title: Regioselectivity and substrate concentration-dependency of involvement of the CYP2D subfamily in oxidative metabolism of amitriptyline and nortriptyline in rat liver microsomes.
abstract: Kinetic analysis of the metabolism of amitriptyline and nortriptyline using liver microsomes from Wister rats showed that more than one enzyme was involved in each reaction except for monophasic amitriptyline N-demethylation. The Vmax values particularly in the high-affinity sites for E-10-hydroxylation of both drugs were larger than those for Z-10-hydroxylations. Their E- and E-10-hydroxylase activities in Dark-Agouti rats, which are deficient for CYP2D1, were significantly lower than those in Wistar rats at a lower substrate concentration (5 microM). The strain difference was reduced at a higher substrate concentration (500 microM). A similar but a smaller strain difference was also observed in nortriptyline N-demethylase activity, and a pronounced sex difference (male > female) was observed in N-demethylation of both drugs in Wistar and Dark-Agouti rats. The reactions with the strain difference were inhibited concentration-dependently by sparteine, a substrate of the CYP2D subfamily, and an antibody against a CYP2D isoenzyme. The profiles of these decreased metabolic activities corresponded to that of the lower metabolic activities in Dark-Agouti rats. These results indicated that a cytochrome P450 isozyme in the CYP2D subfamily was involved in E- and Z-10-hydroxylations of amitriptyline and nortriptyline in rat liver microsomes as a major isozyme in a low substrate concentration range. It seems likely that the CYP2D enzyme contributes to nortriptyline N-demethylation.
authors.affiliation: Laboratory of Biopharmaceutics, Faculty of Pharmaceutical Sciences, Chiba University, Japan.
authors.forename: Y
authors.initials: Y
authors.lastname: Masubuchi
authors.forename: T
authors.initials: T
authors.lastname: Iwasa
authors.forename: S
authors.initials: S
authors.lastname: Fujita
authors.forename: T
authors.initials: T
authors.lastname: Suzuki
authors.forename: T
authors.initials: T
authors.lastname: Horie
authors.forename: S
authors.initials: S
authors.lastname: Narimatsu
mesh_terms: D000639:Amitriptyline / Q000378:metabolism* / Q000493:pharmacokinetics; D000818:Animals; D000928:Antidepressive Agents / Q000378:metabolism* / Q000493:pharmacokinetics; D001711:Biotransformation; D019389:Cytochrome P-450 CYP2D6 / Q000378:metabolism*; D005260:Female; D006801:Humans; D006900:Hydroxylation; D066298:In Vitro Techniques; D007700:Kinetics; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology / Q000378:metabolism*; D009661:Nortriptyline / Q000378:metabolism* / Q000493:pharmacokinetics; D010084:Oxidation-Reduction; D051381:Rats; D017208:Rats, Wistar; D012727:Sex Characteristics; D013034:Sparteine / Q000494:pharmacology
chemical_list: D000928:Antidepressive Agents; D000639:Amitriptyline; D013034:Sparteine; D009661:Nortriptyline; D019389:Cytochrome P-450 CYP2D6