Interactions between human UGT1A1, UGT1A4, and UGT1A6 affect their enzymatic activities.

Protein-protein interactions between human UDP-glucuronosyltransferase (UGT) 1A1, UGT1A4, and UGT1A6 were investigated using double expression systems in HEK293 cells (UGT1A1/UGT1A4, UGT1A1/UGT1A6, and UGT1A4/UGT1A6). The substrates specific for UGT1A1 (estradiol and bilirubin), UGT1A4 (imipramine and trifluoperazine), and UGT1A6 (serotonin and diclofenac) were used to determine the effects of the coexpression of the other UGT1A isoforms on the enzymatic activity. The coexpression of UGT1A4 and UGT1A6 decreased the S(50) and V(max) values of UGT1A1-catalyzed estradiol 3-O-glucuronide formation and increased the V(max) value of UGT1A1-catalyzed bilirubin O-glucuronide formation. The coexpression of UGT1A1 decreased the V(max) value of UGT1A4-catalyzed imipramine N-glucuronide formation but had no effect on UGT1A4-catalyzed trifluoperazine N-glucuronide formation. The coexpression of UGT1A6 had no effect on UGT1A4-catalyzed imipramine N-glucuronide formation but increased the K(m) and V(max) of UGT1A4-catalyzed trifluoperazine N-glucuronide formation. The coexpression of both UGT1A1 and UGT1A4 increased the V(max) values of UGT1A6-catalyzed serotonin and diclofenac O-glucuronide formation. Thus, the effects of the coexpression of other UGT1A isoforms on the kinetics of specific activities were different depending on the UGT1A isoforms and substrates. Native polyacrylamide gel electrophoresis analysis of the double expression systems showed multiple bands at approximately 110 kDa, indicating the existence of heterodimers as well as homodimers of UGTs. In conclusion, we found that human UGT1A1, UGT1A4, and UGT1A6 interact with each other, possibly by heterodimerization, and that their effects on the enzymatic activities are complex depending on the isoforms and substrates.