Chapter 12 Metabolism of bile acids in intestinal microflora

This chapter discusses the metabolism of bile acids in intestinal microflora. The intestinal microflora of man and animals can biotransform bile acids into a number of different metabolites. Normal human feces may contain more than 20 different bile acids formed from the primary bile acids, cholic acid, and chenodeoxycholic acid. Known microbial biotransformations of these bile acids include the hydrolysis of bile acid conjugates yielding free bile acids, oxidation of hydroxyl groups at C-3, C-6, C-7, and C-12 and the reduction of oxo groups to give epimeric hydroxy bile acids. In addition, certain members of the intestinal microflora 7α- and 7β-dehydroxylate primary bile acids yield deoxycholic acid and lithocholic acid. Moreover, 3-sulfated bile acids are converted into a variety of different metabolites by the intestinal microflora. The hydrolysis of bile acid conjugates is probably the initial reaction catalyzed by intestinal bacteria. Therefore, primarily free bile acids are isolated from the feces of man and animals. The bulk of the free bile acids in feces of man is deoxycholic acid and lithocholic acid generated by the 7α-dehydroxylation of cholic acid and chenodeoxycholic acid, respectively. A portion of fecal acids is absorbed from the intestinal tract, returned to the liver where they are conjugated and again secreted via biliary bile. Therefore, the final composition of biliary bile acids is the result of a complex interaction between liver enzymes and enzymes in intestinal bacteria.