Phytoestrogens inhibit human 17β-hydroxysteroid dehydrogenase type 5

The 17 beta -hydroxysteroid dehydrogenase type 5 (17 beta -HSD 5) is involved in estrogen and androgen metabolism. In our study we tested the influence of environmental hormones, such as phytoestrogens (flavonoids, coumarins, coumestans), on reductive and oxidative 17 beta -HSD activity of the human 17 beta -hydroxysteroid dehydrogenase type 5 (17 beta -HSD 5). These dietary substances were shown to be potent inhibitors of aromatase, different 17 beta -HSDs and seem to play an important rule in delay of development of hormone dependent cancers. Our studies show that reductive and oxidative activity of the enzyme are inhibited by many dietary compounds, especially zearalenone. coumestrol, quercetin and biochanin A. Among the group of flavones inhibitor potency is growing with increasing number of hydroxylations. We suggest that these substances are bound to the hydrophilic cofactor-binding pocket of the enzyme. An interesting inhibition pattern is observed for 18 beta -glycyrrhetinic acid, which has no influence on the oxidative but only on the reductive reaction. This indicates that this substrate binds to pH- and cofactor-depending sites at the active center of the enzyme. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.