One-electron oxidation pathway of peroxynitrite decomposition in human blood plasma: Evidence for the formation of protein tryptophan-centred radicals

Exposure of human blood plasma to peroxynitrite in the presence of 3,5-dibromo-4-nitrosobenzenesulphonic acid (DBNBS) resulted,in the trapping of a strongly immobilized nitroxide radical adduct. The adduct was due to protein-centred radicals derived not only from serum albumin but also from other major plasma proteins (fibrinogen, IgG, alpha(1)-antitrypsin and transferrin). Urate significantly protected plasma from the peroxynitrite-induced DBNBS-plasma protein adduct, whereas ascorbate and glutathione were protective at concentrations exceeding those usually found in plasma. Alkylation of plasma -SH groups did not affect the intensity of DBNBS-plasma protein adduct, whereas bicarbonate increased its formation; thus showing a pro-oxidant effect. The DBNBS-plasma protein adduct provided little structural information, but subsequent non-specific-protease treatment resulted in the detection of an isotropic three-line spectrum, indicating the trapping of radicals centred on a tertiary carbon. The nitrogen hyperfine coupling constant of this adduct and its superhyperfine structure were similar to those of DBNBS-tryptophan peptides with the alpha-amino group of tryptophan linked in the amide bond, consistent with a radical adduct formed at C-3 of the indole ring of tryptophan-containing peptides. DBNBS was unable to trap radicals derived from peroxynitrite-treated tyrosine or tyrosine-containing peptides. Methionine treated with peroxynitrite resulted in the trapping of at least two DBNBS-methionine adducts with hyperfine structures different from that of protease-treated DBNBS-plasma proteins. These results demonstrate that peroxynitrite induced in blood plasma the formation of protein radicals centred on tryptophan residues and underline the relevance of the one-electron oxidation pathway of peroxynitrite decomposition in biological fluids.