Binding of thyroxine and triiodothyronine by nuclei of isolated tadpole liver cells.

The binding of L-triiodothyronine (T3) and L-thyroxine (T4) to cytoplasm and nuclei has been studied in isolated Rana catesbeiana tadpole liver cells. Nuclear binding for both thyroid hormones occurred more slowly at 4 C than at 25 C, but reached the same level as at 25 S. Scatchard analyses suggest high affinity, saturable binding sites for both hormones in the nuclear but not in the cytoplasmic fraction. Apparent equilibrium dissociation constants were 6.8 X 10(-10)M and 4.6 X 10(-10)M for T3 and T4, respectively. The maximum number of binding sites per nucleus for T3 was about 12,300 and for T4 about 2,300. Unlabeled T3 competed for the binding of [125I]T3 to nuclei more effectively than unlabeled T4. No difference in the competition of [125I]T4 binding with non-radioactive T3 or T4 was found. Chromatographic analysis of the bound nuclear radioactivity demonstrated no chemical modification for either hormone.