MetMyoglobin-Catalyzed Exogenous and Endogenous Tyrosine Nitration by Nitrite and Hydrogen Peroxide

Metmyoglobin catalyzes the nitration of various phenolic compounds in the presence of nitrite and hydrogen peroxide. The reaction rate depends on the reactant concentrations showing saturation behavior. Two competing paths are responsible for the reaction. In the first one, myoglobin reacts according to a peroxidase-like cycle forming two active intermediates, which can induce one-electron oxidation of the substrates. The MbFeIV=O intermediate oxidizes nitrite to nitrogen dioxide which, after reaction with the phenol or with a phenoxy radical, yields the nitrophenol. In the second mechanism, hydrogen peroxide reacts with the iron-bound nitrite to produce an active nitrating species, which we assume to be a protein bound peroxynitrite species, MbFeIII-N(O)OO. The high nitrating power of the active species is shown by the fact that the catalytic rate constant is essentially independent on the redox properties of the phenol. The occurrence of one or other of these mechanisms depends on the nitrite concentration: at low [NO2-] the nitrating agent is nitrogen dioxide, whereas at high [NO2-] the peroxynitrite path is dominant. The myoglobin derivative that accumulates during turnover depends on the mechanism. When the path involving NO2• is dominant, the spectrum of the MbFeIV=O intermediate is observed. At high nitrite concentration, the Soret band appears at 416 nm, which we attribute to an iron-peroxynitrite species. The metMb/NO2-/H2O2 system competitively nitrates the heme and the endogenous tyrosine at position 146 of the protein. Phenolic substrates protect Tyr146 from nitration by scavenging the active nitrating species. The exposed Tyr103 residue is not nitrated under the same conditions.