Читать книгу Autoimmune Liver Disease - Группа авторов - Страница 29

BAs are synthesized from cholesterol. In humans, the “primary” BAs are cholic acid (CA) and chenodeoxycholic acid (CDCA). Before secretion into the bile, both CA and CDCA are conjugated to the amino group of taurine or glycine. Conjugation enhances the hydrophilicity of the BA, the major function of this being to decrease the passive diffusion of BAs across the cell membranes during their transit through the biliary tree and intestine. Therefore, conjugated BAs are absorbed only if a specific membrane carrier is present. The process of bile formation depends on the liver synthesis and the canalicular secretion of BAs. The active transport of BAs across the canalicular membranes of hepatocytes is a primary driving force for bile flow. The majority of the BAs in the intestine are absorbed intact. Approximately 15% are deconjugated by the bacterial flora in the distal small intestine, with the production of “secondary” BAs by the conversion of CA to deoxycholic acid and of CDCA to lithocholic acid. Most of the conjugated and deconjugated BAs are reabsorbed in the distal intestine and undergo enterohepatic circulation that maintains the BA pool. Thus, at least 12 major conjugated primary and secondary bile salt species are contained in human bile, although primary bile salts are usually predominant.