MECHANISM OF CARBON DIOXIDE-CATALYZED OXIDATION OF TYROSINE BY PEROXYNITRITE

Peroxynitrite ion (ONO2-) reacted rapidly with CO2 to form a short-liv ed intermediate provisionally identified as the ONO2CO2- adduct. This adduct was more reactive in tyrosine oxidation than ONO2- itself and p roduced 3-nitrotyrosine and 3,3'-dityrosine as the major oxidation pro ducts. With tyrosine in excess, the rate of 3-nitrotyrosine formation was independent of the tyrosine concentration and was determined by th e rate of formation of the ONO2CO2- adduct. The overall yield of oxida tion products was also independent of the concentration of tyrosine an d medium acidity; approximately 19% of the added ONO2- was converted t o products under all reaction conditions. However, the 3-nitrotyrosine /3,3'-dityrosine product ratio depended upon the pH, tyrosine concentr ation, and absolute reaction rate. These data are in quantitative agre ement with a reaction mechanism in which the one-electron oxidation of tyrosine by ONO2CO2- generates tyrosyl and NO2 radicals as intermedia ry species, but are inconsistent with mechanisms that invoke direct el ectrophilic attack on the tyrosine aromatic ring by the adduct. Based upon its reactivity characteristics, ONO2CO2- has a lifetime shorter t han 3 ms and a redox potential in excess of 1 V, and oxidizes tyrosine with a bimolecular rate constant greater than 2 x 10(5) M(-1) s(-1). In comparison, in CO2-free solutions, oxidation of tyrosine by peroxyn itrite was much slower and gave significantly lower yields (similar to 8%) of the same products. When tyrosine was the Limiting reactant, 3, 5-dinitrotyrosine was found among the reaction products of the CO2-cat alyzed reaction, but this compound was not detected in the uncatalyzed reaction.