Characterization of sulfur-centered radical intermediates formed during the oxidation of thiols and sulfite by peroxynitrite - ESR-SPIN trapping and oxygen uptake studies

Using a novel phosphorylated spin trap, 5-diethoxy-phosphoryl-5- methyl-1-pyrroline N-oxide (DEPMPO), an analog of the commonly used trap 5,5'-dimethyl-1-pyrroline N-oxide (DMPO), we have investigated the reactions of sulfur-centered radicals produced from the oxidation of thiols and sulfite by peroxynitrite. The predominant species trapped in all cases are the corresponding sulfur-centered radicals, i.e. glutathionyl radical (GS(.)) from glutathione (GSH), N-acetyl-DL-penicillamine thiyl radical (S-.-NAP) from N-acetyl-DL-penicillamine (NAP) and sulfite anion radical (SO3radical anion) from sulfite. These radicals consume molecular oxygen forming either peroxyl or superoxide anion radicals. GS(.), S-.-NAP, and SO3radical anion-derived radicals react with ammonium formate to form the carbon dioxide anion radical (CO2radical anion). Further support of spin adduct assignments and radical reactions are obtained from photolysis of S-nitrosoglutathione and S-nitroso-Nacetyl-DL-penicillamine. We conclude that the direct reaction of peroxynitrite with thiols and sulfite forms thiyl and sulfite anion radicals, respectively, by a hydroxyl radical-independent mechanism. Pathological implications of thiyl radical formation and subsequent oxyradical-mediated chain reactions are discussed. Oxygen activation by thiyl radicals formed during peroxynitrite-mediated oxidation of glutathione may limit the effectiveness of GSH against peroxynitrite-mediated toxicity in cellular systems.