Competitive inhibition of coumarin 7-hydroxylation by pilocarpine and its interaction with mouse CYP 2A5 and human CYP 2A6

Br J Pharmacol. 1995 Nov;116(6):2625-30. doi: 10.1111/j.1476-5381.1995.tb17217.x.


1. We have shown earlier that pilocarpine strongly inhibits mouse and human liver coumarin 7-hydroxylase activity of CYP 2A and pentoxyresorufin O-deethylase activity of CYP 2B in vitro. Since pilocarpine, like coumarin, contains a lactone structure we have studied in more detail its inhibitory potency on mouse and human liver coumarin 7-hydroxylation. 2. Pilocarpine was a competitive inhibitor of coumarin 7-hydroxylase in vitro both in mouse and human liver microsomes although it was not a substrate for CYP 2A5. Ki values were similar, 0.52 +/- 0.22 microM in mice and 1.21 +/- 0.51 microM in human liver microsomes. 3. Pilocarpine induced a type II difference spectrum in mouse, human and recombinant CYP 2A5 yeast cell microsomes, with Ka values of 3.7 +/- 1.6, 1.6 +/- 1.1 and 1.5 +/- 0.1 microM, respectively. 4. Increase in pH of the incubation medium from pH 6 to 7.5 increased the potency of inhibition of coumarin 7-hydroxylation by pilocarpine. 5. Superimposition of pilocarpine and coumarin in such a way that their carbonyls, ring oxygens and the H-7' of coumarin and N-3 of pilocarpine overlap yielded a common molecular volume of 82%. 6. The results indicate that pilocarpine is a competitive inhibitor and has a high affinity for mouse CYP 2A5 and human CYP 2A6. In addition the immunotype nitrogen of pilocarpine is coordinated towards the haem iron in these P450s.

Publication types

  • Comparative Study

MeSH terms

  • Animals
  • Aryl Hydrocarbon Hydroxylases*
  • Binding Sites
  • Binding, Competitive
  • Coumarins / metabolism*
  • Cytochrome P-450 CYP2A6
  • Cytochrome P-450 Enzyme Inhibitors*
  • Cytochrome P-450 Enzyme System / metabolism
  • Enzyme Inhibitors / pharmacology*
  • Female
  • Humans
  • Hydroxylation / drug effects
  • Kinetics
  • Male
  • Mice
  • Mice, Inbred DBA
  • Microsomes, Liver / enzymology
  • Mixed Function Oxygenases / antagonists & inhibitors*
  • Mixed Function Oxygenases / metabolism
  • Models, Chemical
  • Parasympathomimetics / metabolism
  • Parasympathomimetics / pharmacology*
  • Pilocarpine / metabolism
  • Pilocarpine / pharmacology*


  • Coumarins
  • Cytochrome P-450 Enzyme Inhibitors
  • Enzyme Inhibitors
  • Parasympathomimetics
  • Pilocarpine
  • Cytochrome P-450 Enzyme System
  • coumarin
  • Mixed Function Oxygenases
  • Aryl Hydrocarbon Hydroxylases
  • Cytochrome P-450 CYP2A6