title: Oxidative metabolism of omeprazole in human liver microsomes: cosegregation with S-mephenytoin 4'-hydroxylation.
abstract: Oxidative metabolism of omeprazole (OPZ) was studied in 14 human liver microsomes in relation to the 4'-hydroxylation capacity of S-mephenytoin. The formation of 5-hydroxyomeprazole and omeprazole sulfone (OPZ-SFN) from OPZ exhibited a biphasic kinetic behavior, indicating that at least two distinct enzymes are involved in either of the metabolic pathways of OPZ. By using a two-enzyme kinetic approach, the activities were described by high (Km1 and Vmax1)- and low-affinity components (Km2 and Vmax2). The respective mean (+/- S.D.) kinetic parameters for 5-hydroxylation and sulfoxidation were: Km1 = 6.0 +/- 2.4 and 10.2 +/- 7.2 microM and Vmax1 = 88.0 +/- 70.2 and 66.9 +/- 53.9 pmol/mg/min; Km2 = 106 +/- 127 and 482 +/- 472 microM and Vmax2 = 116 +/- 88 and 299 +/- 131 pmol/mg/min. Among these kinetic parameters, only the Vmax1 of 5-hydroxylation gave a close correlation with the corresponding parameter of S-mephenytoin (rs = 0.911, P < .01). In addition, OPZ and S-mephenytoin inhibited competitively each other's metabolism with the respective Ki values of 2.0 and 162 microM. Interestingly, OPZ-SFN also inhibited competitively 4'-hydroxylation of S-mephenytoin with a Ki value of 8.2 microM. Moreover, polyclonal antibodies raised against S-mephenytoin 4'-hydroxylase (P450 form) partially inhibited the 5-hydroxylation of OPZ, whereas no inhibition was observed for the sulfoxidation. These findings suggest that S-mephenytoin 4'-hydroxylase is an enzyme primarily responsible for the 5-hydroxylation of OPZ and further metabolism of OPZ-SFN, but not for the sulfoxidation of OPZ in human liver microsomes.(ABSTRACT TRUNCATED AT 250 WORDS)
authors.affiliation: Division of Clinical Pharmacology, National Medical Center, Tokyo, Japan.
authors.forename: K
authors.initials: K
authors.lastname: Chiba
authors.forename: K
authors.initials: K
authors.lastname: Kobayashi
authors.forename: K
authors.initials: K
authors.lastname: Manabe
authors.forename: M
authors.initials: M
authors.lastname: Tani
authors.forename: T
authors.initials: T
authors.lastname: Kamataki
authors.forename: T
authors.initials: T
authors.lastname: Ishizaki
mesh_terms: D053799:2-Pyridinylmethylsulfinylbenzimidazoles; D001189:Aryl Hydrocarbon Hydroxylases; D065731:Cytochrome P-450 CYP2C19; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D006801:Humans; D006900:Hydroxylation; D066298:In Vitro Techniques; D007700:Kinetics; D008862:Microsomes, Liver / Q000201:enzymology / Q000378:metabolism*; D006899:Mixed Function Oxygenases / Q000378:metabolism*; D009853:Omeprazole / Q000031:analogs & derivatives / Q000378:metabolism*; D010084:Oxidation-Reduction; D013455:Sulfur / Q000378:metabolism
chemical_list: D053799:2-Pyridinylmethylsulfinylbenzimidazoles; D013455:Sulfur; C040450:omeprazole sulfone; D003577:Cytochrome P-450 Enzyme System; C049963:5-hydroxymethylomeprazole; D006899:Mixed Function Oxygenases; D001189:Aryl Hydrocarbon Hydroxylases; C045793:CYP2C19 protein, human; D065731:Cytochrome P-450 CYP2C19; D009853:Omeprazole