Our previous reports had shown that 3,5,
3′-triiodothyronine (
T3) induced the generation of a 52-kDa monomer protein, i.e., TIP (thyroid hormone-induced protein) in the perch ovarian follicle. TIP, in turn, increased progesterone formation by stimulating
Δ5-3β-HSD activity (
3β-hydroxysteroid dehydrogenase/
Δ5–Δ4 isomerase) [Eur. J. Endocrinol. 134 (1996) 128–135; Gen. Comp. Endocrinol. 113 (1999) 212–220]. In the present investigation, perch ovarian follicles were incubated in the absence (control) or the presence of
T3 or gonadotropin (GTH) or human chorionic gonadotropin (hCG). RNAs were isolated and allowed to hybridize with a radiolabeled TIP oligonucleotide probe prepared on the basis of the N-terminal 17-amino-acid sequence of TIP. Only RNA from
T3-incubated follicles hybridized with the probe, while RNA from control or GTH- or hCG-incubated follicles did not hybridize with the probe. The transcript size of TIP mRNA was
1.8kb. mRNA isolated from
T3-incubated ovarian follicles subjected to in vitro translation and Western blot analysis clearly identified a 52-kDa protein which was not found with the mRNA from the control follicles. However, both TIP and GTH stimulated progesterone secretion from perch ovarian follicles in vitro. GTH stimulation of
Δ5-3β-HSD was due to the stimulation of enzyme protein synthesis as a more than twofold increase in
Δ5-3β-HSD occurred in response to GTH. But TIP did not stimulate synthesis of
Δ5-3β-HSD protein. However, in vitro incubation of
Δ5-3β-HSD enzyme with TIP in the presence of NAD and substrate (pregnenolone) greatly stimulated enzyme activity, while incubation with GTH had no effect, indicating a modulation of
Δ5-3β-HSD protein from a less active to a more active state by TIP. This has been supported by another observation, in which TIP (52kDa) and
Δ5-3β-HSD (45kDa) incubation resulted in a complex of 99kDa. This suggests a protein–protein interaction in the process of
Δ5-3β-HSD activation by TIP. The present work, therefore, shows some new and interesting aspects of thyroid hormone regulation of the reproductive control mechanism.