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Thyroid hormone induces the generation of a novel putative protein in piscine ovarian follicle that stimulates the conversion of pregnenolone to progesterone

Bhattacharya S, Guin S, Bandyopadhyay A, Jana NR, Halder S. Thyroid hormone induces the generation of a novel putative protein in piscine ovarian follicle that stimulates the conversion of pregnenolone to progesterone. Eur J Endocrinol 1996:134:128–35. ISSN 0840–4643.

Ovarian follicles were collected from the perch belonging to the vitellogenic stage and incubated in vitro for 4 h in the absence (control) or presence of triiodothyronine (T₃). Addition of T₃ (40 ng/ml) to the follicle incubation caused a two-fold increase of [³H] pregnenolone conversion to radiolabelled progesterone (P₄) as compared to the control. The increase in P₄ formation in the ovarian follicle could be blocked completely by the inhibitors of protein synthesis, actinomycin D and cycloheximide (50 µg/ ml), suggesting a protein or peptide mediator of the T₃ stimulatory effect. To search for this mediator, ovarian follicles from the control or T₃ incubate were homogenized and ultracentrifuged and different fractions were added separately to fresh follicle incubations. Only the 100 000g supernatant from T₃ incubate showed a significant (p < 0.01) increase in P₄ formation, while the corresponding supernatant from control follicle incubations had no such stimulatory effect. Trypsin or heat destroyed this augmentory effect. Based on its ability to stimulate the conversion of radiolabelled pregnenolone to P₄ in the ovarian follicle, the T₃-induced protein (TIP) was purified to homogeneity by employing Sephadex G-75 gel filtration, FPLC Mono-Q and FPLC Superose-6 chromatography. Sodium dodecyl sulphate polyacrylamide gel electrophoresis of purified TIP showed it to be a 52 K monomer protein. Addition of TIP in increasing concentrations to follicle incubations caused a linear increase in P₄ formation. Experiments with radiolabelled TIP ([¹²⁵ I] TIP) indicate its entry through the follicular cell membrane within the limited period of incubation. Results suggest that TIP activates ovarian 3β-hydroxysteroid dehydrogenase enzyme, thus effecting a greater conversion of pregnenolone to P₄.

S. Bhattacharya, Department of Zoology, Visva-Bharati University. Santiniketan 731 235, West Bengal, India

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