Bile acids Taurine

A major and striking physicochemical difference between lithocholic acid and the dihydroxy and trihydroxy bile acids is the insolubility of the sodium salts of the former (10, 45). Sodium salts of the common bile acids (taurine and glycine conjugates of cholic acid, deoxycholic acid, and che-nodeoxycholic acid) are very soluble in water and physiological saline, even at 0°C. The solubility of the ammonium, lithium, sodium, potassium, rubidium, and cesium salts of lithocholic acid (NH4L, LiL, NaL, RbL, CsL) have been studied in water as a function of temperature (45). 

Mammalian bile contains sodium salts of conjugated bile acids, e.g. glycocholic acid and taurocholic acid, in which cholic acid is combined (amide linkage) with glycine and taurine respectively. 

CJH4O5, H02CCH(0H)C02H. Colourless crystals with IH O lost at 60 C. M.p. IhO C (decomp.). Prepared by heating dinitrotartaric acid in aqueous alcohol, taurine, aminoethylsulpbonic acid, C2H7NO3S, NHj CHj CH SOjH. Crystallizes in columns, decomposing at 317 C. In combination with cholic acid it forms one of the bile acids. It is formed in the liver from cysteine. 

Bile acids, which exist mainly as bile salts, are polar carboxylic acid derivatives of cholesterol that are important in the digestion of food, especially the solubilization of ingested fats. The Na and salts of glycocholic acid and tauro-cholic acid are the principal bile salts (Ligure 25.41). Glycocholate and tauro-cholate are conjugates of cholic acid with glycine and taurine, respectively. 

L-Cysteine is a precursor of the thioethanolamine portion of coenzyme A and of the taurine that conjugates with bile acids such as taurocholic acid (Chapter 26). 

The two most important bile acids, cholic acid C24H40Os and desoxy-cholic acid C24H40O4, occur in ox bile in combination, partly with glycine and partly with taurine as glyco- and taurocholic and glyco- and tauro-desoxycholic acids. The linkage between the amino acids and the bile acids is of an amide nature. On hydrolysis the nitrogenous constituents are split off. 

Bile salts consist of taurine linked to bile acids. The salts are essential for digestion and absorption of fat and also of fat-soluble vitamins and cholesterol. 

Table 1.1 lists some of the characteristics of the more common bile acids, which are divided into 3 main classes free bile acids, glycine and taurine conjugates. 

The first studies of specificity were carried out using cholate, the glycine and taurine conjugates and taurine conjugates of the dihydroxy bile acids cheno-deoxycholate and ursodeoxycholate. Kramer and colleagues prepared plasma membrane vesicles from rat liver and compared bile-acid transport with values from CHO cells stably expressing NTCP. This work established that transport by the liver enzyme was maximal when 2 hydroxyls were present,... 

Alternative potential strategies for reduction of mucosal secondary bile acid exposure are to target deconjugation of glycine/taurine bile salts by bacterial bile salt hydrolases and/or bacterial 7-dehydroxylation of primary bile acids to secondary bile acids. Sequestration of bile acids in the intestinal lumen using probiotic bacteria has also been proposed as an area for future research. ... 

FI GURE 4.3 Structures of allocholic acid (petromyzonol sulfate is identical apart from replacement of the carboxyl group with sulfate), taurine-conjugated bile acid, and geosmin. 

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. 

Before leaving the liver, a large proportion of the bile acids are activated with CoA and then conjugated with the amino acids g/ycine or taurine (2 cf A). In this way, cholic acid gives rise to glycocholic acid and taurocholic acid. The liver bile secreted by the liver becomes denser in the gallbladder as a result of the removal of water (bladder bile 3). 

Free bile acids Glycine conjugates Taurine conjugates... 

Fig. 5.4.15a,b Parts of two-dimensional total correlated spectroscopy spectra of human bile (a) and a mixture of standard solutions of GCA, GDCA, TCA and TDCA (b). These spectra show the connectivity of amide proton signals to H-25 and/or H-26 protons of glycine and/or taurine conjugated bile acids (reprinted from [40])... 

Fig. 5.4.16 Typical room temperature nuclear magnetic resonance spectra of human BA and standards of glycine- and taurine-conjugated bile acid, (reprinted from [40])...

Ijare OB, Somashekar BS, Gowda GA, Sharma A, Kapoor VK, Khetrapal CL (2005) Quantification of glycine and taurine conjugated bile acids in human bile using H NMR spectroscopy. Magn Reson Med 53 1441-1446... 

Before the bile acids leave the liver, they are conjugated to I molecule of either glycine or taurine (an end-product of cystene metabolism) by an amide bond between the carboxyl group of tit J bile acid and the amino group of the added compound. These netfl... 

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