Heterogenous effects of natural flavonoids on monooxygenase activities in human and rat liver microsomes

The in vitro effects of nine flavonoids on different monooxygenase activities from human and rat liver were investigated. Flavonoids belonging to different chemical classes [flavones (chrysin, tangeretin, flavone, 5,6-benzoflavone, 7,8-benzoflavone), flavonols (quercetin), and flavanones (pinocembrin, eriodictyol, flavanone)] were chosen. Ethoxyresorufin O-deethylase (EROD) and benzyloxyresorufin O-debenzylase (BROD) were selected as marker activities of P450 isoenzymes involved in carcinogen activation. Human EROD activity was inhibited by all flavonoids. Flavonoids without hydroxyl groups were more effective than those with hydroxyl groups. Flavonoids with an unsaturated flavone nucleus were more inhibitory than the corresponding saturated analogues. Similar structure-activity relationships were found in microsomes from methylcholanthrene-treated rats. The effects of flavonoids on human BROD activity were quite different from those observed on EROD activity. Nonhydroxylated flavones were effective activators. Hydroxylated flavonoids showed a biphasic effect, being activators at low concentrations and inhibitors at higher concentrations. The effects of flavonoids on BROD activity were rather similar in man and control rats.