Abstract
Differential induction with phenobarbital (PB) and 3-methylcholanthrene (3-MC) suggests at least two functionally distinct UDP glucuronosyltransferases (UDP-GT) which have different acceptor selectivities. One form is induced by 3-MC and preferentially conjugates group 1 acceptors, such as p-nitrophenol and 1-naphthol. Another UDP-GT is induced by PB and glucuronidates group 2 aglycones, morphine and chloramphenicol. To further study this functional heterogeneity, male Sprague-Dawley rats were pretreated with the following microsomal enzyme inducers: 7,8-benzoflavone (BF); benzo( a)pyrene (BP); 3-MC; 2,3,7,8-tetrachlorodibenzo- p-dioxin (TCDD); butylated hydroxyanisole (BHA); isosafrole; PB; pregnenolone-16α-carbonitrile (PCN); trans-stilbene oxide (TSO). The effect of induction on UDP-GT activity was determined with nine acceptors. Conjugation of group 1 aglycones, naphthol and p-nitrophenol, was increased by 3-MC (185 and 80%, respectively) whereas PB was ineffective. Conjugation of group 2 acceptors, morphine and chloramphenicol, was stimulated by PB (120 and 250%, respectively) while 3-MC had little effect. BP and TCDD enhanced glucuronidation of group 1 aglycones. ISF and TSO induced conjugation of both acceptor groups but were more effective for group 2. BF and BHA had negligible effects on UDP-GT activity. Since glucuronidation of valproic acid was increased only by PB and TSO treatment, this aglycone is probably a group 2 acceptor. Conjugation of digitoxigenin-monodigitoxoside (DIG) was stimulated by PB (200%) and PCN (1200%). PCN did not induce glucuronidation of group 1 acceptors but did have a slight effect on group 2 aglycones (130 and 40% for chloramphenicol and morphine, respectively). The 12-fold increase in DIG conjugation by PCN pretreated rats suggests that PCN may induce another group (form) of UDP-GT which preferentially glucuronidates DIG. Differential induction of UDP-GT activities within each group of acceptors indicates possible additional heterogeneity of the transferase.
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