Metabolic fate of an oral tracer dose of [C-13]docosahexaenoic acid triglycerides in the rat

The appearance of C-13 in rat lipoprotein, blood cells, and brain Lipids was followed as a function of time after the ingestion of triglycerides (TG) containing [C-13]22:6n-3. The time course of C-13 abundance in 22:6n-3 of various lipid pools, measured by gas chromatography combustion-isotope mass spectrometry, established precursor-product relationships within lipids. The [C-13]22:6n-3 was rapidly incorporated into very low density lipoprotein-chylomicron-TG and unesterified fatty acids bound to albumin, with a concomitant maximal appearance at 3 h and further decline. Lysophosphatidylcholines (lysoPC) bound to albumin were also enriched in [C-13]22:6n-3, and their labeling appeared to be mainly due to hepatic secretion at the earliest time points. From 12 h postingestion, the synthesis of [C-13]22:6n-3-lysoPC was twice as high as that of unesterified [C-13] 22:6n-3, making lysoPC a potential source of 22:6n-3 supply for tissues. The labeling of platelets, red blood cells, and brain phospholipids presented different kinetics, presumably involving the two lipid forms of [C-13]22:6n-3 bound to albumin, to different extents. We conclude that [C-13]22:6n-3 esterified in TG is rapidly redistributed within blood lipoproteins and the albumin fraction and that its incorporation in lipid species bound to albumin influences its uptake by target tissues.