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 superoxid e (0(2)(-.)) is assumed to decompose upon protonation in a first order process via intramolecular rearrangement to NO3-. The present study w as carried out to elucidate the origin of NO-2 found in decomposed per oxynitrite solutions. As revealed by stopped-flow spectroscopy, the de cay of peroxynitrite followed first-order kinetics and exhibited a pK( a) of 6.8 +/- 0.1. The reaction of peroxynitrite with NO was considere d 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 N O2- formation under physiological conditions. Moreover, pure peroxynit rite decomposed to NO-2 without apparent release of NO. Determination of NO-2 and NO3- in solutions of decomposed peroxynitrite showed that the relative amount of NO2- increased with increasing pH, with NO2- ac counting for about 30% of decomposition products at pH 7.5 and NO3- be ing the sole metabolite at pH 3.0. Formation of NO2- was accompanied b y release of stoichiometric amounts of O-2 (0.495 mol/mol of NO2-). Th e two reactions yielding NO2- and NO3- showed distinct temperature dep endences from which a difference in E(act) of 26.2 +/- 0.9 kJ mol(-1) was calculated. The present results demonstrate that peroxynitrite dec omposes with significant rates to NO2- plus O-2 at physiological pH. T hrough formation of biologically active intermediates, this novel path way of peroxynitrite decomposition may contribute to the physiology an d/or cytotoxicity of NO and superoxide.