Phospholipids play a central role in the biochemistry of all living cells. These molecules constitute the lipid bilayer defining the outer confines of a cell, but also serve as the structural entities which confine subcellular components. Mass spectrometry has emerged as a powerful tool useful for the qualitative and quantitative analysis of complex phospholipids, including glycerophospholipids and the sphingolipid, sphingomyelin. Collision induced decomposition of both positive and negative molecular ion species yield rich information as to the polar head group of the phospholipid and the fatty-acyl substituents esterified to the glycerophospholipid backbone. This review presents the current level of understanding of the mechanisms involved in the formation of various product ions following collisional activation of molecular ion species generated by electrospray ionization of the common glycerophospholipids, including phosphatidic acid, phosphatidylethanolamine, phosphatidylcholine, phosphatidylinositol, phosphatidylglycerol, phosphatidylserine, cardiolipin, and sphingomyelin. Recent advances in the application of matrix assisted laser desorption ionization is also considered. Several applications of mass spectrometry applied to phospholipid analysis are presented as they apply to physiology as well as pathophysiology.

Exposure of the lung to concentrations of ozone found in ambient air is known to cause toxicity to the epithelial cells of the lung. Because of the chemical reactivity of ozone, it likely reacts with target molecules in pulmonary surfactant, a lipid-rich material that lines the epithelial cells in the airways. Phospholipids containing unsaturated fatty acyl groups and cholesterol would be susceptible to attack by ozone, which may lead to the formation of cytotoxic products. Whereas free radicalderived oxidized cholesterol products have been frequently studied for their cytotoxic effects, ozonized cholesterol products have not been studied, although they could reasonably play a role in the toxicity of ozone. The reaction of ozone with cholesterol yielded a complex series of products including 3␤-hydroxy-5-oxo-5,6-secocholestan-6-al, 5-hydroperoxy-B-homo-6-oxa-cholestan-3␤,7a-diol, and 5␤,6␤-epoxycholesterol. Mass spectrometry and radioactive monitoring were used to identify the major cholesterol-derived product during the reaction of 2 ppm ozone in surfactant as 5␤,6␤-epoxycholesterol, which is only a minor product during ozonolysis of cholesterol in solution. A dose-dependent formation of 5␤,6␤-epoxycholesterol was also seen during direct exposure of intact cultured human bronchial epithelial cells (16-HBE) to ozone. Studies of the metabolism of this epoxide in lung epithelial cells yielded small amounts of the expected metabolite, cholestan-3␤,5␣,6␤-triol, and more abundant levels of an unexpected metabolite, cholestan-6-oxo-3␤,5␣-diol. Both 5␤,6␤-epoxycholesterol and cholestan-6-oxo-3␤,5␣-diol were shown to be cytotoxic to cultured 16-HBE cells. A possible mechanism for cytotoxicity is the ability of these oxysterols to inhibit isoprenoid-based cholesterol biosynthesis in these cells.Human exposure to 0.2 ppm levels of ozone in ambient air has been shown to cause numerous pulmonary effects such as increased airway inflammation and decreased pulmonary function (1, 2). Studies of ozone in animals using up to 3 ppm ozone have been shown to cause increased airway hyperresponsiveness and epithelial cell death. It has been hypothesized that the very high chemical reactivity of ozone limits the distribution of this gas in the pulmonary system, preventing direct exposure to the cellular components of the lung. In part ozone may react with the various components of the epithelial cell lining fluid in the lung, also known as pulmonary surfactant, which includes proteins, lipids, and single electron antioxidant agents such as ascorbic acid (3-5). Because of the very high reactivity of ozone with lipids containing double bonds, considerable emphasis has been placed on the reaction of ozone with lipid compounds in the lungs and the possibility that the adverse effects of ozone are mediated by lipid-ozonized products (6). Evidence in support of this theory has been accumulating with the identification of biologically active phospholipids (7) such as 1-hexadecanoyl-2-(9-oxo-nonanoyl)-glycerophosphocholine, found following oz...

Ozone toxicity in the lung is thought to be mediated by products derived from the reaction of ozone with components of the lung epithelial lining fluid. Cholesterol is an abundant component of this epithelial lining fluid, and it is susceptible to ozonolysis, yielding several stable products including 3␤-hydroxy-5-oxo-5,6-secocholestan-6-al and 5␤,6␤-epoxycholesterol. Both 5␤,6␤-epoxycholesterol and its metabolite, cholestan-6-oxo-3,5-diol, have been shown to cause cytotoxicity in vitro, suggesting that they may be potential mediators of ozone toxicity in vivo. An ozone-sensitive mouse strain, C57BL/6J, was exposed to varying concentrations of ozone (0.5-3.0 ppm), and subsequently the levels of these cholesterol ozonolysis products were quantitated by electrospray ionization mass spectrometry in bronchoalveolar lavage fluid, lavaged cells, and lung homogenate. An ozone dosedependent formation of these biologically active oxysterols was observed in vivo, supporting a role for these compounds in ozone toxicity. Since the 5␤,6␤-epoxycholesterol metabolite, cholestan-6-oxo-3,5-diol, was isobaric with other cholesterol ozonolysis products, 3␤-hydroxy-5-oxo-5,6-secocholestan-6-al and its aldol condensation product, 3␤-hydroxy-5␤-hydroxy-B-norcholestan-6␤-carboxaldehyde, detailed mass spectral analysis using electron impact ionization was utilized to differentiate these isobaric cholesterol ozonolysis products. The specific detection of cholestan-6-oxo-3,5-diol in lung homogenate after ozone exposure established formation of 5␤,6␤-epoxycholesterol within the lung after exposure to 0.5 ppm ozone.

Cholesterol is the most abundant neutral lipid in the epithelial lining fluid of the lower airways of the lung also known as pulmonary surfactant and a potential target for reaction with ambient ozone when inspired into the human lung. The isolated double bond of cholesterol has been shown to be susceptible to attack by ozone, but the major reaction product from cholesterol ozonolysis had been remarkably difficult to structurally characterize. Recently, NMR and X-ray crystallography have been used to suggest the formation of a hydroperoxy, hydroxy hemiacetal product, using various derivatives and models of cholesterol to stabilize this chemically reactive product. Electrospray ionization mass spectrometry was used to study the somewhat unstable ozonolysis product of cholesterol which was found to display unique ionization and fragmentation properties when collisionally activated. The electron-deficient carbon atoms of this highly oxygenated product permitted covalent attachment of an acetate anion during negative ion electrospray ionization, leading to the formation of abundant adduct ions at m/z 511. Surprisingly, positive ions were not readily formed. Collision induced dissociation of the adduct anion yielded a major ion at m/z 477, corresponding to the loss of hydrogen peroxide. The most abundant fragment ion following collisional activation was observed at m/z 93, resulting from a complex rearrangement subsequent to the attack of the hydroperoxide anion on the carbon center of the acetate adduct. Based on the interpretation of the tandem mass spectral data, the major cholesterol ozonization product was characterized as a hydroperoxy, hydroxy hemiacetal derivative, which was consistent with the NMR and X-ray crystallographic studies which were carried out on the more stable methyl ether derivative.