Mode of action of coumarin in immune cells

In order to characterize further the mode of action of coumarin, binding studies were undertaken using human monocytes and radioactively labelled drug. Since coumarin is only a small compound and we wanted to exclude possible artefacts due to variations in size or conformation, the drug was produced by synthesis in the presence of radioactive 14C. Adding increasing amounts of a mixture of labelled and unlabelled drug to monocytes resulted in saturating conditions only at rather high concentrations. Performing Scatchard analysis demonstrated that binding sites for coumarin appeared to be present in relatively high numbers (7.5 x 10(8)/cell) but their affinity was rather low (K alpha approximately 2 x 10(2) M-1). Inhibition studies with 7-hydroxycoumarin revealed that an approximately four times higher molar concentration of the derivative was necessary to cause 50% displacement of coumarin from its binding site. These results indicate that binding of the drug to cells is characterized by high-capacity but low-affinity conditions. This would be compatible with the hypothesis that coumarin interacts with ubiquitous intracellular receptor proteins able to interact with aromatic hydrocarbons, which might form the basis for enzyme induction, and leads to the effects observed in vitro and in vivo.