Flavonoid inhibition of overexpressed human 3β-hydroxysteroid dehydrogenase type II

Abstract

The inhibitory effects of various flavonoids on human 3β-hydroxysteroid dehydrogenase/Δ 5-Δ 4-isomerase type II (3β-HSD type II), overexpressed in baculovirus, were investigated, and the structure–inhibition relationship was examined. The isoflavone derivatives daidzein, genistein, formononetin and biochanin A inhibited 3β-HSD type II activity at a concentration of 10 μM and of these, genistein was the most potent inhibitor. 6-Hydroxyflavone (6-HF), a synthetic flavone, also strongly inhibited 3β-HSD activity but 5-HF, 7-HF and other natural flavones were less potent. Energy minimization structures of the flavonoids, as produced using MOE software, showed that isoflavones and flavones have an almost flat A–C ring structure, and that flavonoids that acted as inhibitors had similar steric structures to DHEA. Genistein, 6-HF and cyanoketone, which is known as a typical 3β-HSD inhibitor, were found to act as competitive inhibitors with K i values of 0.12 μM, 0.19 μM and 0.67 nM, respectively. Furthermore, the LUMO (lowest unoccupied molecular orbital (LUMO)) values, as calculated using WinMOPAC (Fujitsu, Japan), of the inhibitors were correlated with the IC 50 values ( r 2=0.84). From these results, it appears that inhibitory effects of flavonoids are due to the combination of steric structure and electron affinity between the active center of 3β-HSD type II and the flavonoid molecule.