title: Human CYP2C9 and CYP2A6 mediate formation of the hepatotoxin 4-ene-valproic acid.
abstract: Cytochrome P450-dependent desaturation of the anticonvulsant drug valproic acid (VPA) results in formation of the hepatotoxin, 4-ene-VPA. Polytherapy with other anticonvulsants which are known P450 inducers increases the flux through this bioactivation pathway. The aim of the present study was to identify specific, inducible forms of human liver P450 which catalyze terminal desaturation of VPA. Oxidized VPA metabolites formed in an NADPH-dependent manner by human liver microsomes were quantified by gas-chromatography/mass spectrometry. In vitro reaction conditions were established which reflected the product profile found in vivo. Production of 4-ene-VPA by microsomal P450s could be inhibited significantly by coumarin, sulfaphenazole and diethyldithiocarbamate, but not by triacetyloleandomycin, quinidine or furafylline. Recombinant human CYP3A4 did not form detectable levels of 4-ene-VPA and, of nine additional isoforms expressed in either HepG2 or lymphoblastoid cells which were screened for VPA desaturase activity, only CYP2C9 and CYP2A6 formed detectable levels of metabolite. Consequently, CYP3A4, the isoform usually associated with induction by anticonvulsants cannot be responsible for the enhanced 4-ene-VPA formation that occurs during polytherapy. Instead, enhanced activity in vivo likely results from induction of CYP2A6 and/or CYP2C9.
authors.affiliation: Department of Medicinal Chemistry, University of Washington, Seattle, Washington 98195, USA.
authors.forename: A J
authors.initials: AJ
authors.lastname: Sadeque
authors.forename: M B
authors.initials: MB
authors.lastname: Fisher
authors.forename: K R
authors.initials: KR
authors.lastname: Korzekwa
authors.forename: F J
authors.initials: FJ
authors.lastname: Gonzalez
authors.forename: A E
authors.initials: AE
authors.lastname: Rettie
mesh_terms: D000293:Adolescent; D000328:Adult; D000927:Anticonvulsants / Q000378:metabolism*; D001189:Aryl Hydrocarbon Hydroxylases; D002648:Child; D065637:Cytochrome P-450 CYP2A6; D065729:Cytochrome P-450 CYP2C9; D003577:Cytochrome P-450 Enzyme System / Q000502:physiology*; D005229:Fatty Acids, Monounsaturated / Q000378:metabolism* / Q000633:toxicity; D005260:Female; D006801:Humans; D008099:Liver / Q000187:drug effects* / Q000378:metabolism; D008297:Male; D008875:Middle Aged; D006899:Mixed Function Oxygenases / Q000502:physiology*; D039181:Steroid 16-alpha-Hydroxylase; D013250:Steroid Hydroxylases / Q000502:physiology*; D014635:Valproic Acid / Q000378:metabolism*
chemical_list: D000927:Anticonvulsants; D005229:Fatty Acids, Monounsaturated; C045022:2-propyl-4-pentenoic acid; D014635:Valproic Acid; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; D013250:Steroid Hydroxylases; C450260:CYP2C9 protein, human; D065729:Cytochrome P-450 CYP2C9; D001189:Aryl Hydrocarbon Hydroxylases; C585710:CYP2A6 protein, human; D065637:Cytochrome P-450 CYP2A6; D039181:Steroid 16-alpha-Hydroxylase