PubMed · 8261293
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M Porter. Hyperbaric oxygen therapy.. https://pubmed.ncbi.nlm.nih.gov/8261293/
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Peroxynitrite reacts with CO2 to from an adduct containing a weak O--O bond that can undergo homolytic and/or heterolytic cleavage to give other reactive intermediates. Because the peroxynitrite/CO2 reaction is fast and physiological concentrations of CO2 are relatively high, peroxynitrite-mediated oxidations of biological species probably involve the peroxynitrite-CO2 adduct and its subsequent reactive intermediates. We have examined the reaction of glutathione with peroxynitrite in the presence and absence of added bicarbonate. In the presence of added bicarbonate, CO2 competes with glutathione for peroxynitrite, resulting in a markedly decreased consumption of glutathione compared with that observed in the absence of added bicarbonate. However, the consumption of glutathione still is much higher than predicted from the assumption that the glutathione-peroxynitrite reaction is the only reaction that can consume glutathione in this system. These results suggest that glutathione partially, but not completely, traps intermediate(s) derived from the peroxynitrite and CO2 reaction. Some rate constants for the trapping of the intermediates are estimated by simulating the reactions, and possible mechanisms for the reaction of peroxynitrite with glutathione in the presence of added bicarbonate are discussed.
N-Nitrosation in oxygenated nitric oxide (NO middle dot) solutions was previously shown to be significantly inhibited by phosphate and chloride presumably by anion scavenging of the nitrosating agent nitrous anhydride, N2O3 (Lewis, R. S., Tannenbaum, S. R., and Deen, W. M. (1995) J. Am. Chem. Soc. 117, 3933-3939). Here, bicarbonate is shown to exhibit this same inhibitory effect. Rate constants for reaction of morpholine, phosphate, and bicarbonate with N2O3 relative to N2O3 hydrolysis at pH 8.9 were determined to be (3.7 +/- 0.2) x 10(4) M-1, (4.0 +/- 0.9) x 10(2) M-1, and (9.3 +/- 1.5) x 10(2) M-1, respectively. The morpholine and phosphate rate constants at pH 8.9 are similar to those reported at pH 7.4 assuring that these results are relevant to physiological conditions. The rate constant for this previously unrecognized reaction of bicarbonate with N2O3 suggests the strong scavenging ability of bicarbonate; accordingly, bicarbonate may contribute to reducing deleterious effects of N2O3. This is biologically important due to substantial bicarbonate concentrations in vivo, approximately 30 mM. Bicarbonate was previously shown to alter peroxynitrite reactivity; however, carbon dioxide is the probable reactive species. Bicarbonate is therefore potentially important in determining the fate of two reactive species generated from nitric oxide, N2O3 and ONOO-, and may thus act as a regulator of NO middle dot-induced toxicity.
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.