title: 4-hydroxyretinoic acid, a novel substrate for human liver microsomal UDP-glucuronosyltransferase(s) and recombinant UGT2B7.
abstract: It is suggested that formation of more polar metabolites of all-trans-retinoic acid (atRA) via oxidative pathways limits its biological activity. In this report, we investigated the biotransformation of oxidized products of atRA via glucuronidation. For this purpose, we synthesized 4-hydroxy-RA (4-OH-RA) in radioactive and nonradioactive form, 4-hydroxy-retinyl acetate (4-OH-RAc), and 5,6-epoxy-RA, all of which are major products of atRA oxidation. Glucuronidation of these retinoids by human liver microsomes and human recombinant UDP-glucuronosyltransferases (UGTs) was characterized and compared with the glucuronidation of atRA. The human liver microsomes glucuronidated 4-OH-RA and 4-OH-RAc with 6- and 3-fold higher activity than atRA, respectively. Analysis of the glucuronidation products showed that the hydroxyl-linked glucuronides of 4-OH-RA and 4-OH-RAc were the major products, as opposed to the formation of the carboxyl-linked glucuronide with atRA, 4-oxo-RA, and 5,6-epoxy-RA. We have also determined that human recombinant UGT2B7 can glucuronidate atRA, 4-OH-RA, and 4-OH-RAc with activities similar to those found in human liver microsomes. We therefore postulate that this human isoenzyme, which is expressed in human liver, kidney, and intestine, plays a key role in the biological fate of atRA. We also propose that atRA induces its own oxidative metabolism via a cytochrome P450 (CYP26) and is further biotransformed into glucuronides via UGT-mediated pathways.
authors.affiliation: Departments of Toxicology and Pharmacology, University of Arkansas for Medical Sciences, Little Rock, Arkansas 72205, USA.
authors.forename: V M
authors.initials: VM
authors.lastname: Samokyszyn
authors.forename: W E
authors.initials: WE
authors.lastname: Gall
authors.forename: G
authors.initials: G
authors.lastname: Zawada
authors.forename: M A
authors.initials: MA
authors.lastname: Freyaldenhoven
authors.forename: G
authors.initials: G
authors.lastname: Chen
authors.forename: P I
authors.initials: PI
authors.lastname: Mackenzie
authors.forename: T R
authors.initials: TR
authors.lastname: Tephly
authors.forename: A
authors.initials: A
authors.lastname: Radominska-Pandya
mesh_terms: D020719:Glucuronides / Q000378:metabolism; D014453:Glucuronosyltransferase / Q000378:metabolism*; D006801:Humans; D007700:Kinetics; D008862:Microsomes, Liver / Q000201:enzymology*; D011994:Recombinant Proteins / Q000378:metabolism; D014212:Tretinoin / Q000031:analogs & derivatives* / Q000378:metabolism / Q000494:pharmacology
chemical_list: D020719:Glucuronides; D011994:Recombinant Proteins; D014212:Tretinoin; C019429:4-hydroxyretinoic acid; D014453:Glucuronosyltransferase