Production of new microbially conjugated bile acids by human gut microbiota

Gut microbes have been recognized to convert the human bile acids by deconjugation, dehydroxylation, dehydrogenation and epimerization of the cholesterol core, but the ability to re-conjugate them with amino acids as an additional conversion has only been recently described. These new bile acids are known as microbially conjugated bile acids (MCBAs). The aim of this study was evaluating the MCBAs diversity produced by the gut microbiota through a metabolomics approach. In this study, fresh fecal samples from healthy donors were evaluated to explore the re-conjugation of chenodeoxycholic and 3-oxo-chenodeoxycholic acids by the human gut microbiota. No significant differences were found between the conversion trend of both BAs incubations. The in vitro results showed a clear trend to first accumulate the epimer isoursochenodeoxycholic acid, and the dehydroxylated lithocholic acid derivatives in samples incubated with chenodeoxycholic and 3-oxo-chenodeoxycholic acid. Also showed a strong trend for the production of microbially conjugated dehydroxylated bile acids, instead of chenodeoxycholic backbone conjugates. Different molecules and isomers of microbial conjugates with valine and leucine were also identified and confirmed by MS/MS. These results document the gut microbiota capability to produce esters of MCBAs on hydroxyls of the sterol backbone in addition to amides at the C24 acyl site. This study opens a new perspective to study the BAs diversity produced by the human gut microbiota.

肠道微生物可通过胆固醇核心的去结合、去羟基化、脱氢及差向异构化反应转化人体胆汁酸（bile acids, BAs），但此前仅在近期才有研究报道肠道微生物具备将胆汁酸与氨基酸重新结合的额外转化能力。这类新型胆汁酸被称为微生物结合型胆汁酸（microbially conjugated bile acids, MCBAs）。
本研究旨在通过代谢组学方法，探究肠道菌群所产生的MCBAs的多样性。本研究采集健康志愿者的新鲜粪便样本，探究人体肠道菌群对鹅脱氧胆酸与3-氧代鹅脱氧胆酸的重新结合转化能力。两种BAs孵育体系的转化趋势未发现显著差异。体外实验结果显示，在以鹅脱氧胆酸及3-氧代鹅脱氧胆酸孵育的样本中，肠道菌群呈现出先积累差向异构体异乌索鹅脱氧胆酸，以及去羟基化的石胆酸衍生物的明确趋势。同时，实验结果还显示菌群更倾向于生成去羟基化的微生物结合型胆汁酸，而非鹅脱氧胆酸母核结合物。研究还通过串联质谱（MS/MS）鉴定并证实了多种与缬氨酸、亮氨酸结合的微生物结合物分子及其异构体。本研究结果证实，人体肠道菌群除可在C24酰基位点形成酰胺键结合型MCBAs外，还能够在固醇母核的羟基位点生成MCBAs酯类衍生物。本研究为探究人体肠道菌群介导的胆汁酸多样性提供了全新的研究视角。