The biochemical basis for the conjugation of bile acids with either glycine or taurine

Abstract

All animals, except for the placental mammals, conjugate their bile acids exclusively with taurine. However, in certain of the placental mammals, glycine conjugates are also found. The basis for the appearance of glycine conjugation among the placental mammals was investigated. The reaction of choloyl-CoA with glycine and taurine, as catalysed by the soluble fraction from guinea-pig liver, had a high affinity for taurine and a poor affinity for glycine. The predominant synthesis of glycine conjugates in the guinea pig can be related to the fact that guinea-pig liver contains an unusually low concentration of taurine and a high concentration of glycine. Rabbits make exclusively glycine conjugates and their livers also contain low concentrations of taurine. However, the biochemical basis for their glycine conjugation is more straightforward than in the guinea pig in that the soluble fraction from rabbit liver has a high affinity for glycine and a poor affinity for taurine. Alternative-substrate-inhibition studies with glycine and taurine in soluble fractions from guinea-pig and rabbit liver revealed that glycine and taurine were mutually inhibitory. This suggests that there is only one enzyme for glycine and taurine conjugation in these tissues. The soluble fractions from bovine liver and human liver also made both glycine and taurine conjugates and evidence is presented that suggests that there is only one enzyme in these tissues too. Even the rat, which excretes mostly taurine conjugates, could make both glycine and taurine conjugates in vitro. However, in contrast with all of the placental mammals studied, the supernatant fraction from liver of the chicken, and other non-mammals, could not make glycine conjugates even in the presence of very high concentrations of glycine.

Roles of Endogenous and Exogenous Taurine and Glycine in the Formation of Conjugated Bi e Acids: Analyses Using Freshly Isolated and Primary Cultured Rat Hepatocytes

“…), and the formation of taurine-or glycine-conjugated bile acids (1,4,5) is completed by these precesses. It has also been reported that in the mammalian liver a single enzyme, bile acid-Co A: amino acid N-acyltransferase, catalyzes the conjugation of bile acids both with taurine and glycine, and this enzyme has a higher affinity for taurine than glycine in the rat liver (6,7).…”

Effect of Taurine in Rat Milk on the Growth of Offspring.

“…The synthesis of taurine from methionine or cysteine via cysteic acid or hypotaurine has been reported in liver, brain, lung, and muscle tissues [9,23,25]. After being synthesized, the amino acid is not incorporated into protein but remains free in the intracellular cytosol [3], and is partly conjugated to form bile salts in the liver [31]. Excess taurine is excreted into the urine by the kidney [22].…”

“…However, as information concerning the biological roles of taurine has accumulated, taurine has been illustrated to play many important physiological roles in animals, especially during their development. The possible functions of taurine currently proposed are as follows: conjugation of bile acids in the liver [31], improvement of fat absorption [1], maintenance of osmolarity [17], modulation of calcium levels [30], stabilization of membranes [12], antiarrhythmic activity in the heart [24], regulation of ion fluxes [7], and stimulation of sperm motility [20]. In the central nervous system, although the role of taurine is not well established, it has been proposed as a neurotransmitter or a neuromodulator [2].…”

The Influence of Taurine on the Bile Acid Maximum Secretory Rate in the Guinea Pig

“…This indicates that the availability of large amounts of taurine did not decrease the percent of free bile acids despite the fact that the single transferase conjugation enzyme has a greater affinity for taurine than for glycine (41). Although the extent of sulfation increases with cholestasis (42), bile salt TAURINE AND BILE ACID MAXIMUM SECRETORY RATE 3 7 sulfotransferase activity has not been found to be inducible (43).…”