Abstract
Bile acid composition in the colon is determined by bile acid flow in the intestines, the population of bile acid-converting bacteria, and the properties of the responsible bacterial enzymes. Ursodeoxycholic acid (UDCA) is regarded as a chemopreventive beneficial bile acid due to its low hydrophobicity. However, it is a minor constituent of human bile acids. Here, we characterized an UDCA-producing bacterium, N53, isolated from human feces. 16S rDNA sequence analysis identified this isolate as Ruminococcus gnavus, a novel UDCA-producer. The forward reaction that produces UDCA from 7-oxo-lithocholic acid was observed to have a growth-dependent conversion rate of 90-100% after culture in GAM broth containing 1 mM 7-oxo-lithocholic acid, while the reverse reaction was undetectable. The gene encoding 7β-hydroxysteroid dehydrogenase (7β-HSDH), which facilitates the UDCA-producing reaction, was cloned and overexpressed in Escherichia coli. Characterization of the purified 7β-HSDH revealed that the kcat/Km value was about 55-fold higher for the forward reaction than for the reverse reaction, indicating that the enzyme favors the UDCA-producing reaction. As R. gnavus is a common, core bacterium of the human gut microbiota, these results suggest that this bacterium plays a pivotal role in UDCA formation in the colon.
Highlights
Bile acid composition in the colon is determined by bile acid flow in the intestines, the population of bile acid-converting bacteria, and the properties of the responsible bacterial enzymes
As a result of screening for Ursodeoxycholic acid (UDCA)-producing bacteria from human feces, 1,482 colonies were isolated; these were evaluated for their ability to convert 7-oxo-lithocholic acid (LCA) to UDCA by thin-layer chromatography (TLC)
The bile acids extracted from the culture medium were characterized by gas chromatography-mass spectrometry (GC-MS); their retention times (RT) corresponded to 7-oxo-LCA (RT, 12.2 min) and UDCA (RT, 12.6 min), respectively
Summary
Bile acid composition in the colon is determined by bile acid flow in the intestines, the population of bile acid-converting bacteria, and the properties of the responsible bacterial enzymes. Hydrophilic ursodeoxycholic acid (UDCA; 3␣, 7-dihydroxy-5-cholan-24-oic acid) is regarded as a chemopreventive bile acid [9, 10], because it protects hepatocytes and bile duct epithelial cells against necrosis and apoptosis induced by more hydrophobic secondary bile acids [11,12,13] (e.g., DCA). We showed that bile acid regulates the rat cecal microbiota composition, as feeding CA to rats resulted in a shift in the relative abundance of two major gut microbiota phyla; such feeding increased Firmicutes and decreased Bacteroidetes [15] This is similar to that reported in the fecal microbiota of a mouse model fed a high-fat diet, and such imbalanced bacterial populations have been argued to trigger the development of metabolic syndrome [16,17,18,19,20,21,22,23]. This article is available online at http://www.jlr.org of the bacteria involved in bile acid conversion is important for host health
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