TY - JOUR
T1 - Oxoammonium cation intermediate in the nitroxide-catalyzed dismutation of superoxide
AU - Krishna, Murali C.
AU - Grahame, David A.
AU - Samuni, Amram
AU - Mitchell, James B.
AU - Russo, Angeld
PY - 1992
Y1 - 1992
N2 - 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.
AB - 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.
KW - Electron paramagnetic resonance
KW - Hydrogen peroxide
KW - Oxoammonium cation
KW - Superoxide dismutase
KW - Superoxide radical
UR - http://www.scopus.com/inward/record.url?scp=0026764147&partnerID=8YFLogxK
M3 - ???researchoutput.researchoutputtypes.contributiontojournal.article???
C2 - 1319064
AN - SCOPUS:0026764147
SN - 0027-8424
VL - 89
SP - 5537
EP - 5541
JO - Proceedings of the National Academy of Sciences of the United States of America
JF - Proceedings of the National Academy of Sciences of the United States of America
IS - 12
ER -