Estrogen-specific 17 beta-hydroxysteroid oxidoreductase type 1 (E.C. 1.1.1.62) as a possible target for the action of phytoestrogens.

Several plant estrogens, especially coumestrol and genistein, were found to reduce the conversion of [3H]estrone to [3H] 17 beta-estradiol catalyzed by estrogen-specific 17 beta-hydroxysteroid oxidoreductase Type 1 (E.C. 1.1.1.62) in vitro. Coumestrol, the most potent inhibitor in our experiments, is the best inhibitor of the enzyme known to date. All compounds with inhibitory effects were also estrogenic. However, structural demands for 17 beta-HSOR Type 1 inhibition and estrogenicity of tested compounds in breast cancer cells (judged by increased cell proliferation) were not identical. Zearalenone and diethylstilbestrol, both potent estrogens, did not inhibit 17 beta-HSOR Type 1. Thus, changes in the estrogen molecule may discriminate between active sites of 17 beta-HSOR Type 1 and estrogen binding sites of the ER. The effects of these compounds in vivo cannot be predicted on the basis of these results. Inhibition of 17 beta-HSOR Type 1 enzyme could lead to a decrease in the availability of the highly active endogenous estrogen. However, these compounds are estrogenic per se, and they may thus replace endogenous estrogens. Additional studies are needed to further understand the role of these plant estrogens in the etiology of hormone-dependent cancers. It is not easily conceivable how the chemopreventive action of Asian diets, possibly mediated by phytoestrogens in soya products, can be based on the inhibition of estrone reduction at the target cells by phytoestrogens or related compounds, unless they are "incomplete estrogens" (i.e., unable to induce all effects typical of endogenous estrogens).