IN-VIVO COMPARTMENTAL METABOLISM OF C-13 DOCOSAHEXAENOIC ACID, STUD LED BY GAS-CHROMATOGRAPHY COMBUSTION ISOTOPE RATIO MASS-SPECTROMETRY

The exchange of docosahexaenoic acid (22:6n-3) within lipid pools in r at and human has been followed as a function of time after the ingesti on of triglycerides (TG) containing 22:6n-3 labeled with C-13 (C-13 22 :6n-3). The C-13 abundance in the fatty acid was measured by gas-chrom atography-combustion isotope ratio mass spectrometry which allowed the detection of 0.001 atom C-13 percent C-12. Th, C-13 22:6n-3 appearanc e was rapid in the TC of very low density lipoprotein plus chylomicron fraction, in which the maximal labeling was observed at 3 and 2 h aft er ingestion in rat and human, respectively. Concomitant with the TC u tilization of this fraction by lipoprotein lipase from tissues, uneste rified C-13 22:6n-3 appeared in the plasma albumin. C-13 22:6n-3 bound to albumin was mostly present in unesterified form before 12 h post-i ngestion while after that period, lysophosphatidylcholine (lysoPC) bou nd to albumin carried higher C-13 22:6n-3 concentrations. These lyse-P C were mostly from hepatic origin and might represent a potential sour ce of 22:6n-3 redistribution to tissues. The C-13 22:6n-3 uptake into rat brain PC and phosphatidylethanolamine was still increasing when th e concentration of plasma unesterified C-13 22:6n-3 had already droppe d to a minimal plateau value and during the period of maximal plasma c irculation of C-13 22:6n-3-lysoPC bound to albumin. In contrast, the u ptake of C-13 22:6n-3 into blood platelet PC occurred during the phase of important circulation of C-13-22:6n-3 bound to albumin, suggesting the in vivo efficiency of the Lands pathway for this fatty acid. It i s concluded that C-13 22:6n-3 esterified in TG is rapidly absorbed and redistributed within plasma lipoproteins and that its redistribution within the two lipid species bound to;albumin might influence its upta ke by platelets and rat brain.