Peroxynitrite reaction with carbon dioxide/bicarbonate: kinetics and influence on peroxynitrite-mediated oxidations

Peroxynitrite is a strong oxidant produced in vivo as the reaction product of superoxide anion and nitric oxide (k approximately 5 x 10(9) M-1 s-1) and can be formed and mediate reactions in the extracellular environment. It has recently been reported that peroxynitrite and carbon dioxide react in a second-order process (S. V. Lymar and K. Hurst (1995) J. Am. Chem. Soc. 117, 8867-8868). Since one of the most abundant constituents of the extracellular milieu is bicarbonate anion (25 mM in plasma) which is in equilibrium with carbon dioxide (1.3 mM in plasma) we have further studied the kinetics of the reaction between peroxynitrite and carbon dioxide/ bicarbonate and the effect of bicarbonate on different peroxynitrite-mediated oxidations. The apparent second-order rate constant for the reaction is (2.3 +/- 0.1) x 10(3) M-1 s-1 at 37 degrees C and pH 7.4 and a pH-independent second-order rate constant of (5.8 +/- 0.2) x 10(4) M-1 s-1 at 37 degrees C was obtained considering peroxynitrite anion and carbon dioxide as the reacting species. The enthalpy and entropy of activation are delta H* = +10.7 +/- 0.8 kcal mol-1 and delta S* = -6.5 +/- 0.5 cal mol-1 K-1, respectively. The presence of bicarbonate had variable influence on peroxynitrite-mediated oxidations. While bicarbonate significantly enhanced peroxynitrite-mediated nitration of aromatics, it partially inhibited the oxidation of thiols, dimethylsulfoxide, oxyhemoglobin, and cytochrome c+2 and totally inhibited the hydroxylation of benzoate. Spontaneous chemiluminescence studies suggest the formation of bicarbonate radicals during the interactions of peroxynitrite with carbon dioxide/ bicarbonate. Our results support that peroxynitrite anion rapidly reacts with carbon dioxide to yield an adduct (ONOOCO2-) which can participate in oxidation and nitration processes, thus redirecting the primary reactivity of peroxynitrite.