Regioselectivity and substrate concentration-dependency of involvement of the CYP2D subfamily in oxidative metabolism of amitriptyline and nortriptyline in rat liver microsomes

J Pharm Pharmacol. 1996 Sep;48(9):925-9. doi: 10.1111/j.2042-7158.1996.tb06003.x.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Amitriptyline / metabolism*
  • Amitriptyline / pharmacokinetics
  • Animals
  • Antidepressive Agents / metabolism*
  • Antidepressive Agents / pharmacokinetics
  • Biotransformation
  • Cytochrome P-450 CYP2D6 / metabolism*
  • Female
  • Humans
  • Hydroxylation
  • In Vitro Techniques
  • Kinetics
  • Male
  • Microsomes, Liver / enzymology
  • Microsomes, Liver / metabolism*
  • Nortriptyline / metabolism*
  • Nortriptyline / pharmacokinetics
  • Oxidation-Reduction
  • Rats
  • Rats, Wistar
  • Sex Characteristics
  • Sparteine / pharmacology


  • Antidepressive Agents
  • Amitriptyline
  • Sparteine
  • Nortriptyline
  • Cytochrome P-450 CYP2D6