Peroxynitrite-Mediated Oxidation of d, l-Selenomethionine : Kinetics, Mechanism and the Role of Carbon Dioxide

Abstract

Peroxynitrite oxidizes d, l-selenomethionine (MetSe) by two competing mechanisms, a one-electron oxidation that leads to ethylene and a two-electron oxidation that gives methionine selenoxide (MetSeO). Kinetic modeling of the experimental data suggests that both peroxynitrous acid and the peroxynitrite anion react with MetSe to form MetSeO with rate constants of 20,460 ± 440 M −1s −1 and 200 ± 170 M −1s −1, respectively at 25°C. The enthalpy (ΔH‡) and entropy (ΔS‡) of activation for the reaction of peroxynitrous acid with MetSe at pH 4.6 are 2.55 ± 0.08 kcal mol −1 and −30.5 ± 0.3 cal mol −1 K −1, respectively. With increasing concentrations of MetSe at pH 7.4, the yield of ethylene decreases and that of MetSeO increases, suggesting, as with methionine, the reactions leading to ethylene and MetSeO have different kinetic orders. We propose that the activated form of peroxynitrous acid, HOONO∗, is the one-electron oxidant and ground-state peroxynitrite is the two-electron oxidant in the reaction of peroxynitrite with MetSe. The peroxynitrite anion rapidly adds to CO 2 to form an adduct, ONOOCO 2 − ( 1), capable of generating potent reactive species, and we therefore examined the role of CO 2 in the peroxynitrite/MetSe system. In presence of added bicarbonate, the yield of ethylene obtained from the reaction of 0.4 mM peroxynitrite with 1.0 mM MetSe increases slightly with an increase in the concentration of bicarbonate from 0 to 5.0 mM and remains constant with a further increase of bicarbonate up to 20 mM. The yield of MetSeO, from the reaction of 10 mM peroxynitrite with 10 mM MetSe, decreases by 35% with an increase in the concentration of bicarbonate from 0 to 25 mM. Kinetic simulations show that the decrease in the yield of MetSeO is due to reaction of the peroxynitrite anion with CO 2. These results suggest that CO 2 partially protects MetSe from peroxynitrite-mediated oxidation and that 1 or its derivatives do not mediate the oxidation of MetSe to MetSeO.