The proximate nitrogen moiety stabilizes otherwise elusive selenenic acid intermediate (6), which is observable by Se-77 NMR (delta 1173 ppm). It is suggested by this direct observation of the selenenic acid intermediate that intramolecular Se-N interaction prevents its deterioration by further oxidation in the catalytic system. (3) Since the formation of an Se-N hypervalent bonding is demonstrated by low-temperature dynamic H-1 NMR experiment for the selenenyl sulfide intermediate (4), it is expected that the nucleophilic attack of benzenethiol (PhSH) should occur preferentially at the sulfur atom of 4, allowing effective production of the selenolate intermediate (5') in the catalytic cycle.