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

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

Gut Health Division, Rowett Research Institute, Aberdeen AB21 9SB

Corresponding Author: 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 formation of conjugated linoleic acids (CLA). Rumenic acid (cis-9,trans-11-18:2; RA), 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, due to labelling at C-13, while 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 labelled at C-13. Increasing the deuterium enrichment in water indicated 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 labelled. Incubations with ruminal digesta provided no support for ricinoleic acid (12-OH,cis-9-18:1) as an intermediate of RA synthesis. It is concluded 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 catalysed by a radical-intermediate enzyme.

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