4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl (Tempol) inhibits peroxynitrite-mediated phenol nitration

Peroxynitrite (PN), a very reactive oxidant formed by the combination of superoxide and nitric oxide, appears to play a role in producing tissue damage in a number of inflammatory conditions. Pharmacological scavenging and decomposition of PN within these areas has therapeutic value in several tissue injury models. Recently, we have been interested in nitroxide free radical-containing compounds as possible scavengers of PN decomposition products. Nitroxides can undergo redox reactions to the corresponding hydroxylamine anion or oxo-ammonium cation in biological systems as shown by its ability to react with superoxide, leading to the formation of hydrogen peroxide and molecular oxygen. We found that 4-hydroxy-2,2,6,6- tetramethylpiperidine-1-oxyl (Tempol) inhibits PN-mediated nitration of phenolic compounds in the presence of a large molar excess of PN, suggesting a catalytic-like mechanism. In these experiments, Tempol inhibited PN- mediated nitration over the pH range of 6.5-8.5. This inhibition was specific for nitration and had no effect on hydroxylation. After the inhibition of PN- mediated nitration, Tempol was recovered from the reaction mixtures unmodified. In addition, Tempol was effective in protecting PC-12 cells from death induced by SIN-1, a PN-generating compound. The exact mechanism of Tempol's interaction with PN is not clear; however, we propose that an intermediate in this reaction may be a nitrogen dioxide radical - Tempol complex. This complex could react with water to form either nitrite or nitrate, or with a phenol radical to produce nitrophenol or nitrosophenol products and regenerate the nitroxide.