The Effect of Bicarbonate on Oxidation by Peroxynitrite: Implication for Its Biological Activity

The effect of bicarbonate on the oxidation rates and yields of the reaction of peroxynitrite with Ni^II(cyclam), ferrocyanide, and ABTS was studied using the stopped-flow technique. In the absence of bicarbonate, Ni^II(cyclam) is oxidized directly by peroxynitrous acid wher eas ferrocyanide and ABTS are oxidized by intermediates that are formed during the self-decomposition of peroxynitrous acid. In the presence of sufficient concentrations of bicarbonate, the rate-determining step of these oxidation processes is the reaction of ONOO^- with CO₂. The latter reaction results in the formation of an adduct ONOOCO₂^-, which is incapable of oxidizing directly these substrates. It is shown that about 33% of this adduct decomposes into highly oxidizing species, most probably into ^.NO₂ and CO₃^.- via the homolysis of the peroxo O-O bond. Under physiological conditions, where the most abundant species is ONOO^- and the concentration of CO₂ exceeds 1 mM, the half-life of peroxynitrite is reduced, and consequently its diffusion distance is shortened. Therefore, indirect oxidation by peroxynitrite through intermediates that are formed during its self-decomposition will not take place in vivo, whereas direct oxidation by peroxynitrite may take place provided that the rate of this reaction competes efficiently with that of peroxynitrite with CO₂. We conclude that the toxicity of peroxynitrite in vivo is most probably governed by ONOOCO₂^-, which has a reactivity different from that of peroxynitrite.