Nitration of γ-tocopherol and oxidation of α-tocopherol by copper-zinc superoxide dismutase/H 2 O 2 /NO 2 − : Role of nitrogen dioxide free radical

Copper-zinc superoxide dismutase (Cu,ZnSOD) is the antioxidant enzyme that catalyzes the dismutation of superoxide (O2 •− ) to O2 and H2 O2 . In addition, Cu,ZnSOD also exhibits peroxidase activity in the presence of H2 O2 , leading to self-inactivation and formation of a potent enzyme-bound oxidant. We report in this study that lipid peroxidation ofl -α-lecithin liposomes was enhanced greatly during the SOD/H2 O2 reaction in the presence of nitrite anion (NO2 − ) with or without the metal ion chelator, diethylenetriaminepentacetic acid. The presence of NO2 − also greatly enhanced α-tocopherol (α-TH) oxidation by SOD/H2 O2 in saturated 1,2-dilauroyl-sn-glycero-3-phosphatidylcholine liposomes. The major product identified by HPLC and UV-studies was α-tocopheryl quinone. When 1,2-diauroyl-sn-glycero-3-phosphatidylcholine liposomes containing γ-tocopherol (γ-TH) were incubated with SOD/H2 O2 /NO2 − , the major product identified was 5-NO2 -γ-TH. Nitrone spin traps significantly inhibited the formation of α-tocopheryl quinone and 5-NO2 -γ-TH. NO2 − inhibited H2 O2 -dependent inactivation of SOD. A proposed mechanism of this protection involves the oxidation of NO2 − by an SOD-bound oxidant to the nitrogen dioxide radical (• NO2 ). In this study, we have shown a new mechanism of nitration catalyzed by the peroxidase activity of SOD. We conclude that NO2 − is a suitable probe for investigating the peroxidase activity of familial Amyotrophic Lateral Sclerosis-linked SOD mutants.