Oxidation of guaiacol by myeloperoxidase: a two-electron-oxidized guaiacol transient species as a mediator of NADPH oxidation

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Abstract
The present study was first aimed at a complete steady-state kinetic analysis of the reaction between guaiacol (2-methoxyphenol) and the myeloperoxidase (MPO)/H2O2 system, including a description of the isolation and purification of MPO from human polymorphonuclear neutrophil cells. Secondly, the overall reaction of the oxidation of NADPH, mediated by the reactive intermediates formed from the oxidation of guaiacol in the MPO/H2O2 system, was analysed kinetically. The presence of guaiacol stimulates the oxidation of NADPH by the MPO/H2O2 system in a concentration-dependent manner. Concomitantly, the accumulation of biphenoquinone (BQ), the final steady-state product of guaiacol oxidation, is lowered, and even inhibited completely, at high concentrations of NADPH. Under these conditions, the stoichiometry of NADPH:H2O2 is 1, and the oxidation rate of NADPH approximates to that of the rate of guaiacol oxidation by MPO. The effects of the presence of superoxide dismutase, catalase and of anaerobic conditions on the overall oxidation of NADPH have also been examined, and the data indicated that superoxide formation did not occur. The final product of NADPH oxidation was shown to be enzymically active NADP+, while guaiacol was generated continuously from the reaction between NADPH and oxidized guaiacol product. In contrast, similar experiments performed on the indirect, tyrosine-mediated oxidation of NADPH by MPO showed that a propagation of the free radical chain was occurring, with generation of both O2-• and H2O2. BQ, in itself, was able to spontaneously oxidize NADPH, but neither the rate nor the stoichiometry of the reaction could account for the NADPH-oxidation process involved in the steady-state peroxidation cycle. These results provide evidence that the oxidation of NADPH does not involve a free nucleotide radical intermediate, but that this is probably due to a direct electron-transfer reaction between NADPH and a two-electron-oxidized guaiacol intermediate.

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