Lipidomics of oxidized polyunsaturated fatty acids

Abstract

Lipid mediators are produced from the oxidation of polyunsaturated fatty acids through enzymatic and free radical-mediated reactions. When subject to oxygenation via cyclooxygenases, lipoxygenases, and cytochrome P450 monooxygenases, polyunsaturated fatty acids give rise to an array of metabolites including eicosanoids, docosanoids, and octadecanoids. These potent bioactive lipids are involved in many biochemical and signaling pathways, with inflammation being of particular importance. Moreover, because they are produced by more than one pathway and substrate, and are present in a variety of biological milieus, their analysis is not always possible with conventional assays. Liquid chromatography coupled to electrospray mass spectrometry offers a versatile and sensitive approach for the analysis of bioactive lipids, allowing specific and accurate quantitation of multiple species present in the same sample. Here we explain the principles of this approach to mediator lipidomics and present detailed protocols for the assay of enzymatically produced oxygenated metabolites of polyunsaturated fatty acids that can be tailored to answer biological questions or facilitate assessment of nutritional and pharmacological interventions.

Graphical abstract

Fig. 1

Schematic showing an abridged overview of the main pathways involved in the production of polyunsaturated fatty acid-derived oxygenated mediators. acCOX-2, acetylated COX-2.

Fig. 2

ESI–MS spectra for the AA-derived mediators PGE₂, 12-HETE, LXA₄, and 11(12)-EET, the EPA-derived 18-HEPE and RvE₁, and the DHA-derived 17-HDHA and RvD₁.

Fig. 3

Flow chart showing the main steps for the LC/ESI–MS/MS mediator lipidomic assay.

Fig. 4

Typical chromatograms showing the chiral separation of the AA-derived 15-epi-LXA₄ and LXA_4, EPA-derived 18(R)- and 18(S)-HEPE, the AA-derived 15(R)- and 15(S)-HETE, and DHA-derived 17(R)- and 17(S)-HDHA by LC/ESI–MS/MS (experimental conditions as described under Protocol; 15-epi-LXA₄, 2.5 ng on the column; all other mediators, 250 pg on the column).