In vitro inhibition of human P450 enzymes by prenylated flavonoids from hops, Humulus lupulus

1. Several unique flavonoid compounds have recently been isolated from hops , Humulus lupulus, and their presence has been detected in beer. Their chem ical structures are similar to other plant-derived compounds, many present in the human diet, that have been shown to have cancer chemopreventive prop erties due, in part,to inhibition of cytochrome P450 enzymes that activate carcinogens. Additionally, preliminary studies have shown these flavonoids (at 100 mu M) to be inhibitory of P450-mediated activation reactions in a v ariety of in vitro systems. Thus, the in vitro effects of these phytochemic als on cDNA-expressed human CYP1A1, CYP1B1, CYP1A2, CYP3A4 and CYP2E1 were currently examined by the use of diagnostic substrates and the carcinogen A FB(1). 2. At 10 mu M, the prenylated chalcone, xanthohumol(XN), almost completely inhibited the 7-ethoxyresorufin O-deethylase (EROD) activity of CYP1A1. At the same concentration, other hop flavonoids decreased the EROD activity by 90.8-27.0 %. 3. At 10 mu M, XN completely eliminated CYP1B1 EROD activity, whereas the o ther hop flavonoids showed varying degrees of inhibitory action ranging fro m 99.3 to 1.8 %. 4. In contrast, the most effective inhibitors of CYP1A2 acetanilide C-hydro xylase activity were the two prenylated flavonoids, 8-prenylnaringenin (8PN ) and isoxanthohumol (IX). which produced > 90 % inhibition when added at c oncentrations of 10 mu M. 5. CYP1A2 metabolism of the carcinogen AFB(1) was also inhibited by IX and 8PN as shown by decreased appearance of dihydrodiols and AFM(1) as analysed by hplc. IX and 8PN also decreased covalent binding of radiolabelled AFB(1 ) to microsomal protein in a concomitant manner. 6. XN, IX and 8PN, however, were poor inhibitors of CYP2E1 and CYP3A4 as me asured by their effect on chorzoxazone hydroxylase and nifedipine oxidase a ctivities respectively. 7. These results suggest that the hop flavonoids are potent and selective i nhibitors of human cytochrome P450 and warrant further in vivo investigatio ns.