Effect of •NO on the Decomposition of Peroxynitrite: Reaction of N2O3 with ONOO-

Abstract

Nitric oxide reacts slowly with ONOO^- (k < 1.3 × 10^-3 M^-1 s^-1), and therefore does not affect the stability of peroxynitrite at pH >12. A chain consumption of peroxynitrite by ^•NO takes place at pH 2O₃ with ONOO^-, where the former is formed through two initiation steps: (i) autoxidation of ^•NO, which is the main process in alkaline solutions, and (ii) spontaneous decomposition of peroxynitrite. The effect of ^•NO on the decomposition of peroxynitrite depends on the rate of the reaction of ONOO^- with N₂O₃ (propagation step) relative to that of the hydrolysis of N₂O₃ (termination step). Therefore, rapid consumption of peroxynitrite occurs upon increasing the peroxynitrite concentration, decreasing the phosphate concentration, and lowering the pH, as the hydrolysis of N₂O₃ is base-catalyzed. The rate constant of the reaction of ONOO^- with N₂O₃ has been determined in alkaline solutions to be (3.1 ± 0.3) × 10⁸ M^-1 s^-1. The decomposition of ONOOH in acidic solutions is hardly affected by ^•NO, as N₂O₃ does not react with ONOOH. Under physiological conditions, the reaction of ^•NO with peroxynitrite cannot give rise to a chain consumption of peroxynitrite as the reaction of N₂O₃ with the relatively low concentrations of peroxynitrite cannot compete with the hydrolysis of N₂O₃.