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 (O2.) is assumed to decompose upon protonation in a first order process via intramolecular rearrangement to NO3 2. The present study was carried out to elucidate the origin of NO2 2 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 6 0.1. The reaction of peroxynitrite with NOwas considered as one possible source of NO2 2, but the calculated second order rate constant of 9.1 3 104 M21 s21 is probably too small to explain NO2 2 formation under physiological conditions. Moreover, pure peroxynitrite decomposed to NO2 2 with-out apparent release of NO. Determination of NO2 2 and NO3 2 in solutions of decomposed peroxynitrite showed that the relative amount of NO2 2 increased with increas-ing pH, with NO2 2 accounting for about 30 % of decompo-sition products at pH 7.5 and NO3 2 being the sole metab-olite at pH 3.0. Formation of NO2 2 was accompanied by release of stoichiometric amounts of O2 (0.495 mol/mol o