Competitive inhibition of coumarin 7‐hydroxylation by pilocarpine and its interaction with mouse CYP 2A5 and human CYP2A6

Abstract

. 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. . Pilocarpine was a competitive inhibitor of coumarin 7‐hydroxlase 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 μm in mice and 1.21 ±0.51 μm in human liver microsomes. . 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 μm, respectively. . Increase in pH of the incubation medium from pH 6 to 7.5 increased the potency of inhibition of coumarin 7‐hydroxylation by pilocarpine. . 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%. . 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. 1995 British Pharmacological Society