Glucuronidation catalyzed by UDP-glucuronosyltransferase (UGT) enzymes is an important pathway in the metabolism of drugs as well as environmental chemicals. In this study, protein-protein interactions between human UGT2B7 and UGT1As and their effects on the enzymatic activities were investigated using double expression systems in HEK293 cells (UGT2B7/UGT1A1, UGT2B7/UGT1A4, UGT2B7/UGT1A6, and UGT2B7/UGT1A9). Native-PAGE analysis clearly revealed that UGT2B7 forms homo-oligomers. Furthermore, hetero-oligomers of UGT2B7 with UGT1As were observed by native-PAGE analysis. Immunoprecipitation assay revealed associations of UGT2B7 with UGT1A1, UGT1A4, UGT1A6, and UGT1A9. The thermal stability of UGT2B7 was significantly increased by the coexpressed UGT1A1, UGT1A4, UGT1A6, and UGT1A9, indicating an interaction between UGT2B7 and the UGT1As. To examine the effects of the protein-protein interactions on the enzymatic activities, kinetic analyses were performed. Coexpression of the UGT1As significantly decreased K(m) and increased V(max) of zidovudine O-glucuronidation by UGT2B7. Coexpression of UGT2B7 also affected the kinetics of estradiol 3-O-glucuronidation by UGT1A1, imipramine N-glucuronidation by UGT1A4, serotonin O-glucuronidation by UGT1A6, and propofol O-glucuronidation by UGT1A9. In conclusion, it was clearly demonstrated that human UGT2B7 interacts with UGT1A enzymes, affecting their kinetics. That such interactions might occur in human liver microsomes underscores the complexities in glucuronidations in human liver.