Quantitative regioselectivity of glucuronidation of quercetin by recombinant UDP-glucuronosyltransferases 1A9 and 1A3 using enzymatic kinetic parameters.

Quercetin has been suggested to exert its pharmacological effects, at least in part, via its metabolites, such as glucuronides. Quantitative regioselectivity analyses are important to understand the contributions of UDP-glucuronosyltransferases (UGTs) to the pharmacological activity of quercetin. The present work obtained active UGT1A9 and UGT1A3 enzymes with a Bac-to-Bac expression system, and quercetin was metabolized by each of them to four monoglucuronides (7-, 3-, 4'- and 3'-glucuronide). Enzymatic kinetic parameters of each glucuronide were calculated to elucidate quantitatively UGT1A9's and UGT1A3's regioselectivities for quercetin. UGT1A3's highest glucuronidation efficiency was observed for the 3'-glucuronide, then the 3-, 4'- and 7-glucuronide. The catalytic efficiency order for UGT1A9 was 3->7->3'->4'-glucuronide.