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Abstract
Increased intake of fish oil rich in the omega-3 fatty acids eicosapentaenoic acid (EPA, C20:5 omega-3) and docosahexaenoic acid (DHA, C22:6 omega-3) reduces the incidence of human disorders such as atherosclerotic cardiovascular disease. However, mechanisms that contribute to the beneficial effects of fish oil consumption are poorly understood. Mounting evidence suggests that oxidation products of EPA and DHA may be responsible, at least in part, for these benefits. Previously, we have defined the free radical-induced oxidation of arachidonic acid in vitro and in vivo and have proposed a unified mechanism for its peroxidation. We hypothesize that the oxidation of EPA can be rationally defined but would be predicted to be significantly more complex than arachidonate because of the fact that EPA contains an addition carbon-carbon double bond. Herein, we present, for the first time, a unified mechanism for the peroxidation of EPA. Novel oxidation products were identified employing state-of-the-art mass spectrometric techniques including Ag(+) coordination ionspray and atmospheric pressure chemical ionization mass spectrometry. Predicted compounds detected both in vitro and in vivo included monocylic peroxides, serial cyclic peroxides, bicyclic endoperoxides, and dioxolane-endoperoxides. Systematic study of the peroxidation of EPA provides the basis to examine the role of specific oxidation products as mediators of the biological effects of fish oil.
Highlights
ES000267, GM15431, ES31125, RR00096, DK48831, and CA77839
Free radical-induced oxidation of arachidonic acid has been well studied and a unified mechanism that leads to the selective reaction monitoring; SIM selective ion monitoring; PPh3, triphenylphosphine
Based on a unifying hypothesis to explain the oxidation of EPA in vitro and in vivo, we have identified predicted novel peroxidation products of this PUFA using various MS methods
Summary
Materials—Eicosapentaenoic acid methyl ester was purchased from Nu Chek Prep (Elysian, MN) and was of the highest purity (Ͼ99ϩ%). [2H4] 15-F2t-IsoP (8-iso-PGF2␣) was purchased from Cayman Chemical Co. (Ann Arbor, MI). The PFB esters, after TLC separation, were either directly analyzed by LC/APCI-MS or were further derivatized to TMS derivatives For the latter, after evaporation of the ethyl acetate, 20 l of BSTFA and 10 l of dimethylformamide were added to the residue, and the mixture was incubated at 37 °C for 20 min. Separation of HpEPA Me and HEPA Me by normal phase HPLC using a semi-preparative column with a flow rate of 10 ml/min and 0.7% IPA in hexane. The starting material 5-HpEPA Me has a precursor ion at m/z 455/457 and CID on m/z 455 gives rise to a predominant peak at m/z 325 (Fig. 3a) The formation of this fragment can be understood based on Hock fragmentation induced by Agϩ to generate an aldehyde in the gas phase [30].
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