Molecular cloning and characterization of 3β-hydroxysteroid dehydrogenase/Δ 5–Δ 4 isomerase cDNAs from Japanese eel ovary

Abstract

3β-Hydroxysteroid dehydrogenase/Δ 5–Δ 4 isomerase (3β-HSD) is a crucial steroidogenic enzyme which catalyzes an essential step in the biosynthesis of all classes of steroid hormones. Two closely related cDNAs, encoding Japanese eel ovarian types I and II 3β-HSD, were cloned and characterized. Both cDNAs putatively encoded 375 amino acid residues sharing high sequence homology with those of rainbow trout (71%) and mammalian (approximately 45–50%) 3β-HSD. Transient expression of types I and II 3β-HSD in COS-7 cells revealed that both proteins possess 3β-hydroxysteroid dehydrogenase as well as Δ 5–Δ 4 isomerase activity for both pregnenolone and dehydroepiandrosterone, with the preference of pregnenolone over dehydroepiandrosterone as substrate, although the type I protein is more active than the type II. By northern blot analysis, a single band of the 3β-HSD transcript of approximately 1.5 kb in length was observed in ovarian tissue and the total transcript abundance of both 3β-HSDs remained constant throughout ovarian development artificially induced by gonadotropin-rich salmon pituitary homogenate. This lack of change in 3β-HSD transcript abundance during ovarian development did not correlate with the fluctuation of its enzymatic activity reported previously, which may suggest that changes in 3β-HSD activity during ovarian development may be, in part, post-transcriptionally regulated in the Japanese eel ovary.