title: Oxoammonium cation intermediate in the nitroxide-catalyzed dismutation of superoxide.
abstract: Dismutation of superoxide has been shown previously to be catalyzed by stable nitroxide compounds. In the present study, the mechanism of superoxide (.O2-) dismutation by various five-membered ring and six-membered ring nitroxides was studied by electron paramagnetic resonance spectrometry, UV-visible spectrophotometry, cyclic voltammetry, and bulk electrolysis. Electron paramagnetic resonance signals from the carbocyclic nitroxide derivatives (piperidinyl, pyrrolidinyl, and pyrrolinyl) were unchanged when exposed to enzymatically generated .O2-, whereas, in the presence of .O2- and reducing agents such as NADH and NADPH, the nitroxides underwent reduction to their respective hydroxylamines. The reaction of 4-hydroxy-2,2,6,6-tetramethyl-1-hydroxypiperidine (Tempol-H) with .O2- was measured and, in agreement with earlier reports on related compounds, the rate was found to be too slow to be consistent with a mechanism of .O2- dismutation involving the hydroxylamine as an intermediate. Voltammetric analyses of the carbocyclic nitroxide derivatives revealed a reversible one-electron redox couple at positive potentials. In contrast, oxazolidine derivatives were irreversibly oxidized. At negative potentials, all of the nitroxides studied exhibited a broad, irreversible reductive wave. The rate of .O2- dismutation correlated with the reversible midpoint redox potential. Bulk electrolysis at positive potentials was found to generate a metastable oxidized form of the nitroxide. The results indicate that the dismutation of .O2- is catalyzed by the oxoammonium/nitroxide redox couple for carbocyclic nitroxide derivatives. In addition to the one-electron mitochondrial reduction pathway, the present results suggest the possibility that cellular bioreduction by a two-electron pathway may occur subsequent to oxidation of stable nitroxides. Furthermore, the cellular destruction of persistent spin adduct nitroxides might also be facilitated by a primary univalent oxidation.
authors.affiliation: Radiation Oncology Branch, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892.
authors.forename: M C
authors.initials: MC
authors.lastname: Krishna
authors.forename: D A
authors.initials: DA
authors.lastname: Grahame
authors.forename: A
authors.initials: A
authors.lastname: Samuni
authors.forename: J B
authors.initials: JB
authors.lastname: Mitchell
authors.forename: A
authors.initials: A
authors.lastname: Russo
mesh_terms: D003497:Cyclic N-Oxides; D004563:Electrochemistry / Q000379:methods; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D007700:Kinetics; D010084:Oxidation-Reduction; D000644:Quaternary Ammonium Compounds; D013053:Spectrophotometry / Q000379:methods; D013113:Spin Labels; D013481:Superoxides / Q000737:chemistry* / Q000378:metabolism; D014216:Triacetoneamine-N-Oxyl; D014969:Xanthine Oxidase / Q000378:metabolism
chemical_list: D003497:Cyclic N-Oxides; D000644:Quaternary Ammonium Compounds; D013113:Spin Labels; D013481:Superoxides; D014216:Triacetoneamine-N-Oxyl; D014969:Xanthine Oxidase; C003959:TEMPO