INHIBITION OF INSECT CYTOCHROMES P450 BY FURANOCOUMARINS

Coumarin, benzofuran, and 16 furanocoumarins were evaluated as inhibit ors of Manduca sexta midgut cytochrome P450-catalyzed O-demethylation of p-nitroanisole. 8-O-Alkylpsoralens with nonpolar alkyl groups of fi ve or fewer carbons had the lowest I-50 values. I-50 values of 5-O-alk ylpsoralens increased with the size of the O-alkyl group and were grea ter than psoralen. Psoralens with polar O-alkyl groups such as alcohol s had high I-50 values. Psoralens with substituents on the furan ring were not inhibitory. Coumarin, which lacks a furan ring, was a reversi ble inhibitor with an I-50 value two orders of magnitude higher than p soralen. Benzofuran which lacks the pyrone ring was a weak mechanism-b ased inhibitor. The angular furanocoumarin angelicin had an I-50 value comparable to 8-O-alkylpsoralens. All of the inhibitory furanocoumari ns tested were mechanism-based irreversible inhibitors. The structure- activity results are consistent with a proposed mechanism of inhibitio n whereby the furanocoumarin is oxidized by cytochrome P450 at the dou ble bond of the furan ring forming an unstable epoxide that can react with cytochrome P450. [H-3]Xanthotoxin labels numerous proteins when i ncubated with microsomes in the presence of NADPH. Although a P450-con taining protein fraction separated by chromatography has the highest t ritium/protein ratio, the lack of specificity limits the usefulness of radiolabeling with furanocoumarins for identification of cytochromes P450. (C) 1994 Academic Press, Inc.