Identification and quantitation of lipid C=C location isomers: A shotgun lipidomics approach enabled by photochemical reaction

The field of lipidomics has been significantly advanced by mass spectrometric analysis. The distinction and quantitation of the unsaturated lipid isomers, however, remain a long-standing challenge. In this study, we have developed an analytical tool for both identification and quantitation of lipid C=C location isomers from complex mixtures using online Paternò–Büchi reaction coupled with tandem mass spectrometry (MS/MS). The potential of this method has been demonstrated with an implementation into shotgun lipid analysis of animal tissues. Among 96 of the unsaturated fatty acids and glycerophospholipids identified from rat brain tissue, 50% of them were found as mixtures of C=C location isomers; for the first time, to our knowledge, the quantitative information of lipid C=C isomers from a broad range of classes was obtained. This method also enabled facile cross-tissue examinations, which revealed significant changes in C=C location isomer compositions of a series of fatty acids and glycerophospholipid (GP) species between the normal and cancerous tissues.

• This study employs Paternò-Büchi (PB) reaction coupled with tandem mass spectrometry (PB-MS/MS) approach for high-confidence distinction and quantitation of a wide variety of unsaturated lipid species, using shotgun lipidomic analysis.
• The generated C=C diagnostic fragment ions not only allows unambiguous determination of C=C locations within lipids but also quantitation with linearity over a wide dynamic range.
• Diseased tissue analysis highlighted altered levels of Δ11 isomers in 18:1 containing lipids in the cancerous mouse breast tissues compared to normal controls, suggesting that changes in lipid C=C isomer compositions may offer a new perspective to identifying malignancies and diseased state.