title: Human UGT1A4 and UGT1A3 conjugate 25-hydroxyvitamin D3: metabolite structure, kinetics, inducibility, and interindividual variability.
abstract: 25-Hydroxyvitamin D3 (25OHD3) is used as a clinical biomarker for assessment of vitamin D status. Blood levels of 25OHD3 represent a balance between its formation rate and clearance by several oxidative and conjugative processes. In the present study, the identity of human uridine 5'-diphosphoglucuronyltransferases (UGTs) capable of catalyzing the 25OHD3 glucuronidation reaction was investigated. Two isozymes, UGT1A4 and UGT1A3, were identified as the principal catalysts of 25OHD3 glucuronidation in human liver. Three 25OHD3 monoglucuronides (25OHD3-25-glucuronide, 25OHD3-3-glucuronide, and 5,6-trans-25OHD3-25-glucuronide) were generated by recombinant UGT1A4/UGT1A3, human liver microsomes, and human hepatocytes. The kinetics of 25OHD3 glucuronide formation in all systems tested conformed to the Michaelis-Menten model. An association between the UGT1A4*3 (Leu48Val) gene polymorphism with the rates of glucuronide formation was also investigated using human liver microsomes isolated from 80 genotyped livers. A variant allele dose effect was observed: the homozygous UGT1A4*3 livers (GG) had the highest glucuronidation activity, whereas the wild type (TT) had the lowest activity. Induction of UGT1A4 and UGT1A3 gene expression was also determined in human hepatocytes treated with pregnane X receptor/constitutive androstane receptor agonists, such as rifampin, carbamazepine, and phenobarbital. Although UGT mRNA levels were increased significantly by all of the known pregnane X receptor/constitutive androstane receptor agonists tested, rifampin, the most potent of the inducers, significantly induced total 25OHD3 glucuronide formation activity in human hepatocytes measured after 2, but not 4 and 24 hours, of incubation. Finally, the presence of 25OHD3-3-glucuronide in both human plasma and bile was confirmed, suggesting that the glucuronidation pathway might be physiologically relevant and contribute to vitamin D homeostasis in humans.
authors.affiliation: Departments of Pharmaceutics (Z.W., T.W., K.E.T.) and Medicinal Chemistry (M.S.), University of Washington, Seattle, Washington 98195-7610; Faculty of Pharmacy (T.H.), Osaka Ohtani University, Tondabayashi, Osaka 584-8540, Japan; Department of Pharmacokinetics and Drug Metabolism (L.Z.D.), Amgen, Seattle, Washington 98119; Department of Biomedical Sciences (N.J.K., J.P.G.), College of Veterinary Medicine, Iowa State University, Ames, Iowa 50011; Heartland Assays, LLC (R.L.H.), Ames, Iowa 50010; and Department of Pharmaceutical Sciences (A.S.C., E.G.S.), St Jude Children's Research Hospital, Memphis, Tennessee 38105.
authors.forename: Zhican
authors.initials: Z
authors.lastname: Wang
authors.forename: Timothy
authors.initials: T
authors.lastname: Wong
authors.forename: Takanori
authors.initials: T
authors.lastname: Hashizume
authors.forename: Leslie Z
authors.initials: LZ
authors.lastname: Dickmann
authors.forename: Michele
authors.initials: M
authors.lastname: Scian
authors.forename: Nicholas J
authors.initials: NJ
authors.lastname: Koszewski
authors.forename: Jesse P
authors.initials: JP
authors.lastname: Goff
authors.forename: Ronald L
authors.initials: RL
authors.lastname: Horst
authors.forename: Amarjit S
authors.initials: AS
authors.lastname: Chaudhry
authors.forename: Erin G
authors.initials: EG
authors.lastname: Schuetz
authors.forename: Kenneth E
authors.initials: KE
authors.lastname: Thummel
mesh_terms: D002112:Calcifediol / Q000378:metabolism*; D014453:Glucuronosyltransferase / Q000378:metabolism*; D006801:Humans; D066298:In Vitro Techniques; D007700:Kinetics; D008862:Microsomes, Liver / Q000378:metabolism
chemical_list: C019787:bilirubin glucuronoside glucuronosyltransferase; C122075:UDP-glucuronosyltransferase, UGT1A3; D014453:Glucuronosyltransferase; D002112:Calcifediol