A 17 beta-estradiol (E(2)) is hydrolyzed to 2-hydroxy-E(2) (2-OHE(2)) and 4-hydroxy-E(2) (4-OHE(2) ) via cytochrome P450 (CYP) 1A1 and 1B1, respectively. In estrogen target tissues including the mammary gland, ovaries, and uterus, CYP1B1 is highly expressed, and 4-OHE(2) is predominantly formed in cancerous tissues. In this study, we investigated the inhibitory effects of chrysoeriol (luteorin-3'-methoxy ether), which is a natural methoxyflavonoid, against activity of CYP1A1 and 1B1 using in vitro and cultured cell techniques. Chrysoeriol selectively inhibited human recombinant CYP1B1-mediated 7-ethoxyresorufin-O-deethylation (EROD) activity 5-fold more than that of CYP1A1-mediated activity in a competitive manner. Additionally, chrysoeriol inhibited E(2) hydroxylation was catalyzed by CYP1B1, but not by CYP1A1. Methylation of 4-OHE(2), which is thought to be a detoxification process, was not affected by the presence of chrysoeriol. In human breast cancer MCF-7 cells, chrysoeriol did not affect the gene expression of CYP1A1 and 1B1, but significantly inhibited the formation of 4-methoxy E(2) without any effects on the formation of 2-methoxy E(2). In conclusion, we present the first report to show that chrysoeriol is a chemopreventive natural ingredient that can selectively inhibit CYP1B1 activity and prevent the formation of carcinogenic 4-OHE(2) from E(2). (C) 2009 Elsevier Ltd. All rights reserved.