Metabolic fate of peroxynitrite in aqueous solution. Reaction with nitric oxide and pH-dependent decomposition to nitrite and oxygen in a 2:1 stoichiometry.

Peroxynitrite, the reaction product of nitric oxide (NO) and superoxide (O-2) is assumed to decompose upon protonation in a first order process via intramolecular rearrangement to NO3-. The present study was carried out to elucidate the origin of NO2- found in decomposed peroxynitrite solutions. As revealed by stopped-flow spectroscopy, the decay of peroxynitrite followed first-order kinetics and exhibited a pKa of 6.8 +/- 0.1. The reaction of peroxynitrite with NO was considered as one possible source of NO2-, but the calculated second order rate constant of 9.1 x 10(4) M-1 s-1 is probably too small to explain NO2- formation under physiological conditions. Moreover, pure peroxynitrite decomposed to NO2- without apparent release of NO. Determination of NO2- and NO3- in solutions of decomposed peroxynitrite showed that the relative amount of NO2- increased with increasing pH, with NO2- accounting for about 30% of decomposition products at pH 7.5 and NO3- being the sole metabolite at pH 3.0. Formation of NO2- was accompanied by release of stoichiometric amounts of O2 (0.495 mol/mol of NO2-). The two reactions yielding NO2- and NO3- showed distinct temperature dependences from which a difference in Eact of 26.2 +/- 0.9 kJ mol-1 was calculated. The present results demonstrate that peroxynitrite decomposes with significant rates to NO2- plus O2 at physiological pH. Through formation of biologically active intermediates, this novel pathway of peroxynitrite decomposition may contribute to the physiology and/or cytotoxicity of NO and superoxide.