Influence of epithelial lining fluid lipids on NO2-induced membrane oxidation and nitration

Within the pulmonary epithelial lining layer (ELF), antioxidants such as ascorbic acid (AH2) and glutathione (GSH) react with inhaled nitrogen dioxide (•NO2) to produce reactive oxygen species (ROS) that induce cellular oxidation. Because the ELF contains unsaturated fatty acids (UFA), which potentially react with •NO2 and/or the antioxidant-derived ROS, we studied the influence of aqueous phase model UFA [egg phosphatidylcholine (EggPC) liposomes] on exposure-induced oxidation and nitration of membranes. Our lung surface model used gas phase •NO2 exposures of immobilized red cell membranes (RCM) overlaid with defined aqueous phases. Acetyl cholinesterase (AChE) activity, TBARS, and 3-nitrotyrosine (3-NT) were used to assess protein and lipid oxidation and RCM nitration, respectively. During •NO2 exposure, AH2 and GSH induced AChE loss and TBARS, which were unchanged with buffer only. Exposures of EggPC generated extensive TBARS but not AChE loss; addition of AH2/GSH to EggPC resulted in smaller AChE declines and fewer TBARS. 3-NT formation occurred with or without EggPC, low concentration antioxidants, SOD, catalase, or DTPA, but was inhibitable by desferrioxamine or high antioxidant concentrations. The data suggest that reaction/diffusion limitations govern •NO2 distribution, that •NO2 per se directly nitrates tyrosine residues within hydrophobic regions, and that the induction of secondary oxidative processes is dependent on nonlinear relationships among •NO2 flux rates, antioxidant concentrations, and diffusivity of secondary reactive species.