Binding of thyroid hormones to isolated hepatic nuclei from Rana catesbeiana tadpoles.

This study is concerned with the characteristics of thyroid hormone binding to isolated hepatic nuclei from premetamorphic tadpoles. Conditions essential for nuclear stability and/or demonstration of saturable binding included 220-mosmol buffers containing 0.1 mM ZnCl2 and removal of most of the melanin granules; binding of T4 and T3 to melanin was significant, but unsaturable. Scatchard analysis of [125I]T3 binding to nuclei in the presence of increasing concentrations of T3 revealed the presence of two sets of saturable sites: a high affinity, low capacity set and a second set which had a lower affinity but approximately 4 times the capacity of the first set. Two sets of T4-binding sites were also detected. The data indicate that the two hormones bind to the same two sets of sites. Thus, both T3 and T4 completely displaced either [125I]T3 or [125I]T4 bound to saturable sites, although more T4 than T3 was required for 50% displacement of either hormone. Moreover, the presence of a partially saturating concentration of T4 in a displacement study of [125I]T3 by cold T3 resulted in decreased affinity of both sets of T3-binding sites. Both sets of sites have a higher affinity for T3 than for T4. It is postulated that the high affinity set of sites consists of hormone receptors.