Enzymatic Reduction of 3-Nitrotyrosine Generates Superoxide

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Abstract
Spin-trapping with 5,5-dimethyl-1-pyrroline 1-oxide (DMPO) was used to demonstrate that 3-nitrotyrosine (nitrotyrosine) promotes the formation of substantial amounts of reactive oxygen species (O2•- and •OH), when incubated with NAD(H)−cytochrome c reductase and a corresponding electron donor. Spin adduct formation is strongly inhibited by the presence of superoxide dismutase (SOD); spin adduct formation requires aerobic conditions. Nitration of leucine enkephalin, a tyrosine-containing pentapeptide, results in a similar generation of O2•- and •OH species. Both nitrotyrosine and nitrated leucine enkephalin stimulate acetylated ferricytochrome c reduction in the presence of NAD(H)−cytochrome c reductase with typical Michaelis−Menten kinetics and Km's of 104 ± 14 and 0.78 ± 0.11 μM, respectively. No stimulation of acetylated ferricytochrome c reduction is observed in the presence of SOD. Catalase and the metal chelators DTPA and deferoxamine mesylate do not influence observed stimulation of acetylated ferricytochrome c reduction by nitrotyrosine. Nitration of two tyrosines (of four) within the sequence of the 6.5-kDa globular protein bovine pancreas trypsin inhibitor (BPTI) fails to stimulate O2•- generation implying steric restrictions for BPTI−reductase interactions. However, nitrated BPTI subjected to trypsin digestion stimulated reduction of acetylated ferricytochrome c. These results suggest that, as with other nitroaromatic compounds, nitrotyrosine may be enzymatically reduced to the corresponding nitro anion radical (ArNO2•-) which is then oxidized by molecular oxygen to yield O2•- and regenerate ArNO2. Thus, once formed in vivo, nitrotyrosine may act to promote oxidative stress by means of repetitive redox cycling.