Ceramide lipids in alive and thermally stressed mussels: an investigation by hydrophilic interaction liquid chromatography-electrospray ionization Fourier transform mass spectrometry

Hydrophilic interaction liquid chromatography coupled to electrospray ionization-Fourier transform mass spectrometry was employed to study ceramide lipids occurring in mussels of sp. Mytilus galloprovincialis. Lipid extracts from alive mussels and mussels deliberately subjected to specific thermal treatments were analyzed. In particular, single and tandem MS measurements were performed on a hybrid quadrupole-Orbitrap mass spectrometer and then complemented by MSn analyses (n = 2, 3) achieved by a linear ion trap mass spectrometer. This approach enabled the characterization of 66 ceramide lipids, encompassing ceramide phosphoethanolamines (CPE), ceramide aminoethylphosphonates (CAEP) and N-monomethylated CAEP. The sphingoid and acyl chains of each ceramide lipid could be distinctly recognized in terms of numbers of carbon atoms and C=C bonds, and indications on the possible location of the latter on the sphingoid chain could be often inferred from fragmentation patterns. The occurrence of several species hydroxylated on the α carbon of the acyl chain was also discovered. On the other hand, the sphingoid chain of ceramide lipids was never found to be involved in oxidation processes, unless forced exposure of the mussel lipid extracts to atmospheric oxygen was performed. CPE(d19:3/16:0) and its hydroxylated form, CPE(d19:3/2-OH-16:0), were found to be the prevailing species among CPE, whereas CAEP(d18:2/16:0), CAEP(d19:3/16:0) and CAEP(d19:3/2-OH-16:0) were the most abundant CAEP. Finally, ceramide lipids showed a remarkably higher stability, compared with glycerophospholipids, in mussels subjected to different thermal treatments. This finding opens interesting perspectives on the role of ceramide-based lipids in the adaptation of aquatic organisms to thermal stresses.