Detection of [U-13C]eicosapentaenoic acid in rat liver lipids using13C nuclear magnetic resonance spectroscopy

Fatty acid carbons are well-resolved in¹³C nuclear magnetic resonance (NMR) spectra of lipid extracts, but application of this methodology to the metabolism of¹³C-labelled fatty acids has not yet been reported. In the present study,¹³C NMR was used to monitor the presence of 98% U-¹³C]eicosapentaenoic acid (EPA) in liver and carcass lipids 24 h after it had been injected into the stomach of a rat. Natural abundance¹³C NMR spectra of liver total fatty acid extracts were obtained from four control rats for comparison. At 24 h post-injection, quantitative high resolution¹³C NMR showed¹³C enrichment in liver fatty acid extracts was present mainly at olefinic and at the n−1 to n−4 carbons, but¹³C signal intensities for C−1 to C−4 of U-¹³C]EPA were markedly reduced or absent. Small¹³C resonances, possibly indicative of some¹³C incorporation into docosahexaenoic acid and saturated or monounsaturated fatty acids, were present in spectra of liver fatty acids. Liver and carcass fatty acid composition was similar in both the controls and the EPA-injected rat, suggesting little accumulation of the injected U-¹³C]EPA after 24 h. We conclude that the carbon-specific data provided by¹³C NMR of lipid extracts may be useful in monitoring the fate of individual carbons during tracer studies using¹³C-labelled fatty acids.