Studies on the Transport and Metabolism of Conjugated Bile Salts by Intestinal Mucosa *

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...

Metabolism of steroid and amino acid moieties of conjugated bile acids in man

“…DISCUSSION Despite considerable variation in the fractional turnover rates of both steroid and amino acid moieties among subjects, glycine degradation (and synthesis) was more rapid than cholic acid loss and 7-dehydroxylation in each subject. Cleavage of the amide bond of cholylglycine is mediated exclusively by bacterial enzymes (13)(14)(15)(16), but the site of such bacterial deconjugation was not indicated by this study.…”

Bile acid metabolism. I. Studies on the mechanisms of intestinal transport.

“…As shown by previous investigators (2,3,5), experiments in this laboratory using everted gut 7.00) solutions of these sacs (3). In the case of taurocholate only ionic diffusion would be expected, whereas with cholate the low pH of the serosal solution would allow both ionic and nonionic diffusion to take place.…”

“…Other investigators (2,3,5,(15)(16)(17) have shown that only segments taken from the terminal half of the intestine are capable of transporting taurocholate and cholate against concentration gradients. However, it should be emphasized that this observation alone does not constitute proof of active transport.…”

Fetal bile salt metabolism. The intestinal absorption of bile salt.

“…The absorption of taurocholate was studied in adult and fetal everted gut rings and sacs (19,20 Thtus, the ileal mechanism for active bile salt transport is not demonstrable in the near-term fetal dog, althouigh bile salt is passively absorbed from both fetal jejtuntum and ileuim. An enterohepatic circulation of bile salt is directly demonstrable in the dog.…”