Stable Isotope-Assisted Liquid Chromatography Mass Spectrometry Reveals Uptake, Metabolism, and Resolvin Biosynthesis from Docosapentaenoic Acid in Atlantic Salmon ( Salmo Salar L.) Hepatocytes

Docosapentaenoic acid (DPA, 22:5n-3) is an intermediate omega-3 polyunsaturated fatty acid with emerging biological significance, yet its metabolic fate in fish remains poorly defined. Stable isotope-assisted liquid chromatography-mass spectrometry (LC-MS) was applied to trace the uptake, intracellular retention, and bioconversion of deuterium-labeled DPA (DPA-d5) in primary hepatocytes from Atlantic salmon (Salmo salar L.). DPA-d5 was efficiently incorporated, and subsequent formation of both docosahexaenoic acid (DHA-d5) and eicosapentaenoic acid (EPA-d5) was observed, providing direct evidence for elongation, desaturation, and retro-conversion pathways. DPA-derived DHA was further incorporated into d-series resolvins (RvD1-d5, RvD2-d5, RvD3-d5, RvD5-d5), establishing a precursor–product relationship between DPA and bioactive lipid mediators under basal conditions. Targeted LC-MS/MS enabled unambiguous discrimination of labeled versus endogenous fatty acids. Results demonstrate the metabolic versatility of DPA in salmon hepatocytes and highlight the capacity of stable isotope-assisted LC-MS to resolve complex fatty acid pathways, including downstream mediator biosynthesis. This analytical workflow provides a powerful framework for investigating the contributions of dietary DPA, or other fatty acids, to n-3 long-chain polyunsaturated fatty acid pools and specialized pro-resolving mediators in fish.