Chenodeoxycholic acid, conjugates

The low concentrations of bile acids in urine have also been measured by radioimmunoassay. In one study, total cholic and chenodeoxycholic acid conjugates were measured after extraction and solvolysis to remove sulfate groups, giving a mean urinary excretion of 0.6 p.mol/24 hours for cholic acid and 1.2 p.mol/24 hours for chenodeoxycholic acid in normal subjects (S7). These estimates can be compared with values of 2.1 xmol/24 hours for conjugated cholic acid and 8.4 xmol/24 hours for sulfoglycolithocholic acid obtained for the urinary excretion of bile acids using commercially available radioimmunoassays (WIO). 

Samuelson, K., Aly, A., Johansson, C., and Norman, A., Serum and urinary bile adds in patients with primary biliary cirrhosis. Scand. J. Gastroentend. 17, 121-128 (1982). Samuelson, K., and Eklund, A., Determination of urinary cholic and chenodeoxycholic acid conjugates with radioimmunoassay. Scand. J. Clin. Lab. Invest. 40,555-561 (1980). 

Cholic acid and chenodeoxycholic acid, known as the primary bile acids, are quantitatively the most important metabolites of cholesterol. After being biosynthesized, they are mostly activated with coenzyme A and then conjugated with glycine or the non-pro-teinogenic amino acid taurine (see p. 62). The acid amides formed in this way are known as conjugated bile acids or bile salts. They are even more amphipathic than the primary products. 

The C>4 bile acids arise from cholesterol in the liver after saturation of the steroid nucleus and reduction in length of the side chain to a 5-carbon add they may differ in the number of hydroxyl groups on the sterol nucleus. The four acids isolated from human bile include cholic acid (3,7,12-tiihydroxy), as shown in Fig. 1 deoxycholic acid (2,12-dihydroxy) chenodeoxycholic acid (3,7-dihydroxy) and lithocholic acid (3-hydroxy). The bile acids are not excreted into the bile as such, but are conjugated through the C24 carboxylic add with glycine or... 

There are four major bile acids (see Figures 47-5 to 47-7). Cholic acid and chenodeoxycholic acid, the primary bile acids, are synthesized in the liver. Bacteria metabolize these primary bile acids to the secondary bile acids—deoxy-cholic acid and lithocholic acid, respectively. Bile acids are conjugated in the liver with the amino acids glycine or taurine. This decreases passive absorption in the biliary tree and proximal small intestine, but permits conservation through active transport in the terminal ileum. This combi-... 

The bile acids are 24-carbon steroid derivatives. The two primary bile acids, cholic acid and chenodeoxycholic acid, are synthesized in the hepatocytes from cholesterol by hy-droxylation, reduction, and side chain oxidation. They are conjugated by amide linkage to glycine or taurine before they are secreted into the bile (see cholesterol metabolism. Chapter 19). The mechanism of secretion of bile acids across the canalicular membrane is poorly understood. Bile acids are present as anions at the pH of the bile, and above a certain concentration (critical micellar concentration) they form polyanionic molecular aggregates, or micelles (Chapter 11). The critical micellar concentration for each bile acid and the size of the aggregates are affected by the concentration of Na+ and other electrolytes and of cholesterol and lecithin. Thus, bile consists of mixed micelles of conjugated bile acids, cholesterol, and lecithin. While the excretion of osmotically active bile acids is a primary determinant of water and solute transport across the canalicular membrane, in the canaliculi they contribute relatively little to osmotic activity because their anions aggregate to form micelles. 

To diagnose CTX, advantage has been taken of the elevated cholestanol levels in plasma. If the ratio of cholestanol to cholesterol is measured by gas-liquid chromatography, a value of over 1% is obtained in patients with CTX compared with less than 0.5% for normal subjects (S2). More recently, capillary gas-liquid chromatography has been used to detect bile alcohol conjugates in the urine of CTX patients and this technique has been useful to assess the efficacy of treatment with orally administered chenodeoxycholic acid (W12). 

As a consequence of the 7a-dehydroxylation process, the bile acid composition of bile in healthy subjects usually comprises around 30 to 40% conjugated cholic acid, 30 to 40% conjugated chenodeoxycholic acid, 10 to 30% conjugated deoxycholic acid, and less than 5% conjugated lithocholic acid, of which the majority is sulfated (H18). 

The liver, and also bacteria in the small and large intestine, can cause other structural modifications to bile acids as they undergo their entero-hepatic cycle. The formation of sulfate esters, already mentioned with respect to lithocholate in Section 4.2.1, is carried out primarily in the liver in man by a sulfotransferase (Lll). Other bile acids can also be sulfoconjugated to a small extent, mainly at the 3a-hydroxyl position. Bacteria, which have been isolated anaerobically from human feces, are known to possess bile acid sulfatase activity, which removes the 3a-sul te group of chenodeoxycholic and cholic acids (H24). The action of this bacterial enzyme probably explains why only trace amounts of sul ted bile acids, which are poorly absorbed in the intestine, are detected in the feces (12). Another type of bile acid conjugate, which has been identified in the urine of healthy subjects and patients with hepatobiliary disease, is the glucuronide (A7, S41). Both the liver and extrahepatic tissues, such as the kidney and small intestinal mucosa, are capable of glucuronidation of bile acids in man (M14). 

Gil. Goswami, S. K., and Frey, C. F., Hiin-layer chromatographic method for the separation of conjugated deoxycholic acid from conjugated chenodeoxycholic acid. J. Chromatogr. 145, 147-150 (1978). 

Scbalm, S. W., Van Berge Henegouwen, G. P., Hofmann, A. F., Cowen, A. E., and Turcotte, J., Radioimmunoassay of bile adds Development, validation, and prelimiiuuy application of an assay for conjugates of chenodeoxycholic acid. Gastroenterology 73, 285-290 (1977). 

The microsomal 12a-hydroxylase seems to be influenced by the flux of bile acids through the liver in a similar way as cholesterol 7a-hydroxylase. Biliary drainage in rats leads to a 2-fold stimulation of 12a-hydroxylase [44] perhaps due to reduced intake of food [252]. However, it was shown later that 12a-hydroxylation of 7a-hydroxy-4-cholesten-3-one was inhibited by feeding rats different taurine-conjugated bile acids at the 1% level [110]. Ahlberg et al. showed that the microsomal 12a-hydroxylase in human liver was inhibited by about 50% after treatment for 8 weeks with chenodeoxycholic acid, 15 mg/kg body weight [HI]. The increased ratio between cholic acid and chenodeoxycholic acid observed after treatment with cholestyramine is also consistent with an inhibitory effect of reabsorbed bile acids on the 12a-hydroxylase [219]. 

The intestinal microflora of man and animals can biotransform bile acids into a number of different metabolites. Normal human feces may contain more than 20 different bile acids which have been formed from the primary bile acids, cholic acid and chenodeoxycholic acid [1-5], Known microbial biotransformations of these bile acids include the hydrolysis of bile acid conjugates yielding free bile acids, oxidation of hydroxyl groups at C-3, C-6, C-7 and C-12 and reduction of oxo groups to give epimeric hydroxy bile acids. In addition, certain members of the intestinal microflora la- and 7j8-dehydroxylate primary bile acids yielding deoxycholic acid and lithocholic acid (Fig. 1). Moreover, 3-sulfated bile acids are converted into a variety of different metabolites by the intestinal microflora [6,7]. 

The hydrolysis of bile acid conjugates is probably the initial reaction catalyzed by intestinal bacteria. Therefore, primarily free bile acids are isolated from the feces of man and animals [1-5]. The bulk of the free bile acids in feces of man is deoxycholic acid and lithocholic acid which are generated by the 7 -dehydroxylation of cholic acid and chenodeoxycholic acid, respectively. A portion of fecal acids is absorbed from the intestinal tract, returned to the liver where they are conjugated and again secreted via biliary bile. Therefore, the final composition of biliary bile acids is the result of a complex interaction between liver enzymes and enzymes in intestinal bacteria. 

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