Catalysis of the oxidation and reduction reactions of steroid and stilbene estrogens by nuclear enzymes

Abstract

We demonstrated for the first time that nuclei were able to convert a stilbene estrogen (diethylstilbestrol) to reactive metabolites, which covalently bind to nuclear proteins and DNA. Depending on the cofactor used, nuclear enzymes catalyzed oxidation and/or reduction of stilbene and steroid estrogens. 2-Hydroxyestradiol (a major metabolite of steroidal estrogen, 17 beta-estradiol) and diethylstilbestrol (DES) were oxidized to 2,3-estradiol quinone and DES quinone, respectively, by peroxide-supported nuclear cytochromes P450. A Lineweaver-Burk plot of rate of formation of DES quinone at various substrate concentrations yielded a Km = 15 microM and Vmax = 10 nmol/mg protein/min. The oxidation of DES to DES quinone by nuclei was drastically decreased by known inhibitors of cytochromes P450. DES quinone was reduced back to DES by nuclei in the presence of NADPH, presumably through cytochrome P450 reductase. The reduction of DES quinone to DES by nuclei was significantly inhibited by antibodies and inhibitors of cytochrome P450 reductase. Under reaction conditions similar to oxidation of DES to DES quinone by nuclei, it was observed that nuclear metabolic products of DES were able to covalently bind to nuclear proteins and DNA. The data reported here establish that DES and a catechol estrogen can be oxidized to quinones and that the quinones may be reduced back to the hydroquinones by nuclear preparations when fortified with an appropriate cofactor and that reactive intermediates are involved based on observed covalent binding to macromolecules. The significance of these events, and their possible role in toxicity/cancer/teratogenicity, however, is not at all clear.