title: Characterization of a common genetic defect of cytochrome P-450 function (debrisoquine-sparteine type polymorphism)--increased Michaelis is Constant (Km) and loss of stereoselectivity of bufuralol 1'-hydroxylation in poor metabolizers.
abstract: In order to define the mechanism of the debrisoquine-sparteine type genetic polymorphism of drug oxidation we studied the kinetics of bufuralol 1'-hydroxylation in liver microsomes from extensive and poor metabolizers and in a purified reconstituted human cytochrome P-450 isozyme with high activity for bufuralol 1'-hydroxylation, P-450[buf]. In extensive metabolizer microsomes the enzymatic reaction displayed apparent Michaelis-Menten kinetics and the (+)-isomer was preferentially metabolized. By contrast, the enzymatic reaction in poor metabolizer microsomes was characterized by a 4- to 5-fold increase in Km and by a loss of stereoselectivity. In a non-membraneous reconstituted system containing NADPH cytochrome P-450 reductase, a NADPH regenerating system and phospholipids, P-450[buf] exhibited an almost complete substrate stereoselectivity for (+)-isomer 1'-hydroxylation. It is concluded that the purified cytochrome P-450[buf] is the target of the debrisoquine-sparteine type oxidation polymorphism and that poor metabolizers have a quantitative or qualitative deficiency of this isozyme.
mesh_terms: D003577:Cytochrome P-450 Enzyme System / Q000235:genetics* / Q000378:metabolism; D004983:Ethanolamines / Q000378:metabolism*; D006801:Humans; D007527:Isoenzymes / Q000378:metabolism; D007700:Kinetics; D008862:Microsomes, Liver / Q000201:enzymology; D011110:Polymorphism, Genetic; D013237:Stereoisomerism
chemical_list: D004983:Ethanolamines; D007527:Isoenzymes; C010831:bufuralol; D003577:Cytochrome P-450 Enzyme System