Reactions of dinitrogen pentoxide and nitrogen dioxide with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine

Endogenous generation of nitro-fatty acid hybrids having dual nitrate ester (RONO2) and nitroalkene (RNO2) substituents

Organic nitrate esters, long-recognized therapies for cardiovascular disorders, have not been detected biologically. We characterize in rat stomach unsaturated fatty acid nitration reactions that proceed by generation of nitro-nitrate intermediates (NO2-ONO2-FA) via oxygen and nitrite dependent reactions. NO2-ONO2-lipids represent ∼70% of all nitrated lipids in the stomach and they decay in vitro at neutral or basic pH by the loss of the nitrate ester group (-ONO2) from the carbon backbone upon deprotonation of the α-carbon (pKa ∼7), yielding nitrate, nitrite, nitrosative species, and an electrophilic fatty acid nitroalkene product (NO2-FA). Of note, NO2-FA are anti-inflammatory and tissue-protective signaling mediators, which are undergoing Phase II trials for the treatment of kidney and pulmonary diseases. The decay of NO2-ONO2-FA occurs during intestinal transit and absorption, leading to the formation of NO2-FA that were subsequently detected in circulating plasma triglycerides. These observations provide new insight into unsaturated fatty acid nitration mechanisms, identify nitro-nitrate ester-containing lipids as intermediates in the formation of both secondary nitrogen oxides and electrophilic fatty acid nitroalkenes, and expand the scope of endogenous products stemming from metabolic reactions of nitrogen oxides.

The impact of lipid oxidation on the functioning of a lung surfactant model

Apart from being responsible for sufficient pulmonary compliance and preventing alveolar collapse, lung surfactant (LS) also forms the first barrier for uptake of inhaled pathogens. As such it is susceptible to damage caused by various deleterious compounds present in air, e.g. oxidants capable of oxidizing unsaturated LS lipids. This study examines the consequences of oxidizing 20% of unsaturated lipids in an LS model: a mixed 1 : 1 DPPC : POPC monolayer. POxnoPC (1-palmitoyl-2-(9-oxo-nonanoyl)-sn-glycero-3-phosphocholine) is considered as the main oxidation product. Experimental surface pressure-area isotherms and polarization-modulation infrared reflection-absorption spectroscopy are employed to probe changes in the macroscopic properties of the unsaturated lipid monolayer induced by oxidation. Microscopic details of the influence of oxidation on the monolayer's phase behavior are elucidated by molecular dynamics simulations at varying lipid packing. We demonstrate that unsaturated lipid oxidation shifts the isotherm towards larger areas and advances monolayer collapse. This is caused by a reversal of the oxidized sn-2 chains of POxnoPC towards the subphase, driven by electrostatic interactions between the aldehyde, glycerin, and water. Increased lipid bulkiness, hindered transition to the LC phase, and transfer of oxidized chain terminals to the subphase have been identified as the most troublesome consequences of this process. They result in the reduction of monolayer stability and its capability to withstand high surface pressures. This may lead to uncontrolled and irreversible loss of lipids from the surface.

The mechanism of oleic acid nitration by *NO(2)

Fatty acid nitration is a recently discovered process that generates biologically active nitro lipids; however, its mechanism has not been fully characterized. For example, some structural details such as vinyl and allyl isomers of the nitro fatty acids have not been established. To characterize lipids that originated from a biomimetic reaction of *NO(2) with oleic acid, we synthesized several isomers of nitro oleic acids and studied their chromatography and mass spectra by various techniques of mass spectrometry. LC/MS analysis performed on a high resolution micro column detected molecular carboxylic anions of various oleic acid nitro isomers (NO(2)OA). Esterification of NO(2)OA with pentafluorobenzyl bromide and diisopropylethylamine as a catalyst produced a unique isoxazole ester derivative exclusively from allyl NO(2)OA isomers via dehydration of the nitro group at ambient temperatures. This new analytical procedure revealed that *NO(2) generated two vinyl and two allyl isomers of NO(2)OA. The vinyl isomers showed high regioselectivity with the 1.8:1 preference for the 10-NO(2)OA isomer that was absent among allylic isomers. The nitration also generated elaidic acid via cis-trans isomerization and diatereoisomers of vicinal nitro hydroxy, nitro keto and alpha-nitro epoxy stearic acids with high stereo and regioselectivity. Nitration of small unilamelar phospholipid vesicles resulted in several phospholipids containing nitro lipids and elaidic acid amenable to hydrolysis by phospholipase A(2).

Trans-arachidonic acids: new mediators of nitro-oxidative stress

A reaction of arachidonic acid with the nitrogen dioxide radical (*NO2) or its precursors (peroxynitrite, nitrous acid, nitrogen trioxide) generates a group of nitro lipids named nitroeicosanoids. A distinct feature of this reaction is abundant formation of four trans isomers of arachidonic acid (TAA) via reversible addition of the NO2 radical to the arachidonic acid cis double bonds. This cis-trans isomerization is biologically relevant because many pathologies that involve NO formation such as inflammation, hyperoxia, hypercapnia or exposure to cigarette smoke increase the TAA levels in cells, tissues and in the systemic circulation. Inflammatory conditions have been known to stimulate formation of a variety of oxidized lipids from unsaturated fatty acid precursors via lipid peroxidation mechanisms; however, nitration-dependent cis-trans-isomerization of arachidonic acid is a characteristic process for *NO2. TAA are likely to function as specific and selective biomarkers of the pathologic conditions that define nitro-oxidative stress. Diet independent biosynthesis of trans fatty acids as a result of disease is our new observation. In the past, experimental feeding and clinical studies have supported the concerns that dietary trans fatty acids are cardiovascular risk factors, however, clinical consequences of the endogenous formation of trans fatty acids are not known but potentially important given available studies on TAA. This review aims to summarize the emerging role of TAA as a unique group of biomarkers that target microcirculation and other systems. A biological mechanism that generates endogenous trans fatty acids poses new challenges for pharmacologic intervention and we suggest approaches that may limit TAA effects.

NO2 interfacial transfer is reduced by phospholipid monolayers

Nitrogen dioxide (NO2) is a ubiquitous, pollutant gas that produces a broad range of pathological and physiological effects on the lung. Absorption of inhaled NO2 is coupled to near-interfacial reactions between the solute gas and constituents of the airway and alveolar epithelial lining fluid. Although alveolar surfactant imparts limited resistance to respiratory gas exchange compared with that contributed by either the pulmonary membrane or uptake in red blood cells, resistance to NO2 flux could have a significant effect on NO2 absorption kinetics. To investigate the effect of interfacial surfactant on NO2 absorption, we designed an apparatus permitting exposure of variably compressed monolayers. Our results suggest that compressed monolayers enriched in 1,2-dipalmitoyl-sn-3-glycero-phosphocholine present significant resistance to NO2 absorption even at surface tensions greater than those achieved in vivo. However, monolayers composed of pure unsaturated phospholipids failed to alter NO2 absorption significantly when compressed, in spite of similar reductions in surface tension. The results demonstrate that phospholipid monolayers appreciably limit NO2 absorption and further that monolayer-induced resistance to NO2 flux is related to physicochemical properties of the film itself rather than alterations within the aqueous and gas phases. On the basis of these findings, we propose that pulmonary surfactant may influence the intrapulmonary gas phase distribution of inhaled NO2.

NO2-induced generation of extracellular reactive oxygen is mediated by epithelial lining layer antioxidants

Nitrogen dioxide (NO2) is an environmental oxidant that causes acute lung injury. Absorption of this aqueous insoluble gas into the epithelial lining fluid (ELF) that covers air space surfaces is, in part, governed by reactions with ELF constituents. Consequently, NO2 absorption is coupled to its chemical elimination and the formation of ELF-derived products. To investigate mechanisms of acute epithelial injury, we developed a model encompassing the spatial arrangements of the lung surface wherein oxidation of cell membranes immobilized below a chemically defined aqueous compartment was assessed after NO2 exposures. Because aqueous-phase unsaturated fatty acids displayed minimal NO2 absorptive activity, these studies focused on glutathione (GSH) and ascorbic acid (AH2) as the primary NO2 absorption substrates. Results demonstrated that membrane oxidation required both gasphase NO2 and aqueous-phase GSH and/or AH2. Membrane oxidation was antioxidant concentration and exposure duration dependent. Furthermore, studies indicated that GSH- and AH2-mediated NO2 absorption lead to the production of the reactive oxygen species (ROS) O-2. and H2O2 but not to .OH and that Fe-O2 complexes likely served as the initiating oxidant. Similar results were also observed in combined systems (GSH + AH2) and in isolated rat ELF. These results suggest that the exposure-induced prooxidant activities of ELF antioxidants generate extracellular ROS that likely contribute to NO2-induced cellular injury.

NO2 reactive absorption substrates in rat pulmonary surface lining fluids

Inhaled 'NO2 is absorbed by a free radical-dependent reaction mechanism that localizes the initial oxidative events to the extracellular space of the pulmonary surface lining layer (SLL). Because 'NO2 per se is eliminated upon absorption, most likely the SLL-derived reaction products are critical to the genesis of 'NO2-induced lung injury. We utilized analysis of the rate of 'NO2 disappearance from the gas phase to determine the preferential absorption substrates within rat SLL. SLL was obtained via bronchoalveolar lavage and was used either as the cell-free composite or after constituent manipulation [(i) dialysis, treatment with (ii) N-ethylmaleimide, (iii) ascorbate oxidase, (iv) uricase, or (v) combined ii + iii]. Specific SLL constituents were studied in pure chemical systems. Exposures were conducted under conditions where 'NO2 is the limiting reagent and disappears with first-order kinetics ([NO2]0 < or = 10 ppm). Reduced glutathione and ascorbate were the principle rat SLL absorption substrates. Nonsulfhydryl amino acids and dipalmitoyl phosphatidylcholine exhibited negligible absorption activity. Whereas uric acid and vitamins A and E displayed rapid absorption kinetics, their low SLL concentrations preclude appreciable direct interaction. Unsaturated fatty acids may account for < or = 20% of absorption. The results suggest that water soluble, low molecular weight antioxidants are the preferential substrates driving 'NO2 absorption. Consequently, their free radicals, produced as a consequence of 'NO2 exposure, may participate in initiating the 'NO2-induced cascade, which results in epithelial injury.

Interaction of nitrogen dioxide with human plasma. Antioxidant depletion and oxidative damage

Nitrogen dioxide (NO2.) is often present in inhaled air and may be generated in vivo from nitric oxide. Exposure of human blood plasma to NO2. caused rapid losses of ascorbic acid, uric acid and protein thiol groups, as well as lipid peroxidation and depletions of alpha-tocopherol, bilirubin and ubiquinol-10. No increase in protein carbonyls was detected. Supplementation of plasma with ascorbate decreased the rates of lipid peroxidation, alpha-tocopherol depletion and loss of uric acid. Uric acid supplementation decreased rates of lipid peroxidation but not the loss of alpha-tocopherol. We conclude that ascorbic acid, protein -SH groups, uric acid and alpha-tocopherol may be important agents protecting against NO2. in vivo. If these antioxidants are depleted, peroxidation of lipids occurs and might contribute to the toxicity of NO2..