The activity, expression and localization of the UDP-glucuronosyltransferases (UGTs) were investigated in human placenta at term. UGT activity (measured with the substrate 4-methylumbelliferone (4-MU)) was observed in all 25 placentas sampled and maximum velocity (V(max)) ranged 13-fold from 5.1+/-0.9 to 66.9+/-17.5 nmol/min/mg protein (mean+/-SD). Substrate affinity (K(m)) ranged 5-fold from 246+/-24 to 1124+/-422 microM. Using reverse transcriptase-polymerase chain reaction (RT-PCR), expression of the isoforms UGT2B4, 2B7, 2B10, 2B11 and 2B15 was observed in all (12/12) placentas sampled and expression of UGT2B17 was noted in 8/12 placentas. Northern analysis of the UGT2B7 isoform in 12 placentas revealed a 10-fold difference in expression with RT-PCR variability and the 13-fold variation observed in UGT activity. The presence of UGT2B4 and 2B7 proteins (52 and 56kDa, respectively) was demonstrated by Western blotting. The sites of placental UGT2B transcription (in situ hybridization) and protein expression (immunohistochemistry) were located in the syncytium of the placental trophoblasts bordering the placental villi. UGT1A proteins could not be observed with immunohistochemistry or Western blotting and expression could not be observed with RT-PCR. Our discovery of UGT expression and activity at the site of maternal-fetal exchange is consistent with a role for UGTs in detoxification of exogenous and endogenous ligands and the maintenance of placental function through clearance and regulation of steroid hormones.