Permeation of Phospholipid Membranes by Peroxynitrite

Quantitative kinetic models have been developed for the reaction between peroxynitrite and membrane lipids in vesicles and for transmembrane oxidation of reactants located within their inner aqueous cores. The models were used to analyze TBARS formation and oxidation of entrapped Fe(CN)6 4- ion in egg lecithin liposomes and several artificial vesicles. The analyses indicate that permeation of the bilayers by ONOOH and NO2 •, a radical formed by homolysis of the ONOOH bond, is unusually rapid but that permeation by ONOO- and CO3 •-, a radical formed when CO2 is present, is negligible. Bicarbonate protects the vesicles against both membrane and Fe(CN)6 4- oxidation by rapid competitive CO2-catalyzed isomerization of ONOOH to NO3 -; this effect is partially reversed by addition of nitrite ion, which reacts with CO3 •- to generate additional NO2 •. Under medium conditions mimicking the physiological milieu, a significant fraction of the oxidants escape to inflict damage upon the vesicular assemblies. Rate constants for several elementary reaction steps, including transmembrane diffusion rates for ONOOH and NO2 •, were estimated from the bicarbonate dependence of the oxidative reactions.