Inhibition of 7-ethoxycoumarin O-deethylase activity in rat liver microsomes by naturally occurring flavonoids: structure-activity relationships

Abstract

1. The inhibitory effects of several naturally occurring flavonoids and related compounds oncytochrome P450-dependent 7-ethoxycoumarin O-deethylase(ECOD) and the structure-activity relationships were studied in liver microsomes from rats treated with 3-methylcholanthrene (MC). 2. All the flavonoids (flavone, apigenin, chrysin, flavonol, fisetin, kaempferol, morin, myrisetin, quercetin, flavanone, hesperetin and naringenin) studied inhibited microsomal ECOD activity in the following order: flavones > flavonols > flavanones, were mixed type inhibitors and had K_i in the range of 0·17-4·5 μM. (±)-Catechin had no effect. 3. The double bond between C₂ and C₃ of the C ring, the keto group and hydroxyl group of this ring in the flavonoids seem to play major roles in inhibiting the ECOD activity. 4. The hydroxyl groups in the C₅ and C₇ positions of A ring in the flavone and the hydroxyl group in the C₃ position of C ring in the flavonol classes, respectively, were important factors for the inhibition of the enzyme. 5. In a series of 3, 5, 7-trihydroxyflavones, the hydroxyl group at the C₄ in the B ring was also an important factor for the inhibition of ECOD activity, but hydroxyl groups in other positions of the B ring had little effect on the inhibition. 6. We conclude that all the flavonoids studied inhibit ECOD activity by interfering with the binding of substrate to the active site and other site(s) of the enzyme and that their structural differences lead to different binding affinities at the active site and possibly to binding at other site(s) of the enzyme for the flavonoids.