Microbiological Transformation of Bile Acids

Publisher Summary This chapter discusses microbiological transformation of bile acids. There is an intimate relationship between bile acid metabolism in vivo and intestinal microorganisms. The conjugated primary bile acids are attacked by intestinal microorganisms in the lower small intestine and cecum and transformed into various metabolites that are absorbed and further transformed by liver enzymes prior to re-excretion into the bile. The liver enzymes mainly catalyze the following reactions: (1) conjugation of free bile acids with taurine or glycine, (2) reduction of keto groups to alcohols, and (3) hydroxylation of the cholane nucleus. Thus, the final composition of the bile acids in normal bile is the result of an intimate interaction between the liver and intestinal microorganisms. The bile-acid transformations in the intestine can be classified as follows: (1) hydrolysis of the conjugated bile acids; (2) elimination of hydroxyl groups, mainly at the C-7 position of the cholane nucleus; (3) oxidation of the hydroxyl groups at C-3, C-7, and C-12 to oxo groups; (4) reduction of oxo groups to both α- and β-hydroxyl groups; and (5) other miscellaneous reactions.