Taurine increases bile acid pool size and reduces bile saturation index in the hamster1

There is evidence that increased availability of taurine enhances the proportion of taurine-conjugated bile acids in bile. To explore the possibility that taurine treatment could also influence hepatic cholesterol and bile acid metabolism, we fed female hamsters for 1 week and measured both the biliary lipid content and the microsomal level of the rate-limiting enzymes of cholesterol and bile acid synthesis. In these animals the cholesterol 7a-hydroxylase activity was significantly greater in respect to controls (P < 0.05). The total HMG-CoA reductase activity, as well as that of the active form, was similarly increased. The stimulation of 7a-hydroxycholesterol synthesis was associated with an expansion of the bile acid pool size in taurine-fed animals. Taurine feeding was observed to induce an increase in bile flow as well as in the rate of excretion of bile acids, whereas the secretion rate of cholesterol in bile was decreased. As a consequence, the saturation index was significantly lower in taunnefed animals (P < 0.05). The possible mechanisms through which taurine exhibits the modification of the enzyme activities and of the biliary lipid composition are discussed.Bellentani, S., M. Pecorari, P. Cordoma, P. Marchegiano, F. Manenti, E. Bosisio, E. De Fabiani, and G. Galli. Taurine increases bile acid pool size and reduces bile saturation index in the hamster. J. Lipid Res. 1987. 28: 1021 1027. Supplementary key words tase cholesterol 7a-hydroxylase biliary saturation index bile acids taurine HMG-CoA reducBile acids are conjugated with glycine and taurine before their secretion in bile. In the hamster and in humans, taurine is primarily of dietary origin (1) and normally the hepatic taurine conjugation of bile acids accounts for about 30-35% of the total bile acid pool in these species. Glycine and taurine bile acid derivatives possess different physicochemical properties which partly account for their functional and metabolic differences. Taurine-conjugated bile acids are more water-soluble and less toxic than glyco-conjugated bile acids (2). In rats, for example, the glyco-, but not the tauro-conjugated form of sulfolithocholate induces cholestasis (3). Furthermore, taurine feeding has been shown to prevent cholestasis induced by lithocholic acid sulfate in the guinea pig (4). A reversal of glycine to taurine conjugation ratio (G/T ratio), which is easily obtained by prolonged feeding of taurine (5, 6), has been suggested to be particularly beneficial in humans during ursodeoxycholate treatment for cholesterol gallstone dissolution (7) inasmuch as the tauro-conjugate of ursodeoxycholic acid is more capable of solubilizing lipids than the glycine conjugate (8). Evidence that taurine administration can also affect cholesterol and bile acid metabolism in humans has already been reported, but results were conflicting. Okamoto et al. (9) found an increase of bile salts and a decrease of cholesterol in the duodenal bile of taurinesupplemented preterm infants. On the other hand, Watkins et al. (10) showed that the rate of bile acid synthesis was unmodified in low-birth-weight infants. Finally, in the only study available in adult humans, Hardison and Grundy (6) , by performing cholesterol balance studies in six volunteers fed taurine, did not find any remarkable modification of cholesterol and bile acid pool size, turnover and synthesis rate. The lack of the effect in this case may, however, depend on the fact that the dietary intake of taurine was not controlled; therefore, it is difficult to quantitate the exact amount of taurine consumed by those