出版社：American Society for Biochemistry and Molecular Biology
摘要：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 ion on C-11 catalyzed by a radical intermediate enzyme.
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