The effect of streptozotocin-induced diabetes on glucuronyltransferase activity
The validity of the streptozotocin-induced diabetic rat as an animal model for juvenile-onset diabetes mellitus was demonstrated. Glucuronyltransferase (GT) activity was measured in hepatic microsomal preparations and perfused livers, isolated from control and streptozotocin treated rats, using four model substrates. Specific high pressure liquid chromatography methods were developed to quantify the conjugates formed. The glucuronidation of p-nitrophenol and a-naphthol (GT1 substrates) was deficient in freshly prepared 'native' microsomes from diabetic rats. The effect was not observed in female diabetic rats. Insulin treatment abolished the streptozotocin-induced defect, suggesting that it was due to hypoinsulinaemia accompanying the diabetic state. The glucuronidation of paracetamol and phenolphthalein (GT2 substrates) was not altered by streptozotocin treatment. Optimal activation by Triton X-100 abolished the streptozotocin-induced defect in microsomal p-nitrophenol glucuronidation. In contrast, optimal activation by the endogenous modulators, UDP N-acetylglucosamine and Mg2+, did not abolish this defect. The affinity of diabetic glucuronyltransferase for UDP N-acetylglucosamine was not altered, but the enzyme could not express maximal activation towards this agent. In diabetic rats, the Vmax of the 'native' p-nitrophenol glucuronyltransferase was decreased, but this was not associated with an alteration in the km values for either UDP-glucuronic acid or for p-nitrophenol. Increased membrane constraint was proposed to cause the diabetes-induced glucuronyltransferase defect. This constraint was removable by optimal detergent activation. 3-methylcholanthrene induction abolished the streptozotocin- induced defect of p-nitrophenol glucuronidation, whereas phenobarbitone did not. This was attributed to the differential effect of these inducers on microsomal membrane composition. Glucuronidation and sulphation of p-nitrophenol were decreased in perfused livers isolated from streptozotocin treated male rats, showing that the effect of diabetes was not an artifact of microsomal preparation. The intrinsic clearance of p-nitrophenol was greater in perfused livers than in microsomal preparations. This was attributed to specific activation of glucuronyltransferase in the hepatocytes by endogenous modulators, for example UDP N-acetyIglucosamine and Mg2+. The results indicate that glucuronide and sulphate conjugation may be deficient in diabetes mellitus. This may lead to increased incidence of drug-induced toxicity in diabetes patients. It is essential to investigate the substrate specificities of human glucuronyltransferases in order to identify compounds which will be eliminated more slowly in diabetes mellitus.