Stereoselective glucuronidation as a probe of induced forms of UDP-glucuronyltransferase in rabbits.

Stereoselective glucuronidation of the chiral drug oxazepam by hepatic microsomes from rabbits pretreated with various inducers was used to characterize induced UDP-glucuronyltransferase (GT) enzymes. The enzymes were induced by pretreatment of rabbits with phenobarbital, DDT, 3-methylcholanthrene, beta-naphthoflavone, ethanol, trans-stilbene oxide, pregnenolone-16 alpha-carbonitrile, Aroclor 1254, and clofibric acid. Hepatic microsomes from induced and control animals were incubated with racemic oxazepam and the activator, Lubrol PX. Relative amounts of the diastereomeric beta-glucuronides were determined by HPLC. Significant differences were found in enantiomeric selectivity among the microsomes from rabbits pretreated with different inducers. The largest difference in diastereomeric glucuronide ratios observed was between control (R/S = 0.76) and beta-naphthoflavone-induced microsomes (R/S = 1.41). This demonstrated a significant change in enantiomeric selectivity by the enzymes, changing from preference for the S enantiomer in controls to its R antipode after induction. Differences in stereoselectivity between two members of a class of inducers such as 3-methylcholanthrene (R/S = 1.16) and beta-naphthoflavone (R/S = 1.41), provided evidence for possible distinctions between these similar inducers. Inducers of GT1 UDP-glucuronyltransferase enzyme forms, such as 3-methylcholanthrene, beta-naphthoflavone, and Aroclor 1254, did not increase the rate of formation of oxazepam glucuronides. All other inducers except DDT increased the rate of oxazepam glucuronidation by 20-53% with the highest rate (0.23 nmol/min X mg protein) occurring after trans-stilbene oxide induction. UDP-N-acetylglucosamine was also used as an activator to provide a comparison of the Lubrol-activated microsomes with the endogenous activator. Relative differences among inducers were approximately the same with both activators.(ABSTRACT TRUNCATED AT 250 WORDS)