Easy Oxidation and Nitration of Human Myoglobin by Nitrite and Hydrogen Peroxide

The modification of human myoglobin (HMb) by reaction with nitrite and hydrogen peroxide has been investigated. This reaction is important because NO2- and H2O2 are formed in vivo under conditions of oxidative and nitrative stress, where protein derivatization has been often observed. The abundance of HMb in tissues and in the heart makes it a potential source and target of reactive species generated in the body. The oxidant and nitrating species produced by HMb/H2O2/NO2- are nitrogen dioxide and peroxynitrite, which can react with exogenous substrates and endogenous protein residues. Tandem mass analysis of HMb modified by stoichiometric amounts of H2O2 and NO2- indicated the presence of two endogenous derivatizations: oxidation of C110 to sulfinic acid (76%) and nitration of Y103 to 3-nitrotyrosine (44%). When higher concentrations of NO2- and H2O2 were used, nitration of Y146 and of the heme were also observed. The two-dimensional gel-electrophoretic analysis of the modified HMbs showed spots more acidic than that of wild-type HMb, a result in agreement with the formation of sulfinic acid and nitrotyrosine residues. By contrast, the reaction showed no evidence for the formation of protein homodimers, as observed in the reaction of HMb with H2O2 alone. Both HMb and the modified HMb are active in the H2O2/NO2--dependent nitration of exogenous phenols. Their catalytic activity is quite similar and the endogenous modifications of HMb therefore have little effect on the reactivity of the protein intermediates.