Phospholipidomics Profiling of Salmon Muscle by MALDI-TOF MS

Phospholipids are considered as nutrients with putative health benefits, which play critical roles in lipid digestion, transport, inflammatory processes and signaling pathways. As to the nutritive value for human body, salmon contains abundant components of polyunsaturated fatty acyl phospholipids, and it has the beneficial function of lowering the blood fat and cholesterol, reducing the risk of cardiovascular disease. However, lipidomics study of salmon is still missing due to the lack of efficient specific analytical method. In this study, a fast lipidomics method of matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF MS) was developed for the analysis of phospholipids from salmon muscle tissue. In brief, 0.1 g salmon sample was accurately weighted, placed in 5 mL polytetrafluoroethylene tube, and mixed with 1.75 mL of chloroform/methanol (2∶1, V/V) solution. After ultrasonic assisted extracting for 10 min, a potion of 1.25 mL water was added and the tube was centrifuged at 8 000 g for 15 min in order to separate the solvent phase. Then, the lower organic phase was recovered and transferred to a new glass tube by pipette. The aqueous phase was re-extracted with 2 mL chloroform for another two times and treated as described before. The collected organic phase were combined and evaporated under nitrogen flow. The sample spots under investigation was directly analyzed using MALDI-TOF MS in positive ion reflection mode at an accelerating potential of 20 kV, with a delayed ion extraction time of 450 ns according to the mass range under observation (m/z 450-1000) allowing for baseline isotopic mass resolution. The unnaturally occurred DMPC (14∶0/14∶0) and DPPE (15∶0/15∶0) were selected as standards for studying their ions in positive ion mode. The method was validated in terms of precision and recovery, and it was found that the recoveries of DMPC and DPPE were in the range of 74%-83%, and relative standard derivation (RSD) was lower than 7%. The RSDs of intra-day and inter-day precision were both lower than 8.5%. After analyzing the lipid extract of salmon, a total of 28 phospholipid molecular species was identified, among which the peak at m/z 806.40 (PC38:6+H⁺ and PE38:4+K⁺) showed the highest abundance. There is a variety of polyunsaturated phospholipids in salmon, for examples, the peak at m/z 824.38 (PC38:8+Na⁺ and PE42:5+H⁺), m/z 832.41 (PC40:7+H⁺, PC38:4+Na⁺ and PE40:5+H⁺). Part of the phospholipids were fully unsaturated, for example, the PC42:11+H⁺ was composed with eicosapentaenoic acyl chain (EPA-) and docosahexaenoic acyl chain (DHA-) at sn-1 and sn-2 position, respectively. The method is precise and robust, which can provide theoretical support for the nutritional study of salmon in view of lipidomics.