title: Contribution of the 7β-hydroxysteroid dehydrogenase from Ruminococcus gnavus N53 to ursodeoxycholic acid formation in the human colon.
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.
authors.affiliation: Laboratory of Microbial Physiology, Research Faculty of Agriculture, Hokkaido University, Sapporo, Hokkaido 060-8589, Japan.
authors.forename: Ja-Young
authors.initials: JY
authors.lastname: Lee
authors.forename: Hisashi
authors.initials: H
authors.lastname: Arai
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Nakamura
authors.forename: Satoru
authors.initials: S
authors.lastname: Fukiya
authors.forename: Masaru
authors.initials: M
authors.lastname: Wada
authors.forename: Atsushi
authors.initials: A
authors.lastname: Yokota
mesh_terms: D000595:Amino Acid Sequence; D001647:Bile Acids and Salts / Q000378:metabolism; D003001:Cloning, Molecular; D003106:Colon / Q000378:metabolism* / Q000382:microbiology; D016680:Genome, Bacterial / Q000235:genetics; D006801:Humans; D006913:Hydroxysteroid Dehydrogenases / Q000737:chemistry / Q000235:genetics / Q000378:metabolism*; D008095:Lithocholic Acid / Q000031:analogs & derivatives / Q000378:metabolism; D008969:Molecular Sequence Data; D045857:Ruminococcus / Q000201:enzymology* / Q000235:genetics; D014580:Ursodeoxycholic Acid / Q000378:metabolism*
chemical_list: D001647:Bile Acids and Salts; C023020:7-ketolithocholic acid; D008095:Lithocholic Acid; D014580:Ursodeoxycholic Acid; D006913:Hydroxysteroid Dehydrogenases; C031932:7 beta-hydroxysteroid dehydrogenase
keywords: bile acid conversion; epimerization; intestinal bacteria; secondary bile acids