title: The role of S-mephenytoin 4'-hydroxylase in imipramine metabolism by human liver microsomes: a two-enzyme kinetic analysis of N-demethylation and 2-hydroxylation.
abstract: 1. The metabolism of imipramine (N-demethylation and 2-hydroxylation) was studied in relation to the activity of S-mephenytoin 4'-hydroxylase in human liver microsomes. 2. Eadie-Hofstee plots for the formation of despiramine and 2-hydroxyimipramine were biphasic, suggesting that at least two enzymes are involved in both the N-demethylation and 2-hydroxylation of imipramine by human liver microsomes. 3. The respective mean (+/- s.d.) kinetic parameters for the N-demethylation and 2-hydroxylation of imipramine derived from a two-enzyme kinetic analysis were: Km1 = 1.1 +/- 0.4 and 1.6 +/- 0.6 microM, Vmax1 = 0.11 +/- 0.03 and 0.15 +/- 0.07 nmol mg-1 min-1, and Vmax1/Km1 = 0.10 +/- 0.02 and 0.09 +/- 0.04 ml mg-1 min-1; Km2 = 214 +/- 84 and 257 +/- 148 microM, Vmax2 = 2.22 +/- 0.69 and 0.53 +/- 0.15 nmol mg-1 min-1, and Vmax2/Km2 = 0.011 +/- 0.001 and 0.003 +/- 0.002 ml mg-1 min-1. 4. With regard to imipramine N-demethylation and 2-hydroxylation at 2 microM (representing high-affinity reactions) and at 400 microM (representing low-affinity reactions), only N-demethylation at 2 microM showed a close correlation with the 4'-hydroxylation of S-mephenytoin (rs = 0.952, P < 0.01; n = 10 livers). 5. Concentrations up to 250 microM S-mephenytoin inhibited the N-demethylation of imipramine (2 microM), but no further inhibition was observed using concentrations from 250 to 750 microM. 6. Imipramine inhibited S-mephenytoin 4'-hydroxylation competitively with a Ki value of 12.5 microM.(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: A
authors.initials: A
authors.lastname: Saitoh
authors.forename: E
authors.initials: E
authors.lastname: Koyama
authors.forename: M
authors.initials: M
authors.lastname: Tani
authors.forename: M
authors.initials: M
authors.lastname: Hayashi
authors.forename: T
authors.initials: T
authors.lastname: Ishizaki
mesh_terms: D001189:Aryl Hydrocarbon Hydroxylases; D065731:Cytochrome P-450 CYP2C19; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D003891:Desipramine / Q000378:metabolism; D006801:Humans; D006900:Hydroxylation; D007099:Imipramine / Q000031:analogs & derivatives / Q000378:metabolism* / Q000493:pharmacokinetics; D007700:Kinetics; D008617:Mephenytoin / Q000494:pharmacology; D008862:Microsomes, Liver / Q000187:drug effects / Q000378:metabolism*; D006899:Mixed Function Oxygenases / Q000378:metabolism*
chemical_list: C033803:2-hydroxyimipramine; D003577:Cytochrome P-450 Enzyme System; D006899:Mixed Function Oxygenases; D001189:Aryl Hydrocarbon Hydroxylases; C045793:CYP2C19 protein, human; D065731:Cytochrome P-450 CYP2C19; D007099:Imipramine; D008617:Mephenytoin; D003891:Desipramine