Hydrogen bonding in steroidogenesis: studies on new heterocyclic analogs of estrone that inhibit human estradiol 17 beta-dehydrogenase

New heterocyclic analogs of estrone are reported that inhibit estradiol 17 beta-dehydrogenase (E2-17 beta DH) from human placenta. The inhibitors are efficiently synthesized in two steps from estrone (or its 3-O-methyl ether), giving fully characterized analogs with pyrazole or isoxazole fused to the 16,17-position on the D ring. Dixon plots of enzyme kinetic data show the heterocyclic steroids are competitive inhibitors of E2-17 beta DH. Correlating molecular structures of the inhibitors with their Ki-values yields a pattern suggesting intermolecular hydrogen bonding stabilizes the (pyrazole)inhibitor-E2-17 beta DH] complexes. A free energy difference of 2.74 Kcal/mol calculated from Ki-value differences between hydrogen bonded (4.08 microM) and non-bonded (425 microM) inhibitor-E2-17 beta DH] complexes is in the range for intermolecular hydrogen bonding. We conclude that specific intermolecular hydrogen bonds stabilize hydroxysteroid-enzyme] complexes, thereby making important contributions to the affinity between hydroxysteroids and steroid-specific enzymes of steroidogenesis.