Simultaneous expression of guinea pig UDP-glucuronosyltransferase 2B21 and2B22 in COS-7 cells enhances UDP-glucuronosyltransferase 2B21-catalyzed morphine-6-glucuronide formation

Although UDP-glucuronosyltransferases (UGTs) act as an important detoxifica tion system for many endogenous and exogenous compounds, they are also invo lved in the metabolic activation of morphine to form morphine-6-glucuronide (M-6-G). The cDNAs encoding guinea pig liver UGT2B21 and UGT2B22, which ar e intimately involved in M-6-G formation, have been cloned and characterize d. Although some evidence suggests that UGTs may function as oligomers, it is not known whether hetero-oligomer formation leads to differences in subs trate specificity. In this work, evidence for a functional heterooligomer b etween UGT2B21 and UGT2B22 is provided by studies on the glucuronidation of morphine in transfected COS-7 cells. Cells transfected with UGT2B21 cDNA c atalyzed mainly morphine-3-glucuronide formation although M-6-G was also fo rmed to some extent. In contrast, cells transfected with UGT2B22 cDNA did n ot show any significant activity toward morphine. When UGT2B21 and UGT2B22 were expressed simultaneously in different ratios in COS-7 cells, extensive M-6-G formation was observed. This stimulation of M-6-G formation was not observed, however, when microsomes containing UGT2B21were mixed with those containing UGT2B22 in the presence of detergent. Furthermore, this effect w as not very marked when human UGT1A1 and UGT2B21 were coexpressed in COS-7 cells. This is the first report suggesting that UGT hetero-oligomer formati on leads to altered substrate specificity.