In vitro metabolism of quinidine: the (3S)-3-hydroxylation of quinidine is a specific marker reaction for cytochrome P-4503A4 activity in human liver microsomes

J Pharmacol Exp Ther. 1999 Apr;289(1):31-7.


The aim of this study was to evaluate the (3S)-3-hydroxylation and the N-oxidation of quinidine as biomarkers for cytochrome P-450 (CYP)3A4 activity in human liver microsome preparations. An HPLC method was developed to assay the metabolites (3S)-3-hydroxyquinidine (3-OH-Q) and quinidine N-oxide (Q-N-OX) formed during incubation with microsomes from human liver and from Saccharomyces cerevisiae strains expressing 10 human CYPs. 3-OH-Q formation complied with Michaelis-Menten kinetics (mean values of Vmax and Km: 74.4 nmol/mg/h and 74.2 microM, respectively). Q-N-OX formation followed two-site kinetics with mean values of Vmax, Km and Vmax/Km for the low affinity isozyme of 15.9 nmol/mg/h, 76.1 microM and 0.03 ml/mg/h, respectively. 3-OH-Q and Q-N-OX formations were potently inhibited by ketoconazole, itraconazole, and triacetyloleandomycin. Isozyme specific inhibitors of CYP1A2, -2C9, -2C19, -2D6, and -2E1 did not inhibit 3-OH-Q or Q-N-OX formation, with Ki values comparable with previously reported values. Statistically significant correlations were observed between CYP3A4 content and formations of 3-OH-Q and Q-N-OX in 12 human liver microsome preparations. Studies with yeast-expressed isozymes revealed that only CYP3A4 actively catalyzed the (3S)-3-hydroxylation. CYP3A4 was the most active enzyme in Q-N-OX formation, but CYP2C9 and 2E1 also catalyzed minor proportions of the N-oxidation. In conclusion, our studies demonstrate that only CYP3A4 is actively involved in the formation of 3-OH-Q. Hence, the (3S)-3-hydroxylation of quinidine is a specific probe for CYP3A4 activity in human liver microsome preparations, whereas the N-oxidation of quinidine is a somewhat less specific marker reaction for CYP3A4 activity, because the presence of a low affinity enzyme is demonstrated by different approaches.

MeSH terms

  • Algorithms
  • Anti-Arrhythmia Agents / metabolism*
  • Antibodies
  • Binding Sites
  • Cytochrome P-450 CYP3A
  • Cytochrome P-450 Enzyme Inhibitors
  • Cytochrome P-450 Enzyme System / immunology
  • Cytochrome P-450 Enzyme System / metabolism*
  • Enzyme Inhibitors / pharmacology
  • Humans
  • Hydroxylation
  • Immunochemistry
  • In Vitro Techniques
  • Isoenzymes / antagonists & inhibitors
  • Isoenzymes / immunology
  • Isoenzymes / metabolism
  • Ketoconazole / pharmacology
  • Kinetics
  • Microsomes, Liver / drug effects
  • Microsomes, Liver / enzymology
  • Microsomes, Liver / metabolism*
  • Mixed Function Oxygenases / antagonists & inhibitors
  • Mixed Function Oxygenases / immunology
  • Mixed Function Oxygenases / metabolism*
  • Models, Biological
  • Oxidation-Reduction
  • Quinidine / metabolism*
  • Saccharomyces cerevisiae / drug effects
  • Saccharomyces cerevisiae / enzymology


  • Anti-Arrhythmia Agents
  • Antibodies
  • Cytochrome P-450 Enzyme Inhibitors
  • Enzyme Inhibitors
  • Isoenzymes
  • Cytochrome P-450 Enzyme System
  • Mixed Function Oxygenases
  • CYP3A protein, human
  • Cytochrome P-450 CYP3A
  • CYP3A4 protein, human
  • Quinidine
  • Ketoconazole