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Originally published In Press as doi:10.1194/jlr.M500127-JLR200 on June 1, 2005

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Journal of Lipid Research, Vol. 46, 1962-1973, September 2005
Copyright © 2005 by American Society for Biochemistry and Molecular Biology

In vivo conversion of 18- and 20-C essential fatty acids in rats using the multiple simultaneous stable isotope method

Yu Hong Lin and Norman Salem, Jr.1

Section of Nutritional Neuroscience, Laboratory of Membrane Biochemistry and Biophysics, National Institute on Alcohol Abuse and Alcoholism, National Institutes of Health, Bethesda, MD 20892-9410

Published, JLR Papers in Press, June 1, 2005. DOI 10.1194/jlr.M500127-JLR200

1 To whom correspondence should be addressed. e-mail: nsalem{at}niaaa.nih.gov

An important question for mammalian nutrition is the relative efficiency of C18 versus C20 essential fatty acids (EFAs) for supporting the tissue composition of n-3 and n-6 pathway end products. One specific question is whether C22 EFAs are made available to tissues more effectively by dietary {alpha}-linolenic acid (18:3n-3) and linoleic acid (18:2n-6) or by dietary eicosapentaenoic acid (20:5n-3) and dihomo-{gamma}-linolenic acid (20:3n-6). To address this question in a direct manner, four stable isotope compounds were given simultaneously in a novel paradigm. A single oral dose of a mixture of 2H5-18:3n-3, 13C-U-20:5n-3, 13C-U-18:2n-6, and 2H5-20:3n-6 was administered to rats given a defined diet. There was a preferential in vivo conversion of arachidonic acid (20:4n-6) to docosatetraenoic acid (22:4n-6) and of 22:4n-6 to n-6 docosapentaenoic acid (22:5n-6) when the substrates originated from the C18 precursors. However, when the end products docosahexaenoic acid (22:6n-3) or 22:5n-6 were expressed as the total amount in the plasma compartment divided by the dosage, this parameter was 11-fold greater for 20:5n-3 than for 18:3n-3 and 14-fold greater for 20:3n-6 than for 18:2n-6.

Thus, on a per dosage basis, the total amounts of n-3 and n-6 end products accreted in plasma were considerably greater for C20 EFA precursors relative to C18.

Abbreviations: 18:2n-6, linoleic acid; 18:3n-3, {alpha}-linolenic acid; 20:4n-6, arachidonic acid; 20:3n-6, dihomo-{gamma}-linolenic acid; 22:6n-3, docosahexaenoic acid; 22:5n-6, n-6 docosapentaenoic acid; 20:5n-3, eicosapentaenoic acid; AUC, area under the curve; CE, cholesteryl ester; Cmax, maximal concentration of labeled fatty acids in plasma; Dmax, maximal percentage of dose; EFA, essential fatty acid; Emax, maximal enrichment of labeled fatty acids at maximal concentration; MESSI, multiple simultaneous stable isotope; PC, phosphatidylcholine; PE, phosphatidylethanolamine; PI, phosphatidylinositol; PL, phospholipid; TG, triglyceride; Tmax, sampling time at maximal concentration

Supplementary key words stable isotope tracer • {alpha}-linolenic acid • eicosapentaenoic acid • docosahexaenoic acid • linoleic acid • dihomo-{gamma}-linolenic acid • arachidonic acid • gas chromatography-mass spectrometry • multiple simultaneous stable isotopes


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