Evidence for superoxide radical production in peroxynitrite decomposition

Peroxynitrite decomposition was investigated by ESR spin trapping. The spin trap used was 5,5-dimethyl-1-pyrroline N-oxide (DMPO). A mixture of peroxynitrite and DMPO generated predominantly DMPO-O₂^- adduct. A combination of SOD and catalase suppressed the formation of DMPO-O₂^-. The DMPO-O₂^- signal reached its maximum at pH lower than 7 and decreased as pH increased. The DMPO-O₂^- signal also depended on peroxynitrite concentration with maximum signal intensity appearing at 4.2 mM. The results demonstrate that peroxynitrite decomposition generates O₂^-. Since reaction of H₂O₂ with NO₂^- generates peroxynitrite, the results point out a pathway for conversion of H₂O₂ to O₂^- via peroxynitrite as an intermediate.