Aim: To investigate the effects of two nonsynonymous SNPs, UGT1A4*2 (rs#: 6755571, 70C>A, P24T) and UGT1A4*3 (rs#: 2011425, 142T>G, L48V), on the function of UGT1A4 against dihydrotestosterone (DHT), transandrosterone (t-AND), lamotrigine (LTG) and tamoxifen (TAM).
Materials & methods: Detailed kinetic experiments were conducted with recombinant UGT1A4(wild-type), UGT1A4(P24T) and UGT1A4(L48V), which were overexpressed in HEK293 cell lines. The kinetic profiles and kinetic parameters (K(m), V(max) and CL(int)) obtained with either UGT1A4(P24T) or UGT1A4(L48V) were compared with those obtained with the wild-type enzyme. The interaction of TAM on UG1A4-catalyzed DHT glucuronidation was also investigated with the three UGT1A4 polymorphic enzymes.
Results: UGT1A4(L48V) had higher enzyme efficiency (CL(int)) compared with wild-type UGT1A4 on DHT glucuronidation; UGT1A4(P24T) and UGT1A4(L48V) had lower CL(int) than wild-type UGT1A4 for t-AND and LTG glucuronidation. The TAM CL(int) with UGT1A4(P24T) and UGT1A4(L48V) glucuronidation and the UGT1A4(P24T)-catalyzed DHT glucuronidation were, on the other hand, similar to those of the wild-type enzyme. With all three enzymes, TAM activated UGT1A4-catalyzed DHT glucuronidation in a concentration-dependent fashion.
Conclusion: Decreased CL(int) of UGT1A4(P24T) and UGT1A4(L48V) on LTG glucuronidation may lead to interindividual variations in LTG metabolism in vivo. However, it is less likely that these polymorphisms would have impact on DHT and t-AND metabolism in vivo because these compounds are glucuronidated by multiple enzymes.