Subcellular distribution and regulation of hepatic bilirubin UDP-glucuronyltransferase

We have investigated the subcellular location and regulation of hepatic bilirubin UDP-glucuronyltransferase, which has been presumed to be located largely in the smooth endoplasmic reticulum. Purity of subcellular membrane fractions isolated from rat liver was assessed by electron microscopy and marker enzymes. Bilirubin UDP-glucuronyltransferase activity was measured by radiochemical assay using a physiologic concentration of [14C]bilirubin, and formation rates of bilirubin diglucuronide and monoglucuronides (C-8 and C-12 isomers) were determined. Activity of the enzyme was widely distributed in subcellular membranes, the majority being found in smooth and rough endoplasmic reticulum, with small amounts in nuclear envelope and Golgi membranes. No measurable activity was found in plasma membranes or in cytosol. Synthesis of bilirubin diglucuronide as a percentage of total conjugates and the ratio of C-8/C-12 bilirubin monoglucuronide isomers formed were comparable in all membranes, suggesting that the same enzyme is present in all locations. However, the regulation of bilirubin UDP-glucuronyltransferase activity differed among intracellular membranes; enzyme activity measured in the presence of the allosteric effector uridine 5'-diphospho-N-acetylglucosamine exhibited latency in smooth endoplasmic reticulum and Golgi membranes, but not in rough endoplasmic reticulum and nuclear envelope. Since rough membranes comprise 60% of hepatocyte endoplasmic reticulum and bilirubin UDP-glucuronyltransferase activity in vitro is maximal in this membrane fraction under presumed physiologic conditions, it is likely that the rough endoplasmic reticulum represents the major site of bilirubin glucuronidation in hepatocytes.