Binding of Thyroid Hormones to Isolated Hepatic Nuclei from Rana catesbeiana Tadpoles*

Abstract

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 [¹²⁸I]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 [¹²⁶I]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.