Simultaneous quantitative determination of deuterium- and carbon-13-labeled essential fatty acids in rat plasma

This study reports methods for the quantitative determination of stable isotope-labeled essential fatty acids ( EFAs) as well as an experiment in which deuterium-labeled linoleic acid (18:2n-6) and alpha-linolenic acid (18:3n-3) were compared with those labeled with carbon-13 in rat plasma in vivo. Standard curves were constructed to compensate for concentration and plasma matrix effects. It was observed that endogenous pools of fatty acids had a greater suppressing effect on the measurements of C-13-U-labeled EFAs relative to those labeled with H-2(5). Using these methods, the in vivo metabolism of orally administered deuterated-linolenate, C-13-U-labeled linolenate, deuterated-linoleate, and C-13-U-labeled linoleate was compared in adult rats (n = 11). There were no significant differences in the concentrations of the H-2 versus C-13 isotopomers of 18:2n-6, 18:3n-3, arachidonic acid (20:4n-6), and docosahexaenoic acid (22:6n-3) in rat plasma samples at 24 h after dosing. Thus, there appears to be little isotope effect for H-2(5) - versus C-13-U-labeled EFAs when the data are calculated using the conventional standard curves and corrected for endogenous fatty acid pool size and matrix effects.