title: Isomers of conjugated linoleic acids are synthesized via different mechanisms in ruminal digesta and bacteria.
abstract: 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 (1)H was incorporated in preference to (2)H. 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.
authors.affiliation: Rowett Research Institute, Bucksburn, Aberdeen, UK. john.wallace@rowett.ac.uk
authors.forename: R John
authors.initials: RJ
authors.lastname: Wallace
authors.forename: Nest
authors.initials: N
authors.lastname: McKain
authors.forename: Kevin J
authors.initials: KJ
authors.lastname: Shingfield
authors.forename: Estelle
authors.initials: E
authors.lastname: Devillard
mesh_terms: D000818:Animals; D001419:Bacteria / Q000378:metabolism*; D017666:Deuterium Oxide / Q000737:chemistry; D005227:Fatty Acids / Q000378:metabolism; D041981:Gastrointestinal Tract / Q000382:microbiology*; D007536:Isomerism; D019787:Linoleic Acid / Q000737:chemistry; D008041:Linoleic Acids / Q000737:chemistry / Q000378:metabolism*; D044243:Linoleic Acids, Conjugated / Q000737:chemistry; D008954:Models, Biological; D020033:Protein Isoforms; D012418:Ruminants; D012756:Sheep; D013997:Time Factors
chemical_list: D005227:Fatty Acids; D008041:Linoleic Acids; D044243:Linoleic Acids, Conjugated; D020033:Protein Isoforms; C046938:9,11-linoleic acid; D019787:Linoleic Acid; D017666:Deuterium Oxide