Deepening of lipidome annotation by associating cross-metathesis reaction with mass spectrometry: application to an in vitro model of corneal toxicity

The in-depth knowledge of lipid biological functions needs a comprehensive structural annotation including a method to locate fatty acid unsaturations, which remains a thorny problem. For this purpose, we have associated Grubbs' cross-metathesis reaction and liquid chromatography hyphenated to tandem mass spectrometry to locate double bond positions in lipid species. The pretreatment of lipid-containing samples by Grubbs' catalyst and an appropriate alkene generates substituted lipids through cross-metathesis reaction under mild, chemoselective, and reproducible conditions. A systematic LC-MS/MS analysis of the reaction mixture allows locating unambiguously the double bonds in fatty acid side chains of phospholipids, glycerolipids, and sphingolipids. This method has been successfully applied at a nanomole scale to commercial standard mixtures consisting of 10 lipid subclasses as well as in lipid extracts of human corneal epithelial (HCE) cell line allowing to pinpoint double bond of more than 90 species. This method has also been useful to investigate the lipid homeostasis alteration in an in vitro model of corneal toxicity, i.e., HCE cells incubated with benzalkonium chloride. The association of cross-metathesis and tandem mass spectrometry appears suitable to locate double bond positions in lipids involved in relevant biological processes.