Effect of Superoxide Dismutase Mimics on Radical Adduct Formation during the Reaction between Peroxynitrite and Thiols—An ESR-Spin Trapping Study

We have reexamined the formation and reactions of radicals formed from peroxynitrite (ONOO−)-mediated oxidation of glutathione (GSH),l-cysteine (Cys),N-acetyl-d,l-penicillamine (NAP), and sodium bisulfite (NaHSO3). Sulfur-centered and superoxide anion radicals were trapped using 5,5-dimethyl-1-pyrroline-N-oxide (DMPO) and the radical adducts were analyzed by electron spin resonance (ESR) spectroscopy. The following sulfur-centered radicals were detected: glutathionyl radical (GS•) from GSH,l-cysteinyl radical (•Scys) froml-cysteine,N-acetyl-d,l-penicillamine thiyl radical (•Snap) from NAP, and sulfite anion radical[formula]from NaHSO3. Additionally the formation of the hydroxyl radical adduct of DMPO (DMPO/•OH) was observed. DMPO/•OH formation was totally inhibited by low-molecular-weight superoxide dismutase (SOD) mimics. This suggests that DMPO/•OH was formed from the decay of the superoxide radical adduct of DMPO. In the presence of SOD mimics, the DMPO-sulfur-centered adducts were more persistent, suggesting that[formula]is partially responsible for the instability of DMPO-thiyl adducts. Sulfur-centered radicals formed during oxidation of thiols and sulfite by peroxynitrite react with ammonium formate to form the carbon dioxide anion radical[formula]We conclude that sulfur-centered radicals produced from the oxidation of thiols and sulfite by peroxynitrite arise from a hydroxyl-radical-independent mechanism. Biological implications of peroxynitrite-mediated oxidation of thiols as well as the use of SOD mimics in biological spin-trapping are discussed.