Induction and activation of rat liver microsomal bile salt glucuronyltransferase

In vivo induction and in vitro activation of the recently described bile salt glucuronyltransferase were investigated in rat. A radioactive assay for the determination of glucuronyltransferase activity was used. 14C-Labeled bile salts served as substrates, and the glucuronides were separated by thin layer chromatography. Lithocholate glucuronyltransferase activity was determined in liver microsomes of phenobarbital- and 3-methylcholanthrene-treated rats and of untreated controls. Pretreatment with phenobarbital induced lithocholate glucuronyltransferase activity to 150.5% of controls. In contrast, 3-methylcholanthrene treatment decreased activity to 29.6% of controls. In vivo activation of lithocholate glucuronyltransferase by Triton X-100 was observed in controls and in the 3-methylcholanthrene group, but not in the phenobarbital group. Substrate activation of the enzyme by lithocholate was demonstrated in microsomes of untreated controls. Pretreatment with 3-methylcholanthrene, but not phenobarbital, increased the latency of lithocholate glucuronyltransferase. The results indicate that rat liver microsomal bile salt glucuronyltransferase activity is increased by in vivo induction with phenobarbital and by in vitro activation with detergents like Triton X-100. The induction of bile salt glucuronide formation by phenobarbital is most likely one of the factors contributing to the increased biliary and fecal excretion of bile salts in patients with cholestasis following phenobarbital therapy.