Heterologous expression of active human uridine diphosphate glucuronosyltransferase 1A3 in Chinese hamster lung cells

Genetic variants of human UGT1A3: functional characterization and frequency distribution in a Chinese Han population

UDP-glucuronosyltransferase 1A3 (UGT1A3) contributes to glucuronidation of many important endogenous compounds and xenobiotics, including some flavonoids. Recently, a total of six single nucleotide polymorphisms (SNPs) have been identified in the human UGT1A3 gene. Among them, four SNPs (A17G, Q6R; T31C, W11R; C133T, R45W; and T140C, V47A) cause amino acid substitutions. Variants caused by these SNPs showed an activity change in estrone metabolism, whereas their activities toward other substrates were not examined. In the present study, three common flavonoids, quercetin, luteolin, and kaempferol, were used as substrates for glucuronidation by wild-type and variant UGT1A3s. Our results demonstrated that the activities of three variants, UGT1A3.2, UGT1A3.3, and UGT1A3.5, were remarkably lower than that of UGT1A3.1. In contrast, UGT1A3.4 exhibited an increase in glucuronidation efficiency of approximately 4 times and a clear preference to quercetin 7- and 3-hydroxyl groups. The frequency distributions of UGT1A3 alleles and SNPs in UGT1A3 in a Chinese Han population were statistically different from the reported value in German-Caucasians (p < 0.05). UGT1A3 variants have an altered glucuronidation activity toward quercetin, luteolin, and kaempferol and may alter human susceptibility to flavonoid exposure.

Uridine diphosphoglucuronosyltransferase pharmacogenetics and cancer

The uridine diphosphoglucuronosyltransferases (UGTs) belong to a superfamily of enzymes that catalyse the glucuronidation of numerous endobiotics and xenobiotics. Several human hepatic and extrahepatic UGT isozymes have been characterized with respect to their substrate specificity, tissue expression and gene structure. Genetic polymorphisms have been identified for almost all the UGT family members. A wide variety of anticancer drugs, dietary chemopreventives and carcinogens are known to be conjugated by members of both UGT1A and UGT2B subfamilies. This review examines in detail each UGT isozyme known to be associated with cancer and carcinogenesis. The cancer-related substrates for several UGTs are summarized, and the functionally relevant genetic polymorphisms of UGTs are reviewed. A number of genotype-phenotype association studies have been carried out to characterize the role of UGT pharmacogenetics in several types of cancer, and these examples are discussed here. In summary, this review focuses on the role of the human UGT genetic polymorphisms in carcinogenesis, chemoprevention and cancer risk.

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.