Metabolism and effect of 7-oxo-lithocholic acid 3-sulfate on bile flow and biliary lipid secretion in rats

Abstract

Significant amounts of oxo and sulfated bile acids are excreted in the urine of patients with cholestatic liver diseases, but these bile acid species are found only in traces in bile. Several possible explanations for this finding include poor canalicular secretion, rapid biotransformation by oxidoreduction and extrahepatic sulfation of oxo bile acids. To assess the physiological actions and hepatic metabolism of oxo and sulfated bile acids, we synthesized 7-oxo-lithocholic acid 3-sulfate, examined the effect of intravenous infusion of this bile acid (5.0 μmol/min/kg body wt) on bile flow and biliary lipid secretion in Sprague-Dawley rats with a bile fistula and compared this effect with that of the unconjugated analog, 7-oxo-lithocholic acid, and the primary bile acid conjugate, chenodeoxycholic acid 3-sulfate. Bile flow and bile acid secretion increased immediately after infusion of all three bile acids. The highly polar 7-oxo-lithocholic acid 3-sulfate significantly reduced the biliary secretion of phospholipids (p < 0.05) and cholesterol (p < 0.05), but by contrast the more hydrophobic analog, 7-oxo-lithocholic acid, caused a significant increase in the biliary phospholipids (p < 0.01) and cholesterol (p < 0.01) secretion. Infusion of chenodeoxycholic acid 3-sulfate also resulted in an increase in secretion of biliary cholesterol and phospholipids, but the magnitude of the change was less than that induced by 7-oxo-lithocholic acid infusion. Biliary lipid secretion was inversely related to the polarity of the infused bile acid. Negative ion fast atom bombardment—mass spectrometry and gas chromatography—mass spectrometry analyses indicated that the major bile acids excreted in the bile after infusion of 7-oxo-lithocholic acid were taurine and glycine conjugates of the parent compound and the corresponding conjugates of its metabolite, ursodeoxycholate, whereas chenodeoxycholate and its conjugates were minor metabolites. By contrast, 7-oxo-lithocholic acid 3-sulfate and chenodeoxycholic acid 3-sulfate were only partially amidated with taurine and glycine and did not undergo further metabolic transformation of the steroid nucleus. These data demonstrate that sulfation of 7-oxo-lithocholic acid significantly alters its physiological properties and hinders biotransformation, including oxidoreduction, hydroxylation and amidation. The findings that the sulfate conjugates of 7-oxo-lithocholic acid and chenodeoxycholic acid are efficiently and preferentially secreted into bile, and not into urine, would strongly indicate that the bile acid sulfates, found in relatively high proportions in the urine of patients with cholestasis, are of extrahepatic and most probably of renal origin.