Mechanism of metabolic activation and DNA adduct formation by the human carcinogen diethylstilbestrol: The defining link to natural estrogens

Diethylstilbestrol (DES) is a human carcinogen, based on sufficient epidemiological evidence. DES is mainly metabolized to its catechol, 3'-hydroxyDES (3'-OH-DES) which can further oxidize to DES-3',4'-quinone (DES-3',4'-Q). Similarly to estradiol-3,4-qui-none, the reaction of DES-3',4'-Q with DNA would form the depu-rinating 3'-OH-DES-6'-N3Ade and 3-OH-DES-6-N7Gua adducts. To prove this hypothesis, synthesis of DES-3',4'-Q bv oxidation of 3'-OH-DES with Ag ₂O was tried; this failed doe to instantaneous formation of a spiro-quinone. Oxidation of 3'-OH-DES by lacto-peroxidase or tyrosinase in the presence of DNA led to the formation of 3-OH-DES-6'-3Ade and 3-OH-DES-6'-N7Gua adducts. These adducts were tentatively identified bv LC- MS MS as 3 -OH-DES-6'-N3Ade, mlz = 418 [M+H] ⁺, and 3'-OH-DES-6'-N7Gua, mlz = 434 [M+H| ⁼. Demonstration of their structures derived from their oxidation by MnO ₂ to the DES quinone adducts and subsequent tautomerization to the dienestrol (DIES) catechol adducts, which are identical to the standard 3' -OH-DIES-6' -N3Ade, mlz = 416 [M+H] ⁺, and 3'-OH-DIES-6'-N7Gua, mlz =432 [M + H] ⁺, adducts. The reaction of DIES-3/ .4' -Q or lactoper-oxidase-aetivated 3' -OH-DIES with DNA did not produce any depurinating adducts, due to the dienic chain being perpendicular to the phenyl planes, which impedes the intercalation of DIES into the DNA. Enzymic oxidation of 3' -OH-DES suggests that the catechol of DES intercalates into DNA and is then oxidized to its quinone to yield N3Ade and N7Gua adducts. These results suggest that the common denominator of tumor initiation by the synthetic estrogen DES and the natural estrogen estradiol is formation of their catechol qui nones, which react with DNA to afford the depurinating N3Ade and N7Gua adducts.