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Journal of Lipid Research, Vol. 48, 2247-2254, October 2007
Copyright © 2007 by American Society for Biochemistry and Molecular Biology

Isomers of conjugated linoleic acids are synthesized via different mechanisms in ruminal digesta and bacteria

R. John Wallace^1,*, Nest McKain^*, Kevin J. Shingfield and Estelle Devillard^*

^* Rowett Research Institute, Bucksburn, Aberdeen AB21 9SB, UK
MTT Agrifood Research Finland, Jokioinen FI-31600, Finland

Published, JLR Papers in Press, July 20, 2007.

¹ To whom correspondence should be addressed. e-mail: john.wallace{at}rowett.ac.uk

Digesta samples from the ovine rumen and pure ruminal bacteria were incubated with linoleic acid (LA) in deuterium oxide-containing buffer to investigate the mechanisms of the formation of conjugated linoleic acids (CLAs). Rumenic acid (RA; cis-9,trans-11-18:2), trans-9,trans-11-18:2, and trans-10,cis-12-18:2 were the major CLA intermediates formed from LA in ruminal digesta, with traces of trans-9,cis-11-18:2, cis-9,cis-11-18:2, and cis-10,cis-12-18:2. Mass spectrometry indicated an increase in the n+1 isotopomers of RA and other 9,11-CLA isomers, as a result of labeling at C-13, whereas 10,12 isomers contained minimal enrichment. In pure culture, Butyrivibrio fibrisolvens and Clostridium proteoclasticum produced mostly RA with minor amounts of other 9,11 isomers, all labeled at C-13. Increasing the deuterium enrichment in water led to an isotope effect, whereby ¹H was incorporated in preference to ²H. In contrast, the type strain and a ruminal isolate of Propionibacterium acnes produced trans-10,cis-12-18:2 and other 10,12 isomers that were minimally labeled. Incubations with ruminal digesta provided no support for ricinoleic acid (12-OH,cis-9-18:1) as an intermediate of RA synthesis. We conclude that geometric isomers of 10,12-CLA are synthesized by a mechanism that differs from the synthesis of 9,11 isomers, the latter possibly initiated by hydrogen abstraction on C-11 catalyzed by a radical intermediate enzyme.

Supplementary key words biohydrogenation • Butyrivibrio fibrisolvens • Clostridium proteoclasticum • Propionibacterium acnes • rumenic acid

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