Effect of ·NO on the decomposition of peroxynitrite: Reaction of N₂O₃ with ONOO^-

Nitric oxide reacts slowly with ONOO^- (k < 1.3 x 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 < 12 through the reaction of N₂O₃ 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) x 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.