INHIBITORY EFFECTS OF NATURALLY-OCCURRING COUMARINS ON THE METABOLIC-ACTIVATION OF BENZO[A]PYRENE AND 7,12-DIMETHYLBENZ[A]ANTHRACENE IN CULTURED MOUSE KERATINOCYTES

Several naturally occurring coumarins to which humans are routinely ex posed have been previously found to be potent inhibitors and inactivat ors of cytochrome P450 (P450) 1A1-mediated monooxygenase in both murin e hepatic microsomes and in a reconstituted system using purified huma n P450 1A1 [Cai et al, (1993) Chem. Res. Toxicol., 6, 872-879 and Cai et al. (1996) Chem. Res. Toxicol., 9, 729-736]. In the present study, several of these coumarins mere investigated for their inhibitory effe cts on the metabolism and metabolic activation of benzo[a]pyrene (B[a] P) and 7,12-dimethylbenz[a]anthracene (DMBA) in cultured mouse keratin ocytes, Initial analysis of B[a]P metabolism in cultured keratinocytes showed that imperatorin, isoimperatorin, coriandrin, and bergamottin, at concentrations of 2 nM equal with B[a]P, reduced the formation of water-soluble metabolites of B[a]P by 33% to 57%. Bergamottin and cori andrin were the most potent inhibitors of the compounds examined. HPLC analysis of organic solvent-soluble metabolites of B[a]P indicated th at all the coumarins tested significantly reduced the formation of ind ividual B[a]P metabolites (including phenols, diols and tetraols). How ever, the greatest effect was on the formation of B[a]P tetraols, Addi tional experiments determined the ability of selected coumarins to blo ck covalent binding of B[a]P and DMBA to DNA in keratinocytes. Bergamo ttin preferentially inhibited the binding of B[a]P to DNA by 56%, whil e coriandrin preferentially inhibited the binding of DMBA to DNA by 48 %. Notably, analysis of individual DNA adducts formed from B[a]P and D MBA indicated that both bergamottin and coriandrin specifically inhibi ted the formation of anti diol-epoxide DNA adducts derived from both h ydrocarbons. The preferential inhibitory effect of bergamottin and cor iandrin on the formation of anti diol-epoxide adducts derived from DMB A was further confirmed by separation of anti- and syn-diol-epoxide-DN A adducts using immobilized boronate chromatography, The current study demonstrates that certain naturally occurring coumarins inhibited met abolic activation of B[a]P and DMBA in cultured mouse keratinocytes an d specifically inhibited the formation of DNA adducts derived from the anti diol-epoxide diastereomers from either hydrocarbon, The current data also suggest that certain naturally occurring coumarins may posse ss anticarcinogenic activity toward polycyclic aromatic hydrocarbons.