Tauroursodeoxycholate increases rat liver ursodeoxycholate levels and limits lithocholate formation better than ursodeoxycholate.

Background & Aims: To explain the greater hepatoprotective effect of tauroursodeoxycholic acid vs. ursodeoxycholic acid, the absorption, hepatic enrichment, and biotransformation of these bile acids (250 mg/day) were compared in rats.
Methods: Bile acids were determined in intestinal contents, feces, urine, plasma, and liver by gas chromatography-mass spectrometry.
Results: The concentration of ursodeoxycholate in the liver of animals administered tauroursodeoxycholic acid (175 +/- 29 nmol/g) was greater (P < 0.05) than in animals administered ursodeoxycholic acid (79 +/- 19 nmol/g). Hepatic lithocholate was substantially higher after ursodeoxycholic acid administration (21 +/- 10 nmol/g) than after tauroursodeoxycholic acid administration (12 +/- 1 nmol/g). A concomitant reduction in the proportion of hydrophobic bile acids occurred that was greatest during tauroursodeoxycholic acid administration. In the intestinal tract, the mass of ursodeoxycholate and its specific metabolites was greater in rats administered tauroursodeoxycholic acid (27.2 mg) than those administered ursodeoxycholic acid (13.2 mg). In feces, the proportion of lithocholate was 21.9% +/- 4.9% and 5.4% +/- 4.0% after ursodeoxycholic acid and tauroursodeoxycholic acid administration, respectively.
Conclusions: Compared with ursodeoxycholic acid, tauroursodeoxycholic acid induces a greater decrease in the percent composition of more hydrophobic bile acids within the pool, limits lithocholate formation, and increases hepatic ursodeoxycholate concentration. These differences are explained by increased hepatic extraction and reduced intestinal biotransformation and not by enhanced absorption of the amidated species.