ABSTRACT:UDP-glucuronosyltransferase 2B7 (UGT2B7) is involved in the glucuronidation of a wide array of clinically important drugs and endogenous compounds in humans. The aim of this study was to identify an isoform-selective probe substrate that could be used to investigate genetic and environmental influences on glucuronidation mediated by UGT2B7. Three potential probe substrates [3-azido-3-deoxythymidine (AZT), morphine, and codeine], were evaluated using recombinant UGTs and human liver microsomes (HLMs; n ‫؍‬ 54). Of 11 different UGTs screened, UGT2B7 was the principal isoform mediating AZT glucuronidation, morphine-3-glucuronidation, and morphine-6-glucuronidation. Codeine was glucuronidated equally well by UGT2B4 and UGT2B7. Enzyme kinetic analysis of these activities typically showed higher apparent K m values for HLMs (pooled and individual) compared with UGT2B7.This difference was least (less than 2-fold higher K m ) for AZT glucuronidation and greatest (3-to 6-fold higher K m ) for codeine glucuronidation. Microsomal UGT2B7 protein content correlated well with AZT glucuronidation (r s ‫؍‬ 0.77), to a lesser extent with morphine-3-glucuronidation (r s ‫؍‬ 0.50) and morphine-6-glucuronidation (r s ‫؍‬ 0.51), but very weakly with codeine glucuronidation (r s ‫؍‬ 0.33). Livers were also genotyped for the UGT2B7*2 (H268Y) polymorphism. No effect of genotype on microsomal glucuronidation or UGT2B7 protein content was observed. In conclusion, although both AZT and morphine can serve as in vitro probe substrates for UGT2B7, AZT appears to be more selective than morphine. Codeine is not a useful UGT2B7 probe substrate because of significant glucuronidation by UGT2B4. The UGT2B7*2 polymorphism is not a determinant of glucuronidation of AZT, morphine, or codeine in HLMs.

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The purpose of this study was to generate, by real-time PCR, a quantitative expression level profile of the 19 human UDP-glucuronosyltransferases (UGTs) of subfamilies 1A, 2A and 2B, in 26 adult and 3 fetal tissues, for better understanding of their roles in xenobiotic and endobiotic metabolism. Adult liver contained the highest level of combined UGTs mRNA, and UGT2B4 was the most abundant isoform in this tissue (40% of total). Other well expressed hepatic UGTs, in decreasing order of mRNA level, were 1A9, 2B7, 1A4, 2B10, 1A1, 1A6, 2B11, 2B15, 1A3, 2A3, 2B17 and 2B28. UGT2B4 was by far the most abundant isoform in the fetal liver (90% of total). The combined UGT mRNA expression in both adult and fetal olfactory epithelium was high, about 20% the adult hepatic level. Interestingly, a large developmental change was found in this tissue from high UGT2A1 and UGT2A2 expression in the fetus to UGT1A6 in the adult. The most abundantly expressed UGTs in the small intestine were 2A3, 1A10, 1A1, 1A6 and 2B7, while 1A10 and 2A3 predominated in the colon. The results provide the most comprehensive data to date on the tissue distribution of the human UGTs.

Bupropion is primarily metabolized in human liver by cytochrome P450 (CYP) 2B6, an isoform that shows high interindividual variability in expression and catalysis. The aim of this study was to identify mechanisms underlying this variability through comprehensive phenotype-genotype analysis of a well-characterized human liver bank (n = 54). There was substantial variability in microsomal bupropion hydroxylation activities (over 45-fold) and CYP2B6 protein content (over 288-fold), with excellent correlation between protein and activity values (rs = 0.88). CYP2B6 mRNA levels showed less variability (13-fold) and poorer correlation (rs = 0.44) to CYP2B6 protein resulting from 20-30% of livers that contained substantial CYP2B6 mRNA, but low CYP2B6 protein. Livers were genotyped for the common coding polymorphisms (Q172H, K262R and R487C) and 14 additional variations identified by sequencing of the gene promoter to -3000 bp. Of 14 haplotypes that were inferred, *1A (reference), *1H (-2320t>c; -750t>c) and *6B (-1456t>c; -750t>c; Q172H; K262R) were most common with frequencies of 0.28, 0.20 and 0.26, respectively. Alcohol use history (P = 0.011) and *6B haplotype (P = 0.011) were identified as significant predictors of bupropion hydroxylation. A consideration of the effects of these variables on CYP2B6 mRNA and protein levels suggests that alcohol use is associated with enhanced CYP2B6 gene transcription, but the presence of at least one *6B allele reduces this effect on bupropion hydroxylation at the post-transcriptional level. In conclusion, the results of this study indicate that interindividual variability in bupropion hydroxylation is a consequence of interactions between environmental and genetic influences on CYP2B6 gene function.