UDP-Glucuronyltransferase in perfused rat liver and in microsomes

Abstract

Summary To elucidate the disparity between glucuronidation rates in vivo and UDP-glucuronyltransferase in vitro after CCl₄ injury, the time course of the effects of CCl₄ (0.25 ml/kg) on kinetic properties of UDP-glucuronyltransferase (l-naphthol as substrate) was examined in rat liver homogenates and microsomes. These experiments were compared with 1-naphthol glucuronidation by the perfused liver which was studied at various time points after CCl₄ administration. Phenobarbital-treated rats were used to enhance the hepatotoxicity of CCl₄. 1. Within 24 h UDP-glucuronyltransferase activity increased 8-fold in liver homogenates and 3-fold in microsomes. During this time the allosteric activator, UDP-N-acetylglucosamine, lost its effect whereas the inhibitor UDP showed greater inhibitory properties, thus counteracting the activation. 2. 1-Naphthol glucuronidation in perfused livers was significantly decreased by 24 h. Sulfate ester formation was little affected. 3. The content of UDP-glucuronic acid was not significantly altered although liver histology revealed about 45% necrotic and prenecrotic cells and an uniform fatty degeneration of hepatocytes after 24 h. The results suggest that during CCl₄ injury, UDP-glucuronyltransferase is activated. At the same time the kinetic properties of the enzyme are altered in a way leading to inefficient glucuronide synthesis, when assays are carried out under conditions presumed to exist in vivo. Nevertheless the capacity to form glucuronides is retained in the acutely injured liver.