Binding of thyroid hormones by nuclei of target tissues during the chick embryo development

In the present studies we have compared the ontogeny of the binding of thyroxine (T4) and triiodothyronine (T3) to isolated nuclei from various target tissues of chick embryo. We observed a marked difference between the patterns of Satchard plots, maximal binding capacities (MBC) and association constants (Ka) of T4 and those of T3. Scatchard plots revealed that T4 and T3 had different binding sites. In liver, brain and lung MBCs and Kas of T3 and T4 were rather similar at day 9, but during the following days (12-19) T3 MBCs and Kas showed small changes, whereas T4 MBC markedly increased (4-5-fold) and T4 Ka significantly declined. In liver, for instance, T3 MBC = 395 +/- 19 (day 9) and 489 +/- 66 fmol/mg protein (day 19); T4 MBC = 631 +/- 6.5 (day 9) and 2201 +/- 516 fmol/mg protein (day 19); T4 Ka = 1.92 +/- 0.01 (day 9) and 0.56 +/- 0.21 X 10(8) M-1 (day 19). These data indicate that, during chick embryogenesis, nuclei of target tissues contain multiple T4 binding proteins, but only a single T3 binding site.