We have found previously that there are 2 kinds of 20α-hydroxysteroid dehydrogenase (20α-HSD) (EC 188.8.131.52) molecule (HSD1 and HSD2) in the mature rat ovary. In the present study, each enzyme activity was measured during pseudopregnancy (PSP) in the rat. Ovarian cytosol was collected in early (day 5), mid (day 9) or late (day 15) PSP. HSD1 activity was low in early and mid PSP and increased markedly at the end of PSP, while HSD2 activity was lower than HSD1 activity in early PSP and decreased as PSP advanced. These results suggest that the regulatory mechanism of the enzyme activity varies between HSD1 and HSD2, and that HSD1 mainly contributes to the increase in 20α-dihydroprogesterone secretion at the end of PSP. Interestingly, when ovaries at early and mid-term PSP were used, the total 20α-HSD activity in the cytosol fraction was increased markedly (163-296%) by passage through a DEAE column. This phenomenon was not observed with ovarian cytosol obtained at late PSP. Thus, we conclude that the ovary contains 2 different kinds of 20α-HSD molecule whose activities seem to be regulated separatedly, and that the HSD1 molecule is responsible for termination of the luteal phase.
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