Aldini R, Roda A, Montagnani M, Roda E
Istituto di Scienze Chimiche, Università di Bologna, Italy.
Ital J Gastroenterol. 1995 Apr;27(3):141-4.
A close structure-activity relationship exists between the transport of bile acids (BA) in the liver and intestine; hepatic and intestinal BA transport can be evaluated and compared by using perfused liver and perfused intestine in the rabbit. The passive intestinal absorption is limited to the unconjugated BA, which occurs throughout the small bowel and colon, and is conditioned by the apical membrane lipid composition. A higher diffusion component is found in the terminal ileum compared to the jejunum, and seems to be related to the higher cholesterol-to-phospholipid ratio of the ileal brush border membranes. The active transport system is well characterized and the brush border membrane receptor, cytosolic BA binding proteins and basolateral anion exchange protein have been identified. Recently, the ileal BA transporter has been cloned from the hamster and human ileum and the main cytosolic BA binding protein was cloned from the rat ileum. In the liver, the active transport predominates on the passive diffusion both for conjugated and unconjugated BA. The maximal transport capacity in the liver is tenfold higher than in the intestine, while the Km values are of the same order of magnitude, i.e. in the millimolar range. Neither system operates at its maximum transport rate with prevalent concentrations of BA in portal blood or luminal content.
胆汁酸（BA）在肝脏和肠道中的转运之间存在密切的构效关系；可以通过使用兔的灌注肝脏和灌注肠道来评估和比较肝脏和肠道的BA转运。被动性肠道吸收仅限于未结合的BA，其在整个小肠和结肠中发生，并受顶端膜脂质组成的制约。与空肠相比，在回肠末端发现更高的扩散成分，这似乎与回肠刷状缘膜中更高的胆固醇与磷脂比例有关。主动转运系统已得到充分表征，并且已鉴定出刷状缘膜受体、胞质BA结合蛋白和基底外侧阴离子交换蛋白。最近，已从仓鼠和人回肠中克隆出回肠BA转运体，并且已从大鼠回肠中克隆出主要的胞质BA结合蛋白。在肝脏中，无论是结合型还是未结合型BA，主动转运均占被动扩散的主导地位。肝脏中的最大转运能力比肠道高十倍，而Km值处于相同的数量级，即在毫摩尔范围内。在门静脉血或管腔内容物中BA的普遍浓度下，这两种系统均未以其最大转运速率运行。
