Intestinal Bacterial Metabolism of Bile Acids and Bile Pigments

Cholesterol is a precursor of bile acids, and both are synthesized in the liver from two carbon units. Bile acids synthesized in the liver are conjugated through an amide bond to either glycine or taurine. The conjugated bile acids are deposited in the bile, and excreted into the upper small intestine. The bacterial conversion of bile acids primarily occurs in the distal ileum and colon. The bacterial reactions on bile acids include: the hydrolysis of the amide bond to release free bile acids from their corresponding glycine and taruine conjugates; an oxidoreduction of the hydroxyl groups at C3, C7, and C12 to form either oxo bile acids or alpha hydroxyl groups after the reduction of the beta groups (inversion products); and dehydroxylation at C7, and to a smaller extent at the C3 and C12 positions (43). The consequence of these reactions is the conversion of primary to secondary bile acids, and the re-absorption of free bile acids from the ileum, and to a lesser extent, from the colon. Only approximately 5% of bile acids are lost in the feces in each cycle as a result of bacterial deconjugation of bile acids (44).

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