PE, or not PE, that is the question: The case of overlooked lyso-N-acylphosphatidylethanolamines

Rationale: Lyso derivatives of N-acyl-1,2-diacylglycero-3-phosphoethanolamines (L-NAPEs) are a lipid class mostly expressed in vegetables during stress and tissue damage that is involved in the synthesis of the lipid mediator N-acylethanolamines. L-NAPEs can be challenging to distinguish from isomeric phosphatidylethanolamines (PEs), especially in extracted complex samples where they could be confused with abundant PEs.

Methods: In this study, hydrophilic interaction liquid chromatography with electrospray ionization hyphenated with (tandem) mass spectrometry (MS) was proposed to distinguish L-NAPEs and PEs as deprotonated molecules, [M - H]^─ , using both high-resolution/accuracy Fourier transform MS and low-resolution linear ion trap (LIT) mass analyzers. MS³ experiments of [M - H - KE]^─ as precursor ions (KE, ketene loss) using the LIT instrument allowed us to distinguish between isomeric L-NAPE and PE species.

Results: Regiochemical rules were proposed working on enzymatically synthesized L-NAPEs. A few key differences in MS/MS spectra, including abnormal intensity of acyl chain losses as fatty acids, the presence of N-acylphosphoethanolamine ions, and diagnostic ions of the polar head, were disclosed. Additionally, MS³ spectra of [M - H - KE]^─ as precursor ions allowed us to confirm the identification of L-NAPE species. The proposed rules were applied to samples extracted from tomato by-products including stems and leaves.

Conclusions: Overall, our methodology is demonstrated as a robust approach to recognizing L-NAPEs in complex samples. L-NAPEs 18:2-N-18:2, 18:2-N-18:3, 18:3-N-18:2, and 18:2-N-18:1 were the prevailing compounds in the analyzed tomato samples, accounting for more than 90%. In summary, a reliable method for identifying L-NAPEs in complex samples is described. The proposed method could prevent overlooking L-NAPEs and overestimating isomeric PE species in future lipid analyses.