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Conjugated -Linolenic Acid Isomers in Bovine Milk and Muscle

M. Plourde^*,1,2, F. Destaillats, P. Y. Chouinard and P. Angers^*

^* Department of Food Science and Nutrition, Institut des Nutraceutiques et des Aliments Fonctionnels/Centre de Recherche en Sciences et Technologie du Lait (INAF/STELA), Université Laval, Québec, Canada G1K 7P4
Nestlé Research Center, Vers-chez-les-Blanc, Lausanne, Switzerland
Department of Animal Sciences, INAF/STELA Université Laval, Québec, Canada G1K 7P4

² Corresponding author: melanie.plourde2{at}usherbrooke.ca

Conjugated linolenic acids (CLnA) are octadecatrienoic fatty acid isomers with at least 2 conjugated double bonds. Various CLnA isomers occur naturally, and some isomers could be formed by ruminants from dietary -linolenic acid. Ruminant biohydrogenation of polyunsaturated fatty acids gives rise to the formation of numerous metabolites having conjugated or nonconjugated structures. The objectives of this study were to identify and characterize CLnA isomers in milk fat and muscle lipid extracts from cattle fed a high-forage diet. The analysis of total fatty acid methyl esters revealed levels of total CLnA of 0.39% in a single milk lipid extract and 0.34% in a single muscle lipid extract. Fatty acid methyl esters were fractionated by argentation thin-layer chromatography. A fraction containing dienoic fatty acids as well as CLnA isomers was isolated and analyzed. The double bond positions of CLnA isomers (cis-9, trans-11, cis-15 and cis-9, trans-13, cis-15 18:3) were confirmed by mass spectrometry of their 4,4-dimethyloxazoline derivatives. Mass spectra of the cis-9, trans-13, cis-15 18:3 isomer was characterized by an intense ion at m/z 236 attributable to the formation of 2 stabilized allylic radical fragments, whereas this intense ion corresponding to the stabilized radical fragments was located at m/z 262 for the cis-9, trans-11, cis-15 18:3 isomer. The gap of 12 amu between m/z 250 and 262 confirmed the occurrence of a double bond in position ¹³. Configuration of the double bonds of standards having similar mass spectra and gas-liquid chromatographic retention times was confirmed by ¹H nuclear magnetic resonance. We also showed that both CLnA isomers were found in the muscle lipid extract, whereas only the cis-9, trans-11, cis-15 18:3 isomer was identified in the milk lipid extract. This study appears to be the first to identify 2 CLnA isomers in bovine muscle lipid extract.

Key Words: conjugated -linolenic acid • mass spectrometry • bovine milk • bovine muscle

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				R. Gervais and P. Y. Chouinard Effects of Intravenous Infusion of Conjugated Diene 18:3 Isomers on Milk Fat Synthesis in Lactating Dairy Cows J Dairy Sci, September 1, 2008; 91(9): 3568 - 3578. [Abstract] [Full Text] [PDF]