Peroxynitrite -mediated oxidations: Nitration and nitrosation

Using a direct ultraviolet second-derivative spectroscopy method, three peroxynitrite preparative methods were investigated for the nitrite and nitrate present either as impurities or produced during peroxynitrite decomposition: (I) ozonation of azide, reaction of hydrogen peroxide with (II) isoamyl nitrite and (III) nitrous acid.;The oxidation of morpholine by peroxynitrite in the presence and absence of added carbonate gives N-nitromorpholine and N-nitrosomorpholine. Nitration and nitrosation of morpholine are catalyzed by low levels of CO2; however, excess CO2 dramatically reduces the yields of nitrosation but not nitration, and the combined yields of the products are about the same under conditions of high and low concentrations of CO2. These data indicate that both nitration and nitrosation by peroxynitrite are free radical processes. The morpholine radical, formed from the reactions of carbonate and/or hydroxyl radicals with morpholine, reacts with either · NO or ·NO2 and serves as a common precursor for both products.;The peroxynitrite-mediated oxidation of alpha-tocopherol and gamma-tocopherol dispersed in 1,2-dilauroyl-sn-glycero-3-phosphocholine liposomes in the presence and absence of added carbonate gives alpha-tocopherylquinone and 5-nitro-gamma-tocopherol, respectively. The formation of the products is consistent with the current understanding of the free radical nature of oxidations of peroxynitrite and its CO2-adducts; the overall reaction involves a one-electron oxidation of alpha-tocopherol and gamma-tocopherol by HO· or CO3·-, followed by the reaction with ·NO2. When alpha-tocopherol and gamma-tocopherol were present in the same liposome and exposed to peroxynitrite, there was preferential oxidation of alpha-tocopherol over gamma-tocopherol. An explanation for the protection of gamma-tocopherol by alpha-tocopherol could be that gamma- and alpha-tocopheryl radicals, formed from the respective reactions of alpha-tocopherol and gamma-tocopherol with HO· or CO3·-, disproportionate to give alpha-tocopherylquinone and regenerated gamma-tocopherol. This is consistent with the lack of protection of gamma-tocopherol by alpha-tocopherol when alpha-tocopherol and gamma-tocopherol dispersed in different liposomes but present in the same incubation mixer are subjected to oxidation by peroxynitrite.