In vitro inhibition of 3-methylcholanthrene-induced rat hepatic aryl hydrocarbon hydroxylase by 8-acyl-7-hydroxycoumarins. Structure-activity relationships and metabolite profiles.
A series of 4,5,6-substituted-8-acyl-7-hydroxycoumarins was synthesized, and their inhibitory potencies towards 3-methylcholanthrene-induced rat hepatic microsomal aryl hydrocarbon hydroxylase activity were studied, both qualitatively and quantitatively. Using the Hansch approach to structure-activity relationships and computer analysis of the data, we have shown that the inhibitory potency could be related to the lipophilicity and molecular size of the compounds. 13C NMR spectroscopy was used to determine electron density at particular carbons of the candidate inhibitors. The electron density was used as an additional parameter in the fitting of potency curves and was found to be of significance. Thus, the potency of a candidate inhibitor could be related to its lipophilicity, molecular size, and electron distribution. The existence of a dipole at the enzyme active site is postulated. The distribution of benzo[a]pyrene metabolites, both in the presence and absence of several derivatives, was studied with high pressure liquid chromatography. There was a significant decrease of the 7,8- and 9,10-dihydrodiols in the presence of these inhibitors. The comparative decrease in each of the dihydrodiols was not related to the potency of the inhibitor. The possibility of epoxide hydrolase inhibition by the 4,5,6-substituted-8-acyl-7-hydroxycoumarins was also examined; no epoxide hydrolase inhibition or stimulation was detected at concentrations 10-100 times greater than those which inhibited aryl hydrocarbon hydroxylase by 50%. The decreased formation of the 7,8- and 9,10-arene oxides was therefore postulated to be responsible for the decreased 7,8- and 9,10-dihydrodiol formations in the presence of inhibitor. Inhibition is postulated to have occurred at the enzyme active site, probably by means of a perturbation of the electron cloud density and the active site such that oxygenation at the 7,8- and 9,10-positions was unfavourable.
D000818:Animals; D001189:Aryl Hydrocarbon Hydroxylases / Q000037:antagonists & inhibitors / Q000096:biosynthesis*; D001564:Benzo(a)pyrene; D001580:Benzopyrenes / Q000378:metabolism; D002851:Chromatography, High Pressure Liquid; D004790:Enzyme Induction; D004851:Epoxide Hydrolases / Q000378:metabolism; D008099:Liver / Q000201:enzymology*; D008297:Male; D008748:Methylcholanthrene / Q000494:pharmacology*; D051381:Rats; D011919:Rats, Inbred Strains; D013329:Structure-Activity Relationship; D014468:Umbelliferones / Q000494:pharmacology*
D001580:Benzopyrenes; D014468:Umbelliferones; D001564:Benzo(a)pyrene; D008748:Methylcholanthrene; D001189:Aryl Hydrocarbon Hydroxylases; D004851:Epoxide Hydrolases