title: Identification and characterization of human UDP-glucuronosyltransferases responsible for the glucuronidation of fraxetin.
abstract: Fraxetin, a major constituent of the traditional medicine plant Fraxinus rhynchophylla Hance (Oleaceae), has been found to possess multiple bioactivities. However, the metabolic pathway(s) of fraxetin in human tissues has not been reported yet. This study aimed to characterize the glucuronidation pathway(s) of fraxetin in human tissues. Fraxetin could be metabolized to two glucuronides in human liver microsomes (HLMs). These two glucuronides were biosynthesized and characterized as 7-O-glucuronide (7-O-G) and 8-O-glucuronide (8-O-G). UGT1A1, -1A6, -1A7, -1A8, -1A9 and -1A10 participated in the formation of 7-O-G, while the formation of 8-O-G was catalyzed selectively by UGT1A6 and UGT1A9. UGT1A9 showed the highest catalytic activities in the formation of 7-O-G and 8-O-G. Both kinetic characterization and inhibition assays demonstrated that UGT1A9 played important roles in fraxetin glucuronidations in HLMs, especially in the formation of the major metabolite 8-O-G. Furthermore, the intrinsic clearance of fraxetin in both human liver microsomes and UGT1A9 was greater than that of 7,8-dihydroxylcoumarin, revealing that the addition of a C-6 methoxy group led to the higher metabolic clearance. In summary, the glucuronidation pathways of fraxetin in human liver microsomes were well-characterized, and UGT1A9 was the major isoform responsible for the glucuronidations of fraxetin.
authors.affiliation: Laboratory of Pharmaceutical Resource Discovery, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
authors.forename: Yang-Liu
authors.initials: YL
authors.lastname: Xia
authors.forename: Si-Cheng
authors.initials: SC
authors.lastname: Liang
authors.forename: Liang-Liang
authors.initials: LL
authors.lastname: Zhu
authors.forename: Guang-Bo
authors.initials: GB
authors.lastname: Ge
authors.forename: Gui-Yuan
authors.initials: GY
authors.lastname: He
authors.forename: Jing
authors.initials: J
authors.lastname: Ning
authors.forename: Xia
authors.initials: X
authors.lastname: Lv
authors.forename: Xiao-Chi
authors.initials: XC
authors.lastname: Ma
authors.forename: Ling
authors.initials: L
authors.lastname: Yang
authors.forename: Sheng-Li
authors.initials: SL
authors.lastname: Yang
mesh_terms: D001711:Biotransformation; D003374:Coumarins / Q000378:metabolism*; D020719:Glucuronides / Q000378:metabolism*; D014453:Glucuronosyltransferase / Q000378:metabolism*; D006801:Humans; D007527:Isoenzymes; D007700:Kinetics; D008099:Liver / Q000201:enzymology*; D008657:Metabolic Clearance Rate; D008862:Microsomes, Liver / Q000201:enzymology; D011994:Recombinant Proteins / Q000378:metabolism; D013379:Substrate Specificity; D000090264:UDP-Glucuronosyltransferase 1A9
chemical_list: D003374:Coumarins; D020719:Glucuronides; D007527:Isoenzymes; D011994:Recombinant Proteins; C000715591:UGT1A9 protein, human; C105671:fraxetin; D014453:Glucuronosyltransferase; D000090264:UDP-Glucuronosyltransferase 1A9