Cytochrome P450 (CYP) 1B1 catalyzes 17beta-estradiol (E(2)) to predominantly carcinogenic 4-hydroxy-E(2), whereas
CYP1A1 and 1A2 convert E(2) to non-carcinogenic 2-hydroxy-E(2). Hence, selective inhibition of CYP1B1 is recognized to be beneficial for the prevention of E(2) related
breast cancer. In this study, we first evaluated the structure-property relationship of 18 major
flavonoids on inhibiting enzymatic activity of
CYP1A1, 1A2 and 1B1 by using an
ethoxyresorufin O-deethylation assay.
Flavones and
flavonols indicated relatively strong inhibitory effects on CYP1s compared with
flavanone that does not have the double bond between C-positions 2 and 3 on the C-ring.
Flavonoids used in this study selectively inhibited CYP1B1 activity. In particular, methoxy types of
flavones and
flavonols such as
chrysoeriol and
isorhamnetin showed strong and selective inhibition against CYP1B1. To understand why selective inhibition was observed, we carried out a molecular docking analysis of these methoxyflavonoids with the 2-3 double bond and CYP1s. The results suggested that
chrysoeriol and
isorhamnetin fit well into the active site of CYP1B1, but do not fit into the active site of
CYP1A2 and 1A1 because of steric collisions between the methoxy substituent of these methoxyflavonoids and Ser-122 in
CYP1A1 and Thr-124 in
CYP1A2. In conclusion, our results demonstrate: (1) strong inhibitory effects of
flavonoids on CYP1 activities require the 2-3 double bond on the C-ring; (2) methoxyflavonoids with the 2-3 double bond had strong and selective inhibition against CYP1B1, suggesting chemopreventive
flavonoids for E(2) related
breast cancer; and (3) binding specificity of these methoxyflavonoids is based on the interactions between the methoxy groups and specific CYP1s residues.