Identification of CYP3A4 and CYP2C8 as the major cytochrome P450 s responsible for morphine N-demethylation in human liver microsomes

Xenobiotica. 2003 Aug;33(8):841-54. doi: 10.1080/0049825031000121608.


1. The aim was to identify the cytochrome P450 (CYP) enzymes responsible for the N-demethylation of morphine in vitro. 2. In human liver microsomes, normorphine formation followed Michaelis-Menten kinetics with mean Km and Vmax of 12.4 +/- 2.2 mM and 1546 +/- 121 pmol min(-1) mg(-1), respectively. In microsomes from a panel of 14 human livers phenotyped for 10 CYP enzymes, morphine N-demethylation correlated with testosterone 6beta-hydroxylation (r=0.91, p<0.001) and paclitaxel 6-alpha hydroxylation (r=0.72, p<0.001), two specific markers of CYP3A4 and CYP2C8, respectively. Normorphine formation decreased when incubated in the presence of troleandomycin or quercetin (by 46 and 33-36%, respectively), which further corroborates the contribution of CYP3A4 and CYP2C8. 3. Among eight recombinant human CYP enzymes tested, CYP3A4 and CYP2C8 exhibited the highest intrinsic clearance. More than 90% of morphine N-demethylation could be accounted for via the action of both CYP3A4 and CYP2C8. 4. The in vitro findings suggest that hepatic CYP3A4, and to a lesser extent CYP2C8, play an important role in the metabolism of morphine into normorphine.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Aryl Hydrocarbon Hydroxylases / metabolism*
  • Cell Line
  • Cytochrome P-450 CYP2C8
  • Cytochrome P-450 CYP3A
  • Cytochrome P-450 Enzyme System / metabolism*
  • DNA, Complementary
  • Humans
  • Hydroxylation
  • Insecta
  • Methylation
  • Microsomes, Liver / enzymology*
  • Microsomes, Liver / metabolism
  • Molecular Structure
  • Morphine / metabolism*
  • Morphine Derivatives / analysis
  • Morphine Derivatives / metabolism


  • DNA, Complementary
  • Morphine Derivatives
  • Morphine
  • Cytochrome P-450 Enzyme System
  • Aryl Hydrocarbon Hydroxylases
  • CYP2C8 protein, human
  • CYP3A protein, human
  • Cytochrome P-450 CYP2C8
  • Cytochrome P-450 CYP3A
  • CYP3A4 protein, human
  • normorphine