No abstract

Bile salts have a central role in the digestion and absorption of fat in the intestinal tract. In addition, since the sterol nucleus is not further metabolized by mammalian cells, the bile salts represent the main excretory products of the body's cholesterol stores (1). The enterohepatic circulation of bile salts is well established, and upwards of 959 of the amount excreted through the bile duct is absorbed from the intestine for subsequent re-excretion by the liver (2). In vitro studies have shown an active transport mechanism to be present in the ileum of a number of species (3, 4); this site of absorption has been confirmed in vivo in the guinea pig and the dog (5, 6).We have studied further aspects of the active transport of bile salts, choosing sodium taurocholate as a model for most of the work. We were unable to find evidence for hydrolysis of the conjugated bile salts during their active transport in vitro. Taurocholate absorption across everted gut sacs exhibted saturation kinetics and was reversibly inhibited by cholate and glycocholate. A striking finding was that at low or absent sodium ion concentrations in the medium, active transport is markedly inhibited. We have compared these effects with those of anoxia and of metabolic inhibitors such as dinitrophenol.* Submitted for publication September 24, 1963; Materials and MethodsCommercial preparations of cholic acid,' glycine,2 and taurine 3 were recrystallized twice before use. The purity of the cholic acid was checked by thin-layer chromatography on silicic acid by the two solvent systems described by Hofmann (7). Solvents employed were reagent grade and were not redistilled; ether was peroxide-free. 2,4-Dinitrophenol 3 (DNP) was recrystallized from water; other inhibitors were phloridzin 4 and ouabain.5 Sodium taurocholate was synthesized by the method of Norman (8). After evaporation, the reaction mixture was dissolved in dilute aqueous alkali and extracted with ether to remove residual tributylamine. The solution was then acidified and unreacted cholic acid extracted with chloroform. Further purification was carried out as described (8) ; however, when there was difficulty with the first recrystallization, a salting out procedure was also employed (9). The purity of the product was checked by melting point determinations and by thin-layer chromatography; chemical assay for any persistent contamination by cholate was carried out by the procedure described below.Glycocholic acid was synthesized by the technique of Bergstr6m and Norman (10) without modification. The purity of the glycocholate was determined by melting point determinations and thin-layer chromatography.Radioactive conjugated bile salts were prepared from nonradioactive cholic acid and either glycine-1-C14 6 or taurine-S'.7 The S3-taurocholate was essentially radiochemically pure. On thin-layer chromatography of the synthesized C14-glycocholic acid, there was a contaminant with the mobility of glycodeoxycholate; this impurity could be removed by reverse-phase column chromatography...

The efficiency of intestinal absorption of bile salts was evaluated by studying the rate of disappearance of radioactivity from the bile of dogs after the intravenous administration of sodium taurocholate-24-C14. Bile was sampled through an indwelling tube in the gall bladder. One day after a high-fat meal normal dogs retained 48% of the radioactivity; dogs with resection of the jejunum retained 48%, whereas those with resection of the ileum retained only 3% in the bile. This is consistent with previous observations that the ileum is the site of bile salt absorption in vitro and in anesthetized animals. Animals with resection of the ileum exhibited significant steatorrhea; however, three-fourths of the ingested fat was absorbed in spite of almost complete failure to absorb bile salts. This indicates that fat and bile salts are not normally absorbed together. Elimination of enterohepatic circulation of bile salts by resection of the ileum contributes to the observed steatorrhea.

No abstract