title: Relationship between phenytoin and tolbutamide hydroxylations in human liver microsomes.
abstract: 1. The metabolic interaction of phenytoin and tolbutamide in human liver microsomes was investigated. 2. Phenytoin 4-hydroxylation (mean Km 29.6 microM, n = 3) was competitively inhibited by tolbutamide (mean Ki 106.2 microM, n = 3) and tolbutamide methylhydroxylation (mean Km 85.6 microM, n = 3) was competitively inhibited by phenytoin (mean Ki 22.6 microM, n = 3). 3. A significant correlation was obtained between phenytoin and tolbutamide hydroxylations in microsomes from 18 human livers (rs = 0.82, P less than 0.001). 4. Sulphaphenazole was a potent inhibitor of both phenytoin and tolbutamide hydroxylations with IC50 values of 0.4 microM and 0.6 microM, respectively. 5. Mephenytoin was a poor inhibitor of both phenytoin and tolbutamide hydroxylations with IC50 values greater than 400 microM for both reactions. 6. Anti-rabbit P450IIC3 IgG inhibited both phenytoin and tolbutamide hydroxylations in human liver microsomes by 62 and 68%, respectively. 7. These in vitro studies are consistent with phenytoin 4-hydroxylation and tolbutamide methylhydroxylation being catalysed by the same cytochrome P450 isozyme(s) in human liver microsomes.
authors.affiliation: Department of Clinical Pharmacology, School of Medicine, Flinders University of South Australia, Bedford Park.
authors.forename: C J
authors.initials: CJ
authors.lastname: Doecke
authors.forename: M E
authors.initials: ME
authors.lastname: Veronese
authors.forename: S M
authors.initials: SM
authors.lastname: Pond
authors.forename: J O
authors.initials: JO
authors.lastname: Miners
authors.forename: D J
authors.initials: DJ
authors.lastname: Birkett
authors.forename: L N
authors.initials: LN
authors.lastname: Sansom
authors.forename: M E
authors.initials: ME
authors.lastname: McManus
mesh_terms: D001667:Binding, Competitive; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000276:immunology; D006801:Humans; D006900:Hydroxylation; D007074:Immunoglobulin G / Q000276:immunology; D066298:In Vitro Techniques; D007700:Kinetics; D008617:Mephenytoin / Q000494:pharmacology; D008862:Microsomes, Liver / Q000378:metabolism*; D006899:Mixed Function Oxygenases / Q000037:antagonists & inhibitors; D010672:Phenytoin / Q000378:metabolism*; D013426:Sulfaphenazole / Q000494:pharmacology; D014044:Tolbutamide / Q000378:metabolism*
chemical_list: D065607:Cytochrome P-450 Enzyme Inhibitors; D007074:Immunoglobulin G; D013426:Sulfaphenazole; D010672:Phenytoin; D003577:Cytochrome P-450 Enzyme System; D014044:Tolbutamide; D006899:Mixed Function Oxygenases; C053553:tolbutamide 4-hydroxylase; D008617:Mephenytoin