Bovine adrenal 3 beta-hydroxysteroid dehydrogenase (EC 1.1.1.145)/5-ene-4-ene isomerase (EC 5.3.3.1): characterization and its inhibition by isoflavones

The isoflavones daidzein, genistein, biochanin A and formononetin inhibit p otently and preferentially the gamma-isozymes of mammalian alcohol dehydrog enase (gamma gamma-ADH), the only ADH isozyme that catalyzes the oxidation of 3 beta-hydroxysteroids. Based on these results, we proposed that these i soflavones might also act on other enzymes involved in 3 beta-hydroxysteroi d metabolism. Recently, we showed that they indeed are potent inhibitors of a bacterial beta-hydroxysteroid dehydrogenase (beta-HSD). To extend this f inding to the mammalian systems, we hereby purified, characterized and stud ied the effects of isoflavones and structurally related compounds on, a bov ine adrenal 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD). This enzyme c atalyzes the oxidation of 3 beta-hydroxysteroids but not 3 alpha-, 11 beta- or 17 beta-hydroxysteroids. The same enzyme also catalyzes 5-ene-4-ene iso merization, converting 5-pregnen 3, 20-dione to progesterone. The K-m value s of its dehydrogenase activity determined for a list of 3 beta-hydroxyster oid substrates are similar (1 to 2 mu M) and that of its isomerase activity , determined with 5-pregnen 3, 20-dione as a substrate, is 10 mu M The k(ca t) value determined for its isomerase activity (18.2 min(-1)) is also highe r than that for its dehydrogenase activity(1.4-2.4 min(-1)). A survey of mo re than 30 isoflavones and structurally related compounds revealed that dai dzein, genistein, biochanin A and formononetin inhibit both the dehydrogena se and isomerase activity of this enzyme. Inhibition is potent and concentr ation dependent. IC50 values determined for these compounds range from 0.4 to 11 mu M, within the plasma and urine concentration ranges of daidzein an d genistein of individuals on vegetarian diet or semi-vegetarian diet. Thes e results suggest that dietary isoflavones may exert their biological effec ts by inhibiting the action of 3 beta-HSD, a key enzyme of neurosteroid and /or steroid hormone biosynthesis. (C) 2000 Elsevier Science Ltd. All rights reserved.