Fig. 2
From: Estrogen- and stress-induced DNA damage in breast cancer and chemoprevention with dietary flavonoid
Schematic representation of typical estrogen metabolism. 17β-Estradiol (E2) is most common estrogen in the breast tissue. Endogenous E2 is hydroxylated to two types of catechol estrogens, 2-hydroxy-E2 (2-OHE2) and 4-hydroxy-E2 (4-OHE2), by Cytochrome P450 (CYP) 1A1 and CYP1B1, respectively. These catechol estrogens are further methylated to individual methoxide E2, 2-methoxy-E2 (2-MeOE2) and 4-methoxy-E2 (4-MeOE2), by catechol-O-methyltransferase (COMT). Methoxide E2 are basically ineffective compounds to DAN damage, and 2-MeOE2 exerts suppressive effects for tumor growth. On the other hand, 4-OHE2 is oxidized by any oxidative enzyme and/or metal ion, and formed E2-3,4-quinone (E2-3,4-Q), which can react with DNA directly