Collision-induced dissociation of F2-isoprostane-containing phospholipids

Free radical-induced lipid peroxidation results in the production of metabolites of arachidonic acid isomeric with prostaglandin F2 alpha. The formation of these compounds, termed F2-isoprostanes, occurs independent of the enzyme cyclooxygenase. The discovery that F2-isoprostanes can exert potent biological activity has suggested that they may mediate, to some extent, the biological responses to oxidant injury. Collision-induced dissociation of the [M-CH3]- ions from oxidized phospholipids isolated by extraction and normal phase high performance liquid chromatography from livers of rats treated with CCl4 to induce lipid peroxidation revealed several molecular species of phospholipids that had the F2-isoprostane esterified to the glycerophosphocholine backbone. Collision-induced dissociation of the [M-CH2CHN(CH3)3]- ion revealed that the F2-isoprostanes were primarily esterified at the sn-2 position of the glycerophospholipid as expected. Furthermore, tandem mass spectrometry of the carboxylate anion from the F2-isoprostane (m/z 353) resulted in the unique loss of 44 u characteristic of a 1,2-cyclic diol moiety such as that found in the PGF2-ring. These observations indicate that intact phospholipids containing fatty acyl groups of the isoprostane structure can be readily detected with tandem mass spectrometry even when present as minor components in a biological extract. Although no specific isomer identification can be made from the complex mixture, these techniques establish the existence of these novel metabolites of arachidonic acid esterified to glycerophospholipids.