Rapid scavenging of peroxynitrous acid by monohydroascorbate

• Published in: Free radical biology & medicine Vol. 35; no. 12; pp. 1529 - 1537
• Main Authors: Kurz, Christophe R, Kissner, Reinhard, Nauser, Thomas, Perrin, Daniel, Koppenol, Willem H
• Format: Journal Article
• Language: English
• Published: United States Elsevier Inc 15.12.2003
• Subjects: Antioxidant defense; Ascorbic acid; Ascorbic Acid - chemistry; Ascorbyl radical; Electron Spin Resonance Spectroscopy; EPR; Free radicals; Homolysis; Hydrogen-Ion Concentration; Kinetics; Oxoperoxonitrate(1-); Peroxynitrite; Peroxynitrous Acid - chemistry; Reaction kinetics; Stopped-flow spectrophotometry; Peroxynitrite; Ascorbic acid; Oxoperoxonitrate(1-); Free radicals; EPR; Reaction kinetics; Stopped-flow spectrophotometry; Homolysis; Ascorbyl radical; Antioxidant defense
• ISSN: 0891-5849; 1873-4596
• DOI: 10.1016/j.freeradbiomed.2003.08.012

The reaction of peroxynitrous acid with monohydroascorbate, over the concentration range of 250 μM to 50 mM of monohydroascorbate at pH 5.8 and at 25°C, was reinvestigated and the rate constant of the reaction found to be much higher than reported earlier (Bartlett, D.; Church, D. F.; Bounds, P. L.; Koppenol, W. H. The kinetics of oxidation of L-ascorbic acid by peroxynitrite. Free Radic. Biol. Med. 18:85–92; 1995; Squadrito, G. L.; Jin, X.; Pryor, W. A. Stopped-flow kinetics of the reaction of ascorbic acid with peroxynitrite. Arch. Biochem. Biophys. 322:53–59; 1995). The new rate constants at pH 5.8 are k 1 = 1 × 10 6 M −1 s −1 and k −1 = 500 s −1 for 25°C and k 1 = 1.5 × 10 6 M −1 s −1 and k −1 = 1 × 10 3 s −1 for 37°C. These values indicate that even at low monohydroascorbate concentrations most of peroxynitrous acid forms an adduct with this antioxidant. The mechanism of the reaction involves formation of an intermediate, which decays to a second intermediate with an absorption maximum at 345 nm. At low monohydroascorbate concentrations, the second intermediate decays to nitrate and monohydroascorbate, while at monohydroascorbate concentrations greater than 4 mM, this second intermediate reacts with a second monohydroascorbate to form nitrite, dehydroascorbate, and monohydroascorbate. EPR experiments indicate that the yield of the ascorbyl radical is 0.24% relative to the initial peroxynitrous acid concentration, and that this small amount of ascorbyl radicals is formed concomitantly with the decrease of the absorption at 345 nm. Thus, the ascorbyl radical is not a primary reaction product. Under the conditions of these experiments, no homolysis of peroxynitrous acid to nitrogen dioxide and hydroxyl radical was observed. Aside from monohydroascorbate's ability to “repair” oxidatively modified biomolecules, it may play a role as scavenger of peroxynitrous acid.