Subcellular localization of UDP-glucuronyltransferase by differential centrifugation. Changes produced by pretreatment of rats with secretin, glucagon, vasoactive intestinal polypeptide and phenobarbitone

Subcellular fractionation of liver homogenates from treated rats was carried out in order to study the mechanism of action of the gastrointestinal polypeptides on glucoronidation. Rats were treated for 90 min with an intravenous infusion of secretin (0.4 cU/h/100 g body weight), glucagon (100 micrograms/h/100 g body weight) and vasoactive intestinal polypeptide (VIP) (300 ng/h/100 g body weight); controls were sham-treated rats. For comparison, another group of animals was treated with a daily injection of phenobarbitone (10 mg/kg), a well-established enzyme inducer. Treatment with the different polypeptides produced minor changes in the subcellular localization of the enzyme. The bulk of activity was always recovered in the microsomal fraction, as identified by both differential centrifugation and the enrichment in specific activity of glucose-6-phosphatase, esterase and NADPH-cytochrome c reductase. Secretin produced a specific increase of bilirubin glucuronidation, more evident in all nuclear fractions. Glucagon increased both bilirubin and p-nitrophenol glucuronidation in all subcellular fractions. VIP had a selective action on p-nitrophenol conjugation of similar extent in nuclear and microsomal fractions. The type of changes observed is suggestive of physicochemical modifications occurring into the cell, perhaps at the membrane environment of different organelles, able to modify the overall conjugation of different substrates by the cell.