In vitro transformation of chenodeoxycholic acid and ursodeoxycholic acid by human intestinal flora, with particular reference to the mutual conversion between the two bile acids.

Nine fecal samples from four healthy subjects were examined for their ability to transform chenodeoxy-cholic acid (CDCA) and ursodeoxycholic acid (UDCA) in in vitro anaerobic broth cultures. Seven samples converted CDCA and UDCA into each other (more than 50% of CDCA was converted into UDCA while 10% or less of UDCA was converted into CDCA), and produced 7-keto-lithocholic acid and lithocholic acid equally from both acids. No alteration of the 7 beta-hydroxy group of UDCA was demonstrated by two fecal samples that failed to perform mutual 7-epimerization, suggesting the conversion of UDCA into lithocholic acid via CDCA. The 3 alpha-hydroxy groups of these substrate and metabolite bile acids were invariably partially epimerized to 3 beta-hydroxy groups by all the fecal samples. Evidence is presented for the prevalence of these 7- and 3-epimerizing organisms among the human intestinal flora.