Inhibitory effects of flavonoids on the reduction of progesterone to 20α-hydroxyprogesterone in rat liver

The first aim of this study is to characterize the reduction of progesterone in rat liver. Progesterone was mainly reduced to 20α-hydroxyprogesterone in the cytosolic fraction of rat liver. The amount of 20α-hydroxyprogesterone formed from progesterone in the cytosolic fraction was significantly larger in the males than in the females and this enzyme reaction proceeded not only in the presence of NADPH, but also in the presence of NADH. Furthermore, we attempted to evaluate the inhibitory effects of 15 flavonoids on the NADPH-dependent reduction of progesterone to 20α-hydroxyprogesterone in liver cytosol of male rats. The order of the inhibitory potencies was luteolin > apigenin > quercetin > myricetin = fisetin = kaempferol. Other flavonoids exhibited lower inhibitory potencies. Energy-minimized molecular models demonstrated that a planar benzopyrone ring (A and C rings) with a coplanar phenyl ring (B ring) is a structural characteristic determining the inhibitory effects of flavonoids other than isoflavones.