Molecular cloning of cDNA for rat ovarian 20α-hydroxysteroid dehydrogenase (HSD1)

20α-Hydroxysteroid dehydrogenase (20α-HSD, EC 1.1.1.149) catalyses the conversion of progesterone into 20α-dihydroprogesterone (20α-OHP). Previously, we purified the enzyme (37 kDa) from rat ovary and determined its N-terminal amino acid sequence. In the present study we succeeded in cloning a full-length 20α-HSD cDNA. mRNA was extracted from immature rat ovaries after successive treatment with equine chorionic gonadotropin (eCG) and human chorionic gonadotropin (hCG). A cDNA library was constructed in λZAP. For screening, a 576 bp probe was amplified by the PCR using mixed primers based on the N-terminal sequence of 20α-HSD, and labelled with [³²P]dCTP. Eight positive clones were isolated from 1.2 x 10⁴ recombinants. Analysis of the nucleotide sequence revealed that one clone of 1.2 kbp cDNA (pHSD12-07) contained a poly-adenylation site and an open reading frame encoding 323 amino acids with the N-terminal sequence of 20α-HSD. The fusion protein of pHSD12-07 produced by Escherichia coli reacted with a specific polyclonal antibody generated against rat ovarian 20α-HSD. In addition, the in vitro transcription-translation product produced by Xenopus oocytes showed 20α-HSD activity and Northern-blotting analysis revealed that the ovaries from normal adult rats contained a 1.2 kb mRNA. Thus we succeeded in isolating a clone encoding the full length of rat ovarian 20α-HSD. The sequence showed high similarity with those of rat liver 3α-hydroxysteroid dehydrogenase (3α-HSD), bovine lung prostaglandin F synthase (PGFS), human liver chlordecone reductase (CDR), frog lens p-crystallin and aldose reductases, indicating that 20α-HSD belongs to the aldo-keto reductase family.