STUDIES ON ACTIVATION IN VITRO OF GLUCURONYLTRANSFERASE

1. 1. The relationship between the different activating principles acting of glucuronyltransferase (UDP-glucuronate glucuronyltransferase EC 2.4.1.17) in vitro has been studied using five acceptor substrates (bilirium o-aminophenol, 4-methylumbelliferone, p-nitrophenol and phenolphthalein) in assay of enzyme activity 2. 2. Preincubation of mouse and rat-liver suspensions in vitro resulted in an increased activity of glucuronyltransferase which was highly variable depending on the acceptor substrate used. While 4-methylumbelliferone glucuronyltransferase exhibited an activity that was 45 times the initial one at maximum, the o-amninophenol enzyme was not activated by this procedure 3. 3. Detergents (Triton X-roo and digitonin) and UDP-N-acetylglucosamine were also found to activate glucuronyltransferase; only bilirubin glucuronyltransferase was not activated by UDP-N-acetylglucosamine 4. 4. Activation of glucuronyltransferase could not be increased by combining preincubation, detergents or UDP-N-acetylglucosamine, whereas activation of ratliver o-aminophenol gluconyltransferase by detergents and UDP-N-acetylglucosamine was strongly potentiated by diethylnitrosamine 5. 5. The slightly different kinetic of nonactivated and activated enzyme that were found at varying substrate (UDP-glucuronic acid, p-nitrophenol concentrations could not explain the activation observed 6. 6. The results are compatible with an activation by preincubation, detergents and UDP-N-acetylglucosamine due to exposition of active sites of glucuronyltransferases that have been nonfunctioning in unactivated tissue homogenates 7. 7. The degree of activation at constant detergent concentration was the same over a wide range of enzyme-protein concentrations 8. 8. The pH optimum for detergent-activated p-nitrophenol glucuronyltransferase was found at 6.2 6.6 while the activity towards bilirubin and o-aminophenol was maximum at pH 7.6. Detergent-activated 4-methylumbelliferone and phenolphthalein glucuronyltransferases, on the other hand, revealed about the same activity in the range between pH 6.2 and 7.6. © 1969.