Taurine Conjugation of Bile Acids

In addition to bile acid conjugation, a variety of other compounds may also be excreted as taurine conjugates, including retinoic acid (Section 2.2.1.3) and a number of xenobiotics. 

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Taurine conjugation with bile acids, phenylacetic acid, and indolylacetic acid seems to be a minor process in most species, but in the pigeon and ferret, it occurs extensively. Other infrequently reported conjugations include serine conjugation of xanthurenic acid in rats excretion of quinaldic acid as quinaldylglycyltaurine and quinaldylglycylglycine in the urine of the cat, but not of the rat or rabbit and conversion of furfural to furylacrylie acid in the dog and rabbit, but not in the rat, hen, or human. The dog and... 

Taurine Osmotic agent in retina and used for conjugation of bile acids dietary essential for cats... 

Taurine was discovered in 1827 in ox hUe, where it is conjugated with the bile acids. It was later shown to be a major excretory product of the sulfur amino acids methionine and cysteine. Until about 1976, it was assumed that it was a metabolic end-product whose only function was the conjugation of bile acids. In the rat, taurine synthesis accounts for 70% to 85% of total cysteine catabolism. 

Conjugation of bile acids with taurine and glycine. 

H7. Hardison, W. C. M., and Grundy, S. M., The effect of ursodeoxycholate and its taurine conjugate on bile acid synthesis and cholesterol absorption. Gastroenterology 87, 130-135 (1984). 

V7. Vessey, D. A., The biochemical basis for the conjugation of bile acids with either glycine or taurine. Biochem. J. 174, 621-626 (1978). 

The formation of conjugates of taurine or glycine is induced by glucocorticoids, specifically dexamethasone [49]. In primary cultures of rat fetal liver dexamethasone increased and maintained taurine conjugation of cholate, chenodeoxycholate, and deoxycholate short-term cultures of adult rat hepatocytes maintained conjugation of bile acids with glycine and taurine [49]. Enzymatic activity was stimulated by low doses of ethynylestradiol [50], and has been verified in fetal liver [51]. 

The amino acid taurine has been suggested as a marker of hepatotoxicity because it is involved in the conjugation of bile acids to bile acids, the conjugation of xenobiot-ics, and the detoxification of reactive metabolites. Taurine is a product of sulfur acid metabolism and may reflect protein synthesis. The measurement of urinary taurine in models of hepatotoxicity has been described (Sanins et al. 1990 Waterfield et al. 1993a, 1993b, 1993c Timbrell, Seabra, and Waterfield 1995 Waterfield, Asker, and Timbrell 1996), but the measurement is reported to show diurnal variation, wide intraanimal variability, and less success at minor levels of hepatic injury (Maxuitenko, North, and Roebuck 1997). 

Hepatic bile acid production is also changed qualitatively. The conjugation of bile acids with glycine is significantly decreased in liver biopsies from cirrhotic patients (194). However, the ratio of glycine- to taurine-... 

The demonstration by Bergstrom et al. (4,5) in 1953 of the conversion of deoxycholic acid to taurocholic acid in the rat in vivo and by rat liver slices paved the way for studies on 7a-hydroxylation and conjugation of bile acids. The early work on the synthesis of bile acid conjugates in vitro utilized slices or homogenates of rat and human liver, and the enzymatic reaction was followed by the incorporation of radioactivity from carboxyl- C-labeled bile acids into the corresponding taurine and glycine conjugates (6,7). The... 

Virtually all markers of peroxisomal function are lipid-boimd or protein-bound they are present in the plasma, but they are not excreted into the urine. Exceptions are the water-soluble glycine-, taurine-, or glucuronic acid conjugates of bile acids which may be found in the urine. The general clinical chemistry lab will not give consistent clues to the existence of peroxisomal disease as there is no readily accessible end product of peroxisomal substrates. 

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). 

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