Further studies were performed to define the kinetic characteristics of nuclear triiodothyronine (T3) binding sites in rat liver. Sequential determination of labeled T3 associated with nuclei and cytoplasm over a 4 hr period allowed analysis of the relationship of T3 in nuclear and cytoplasmic compartments. A rapid interchange of hormone between nuclei and cytoplasm was demonstrated, and in vitro incubation experiments with nuclei yielded no evidence favoring metabolic transformation of T3 by the nuclei. In vivo displacement experiments were performed by subcellular fractionation of liver 1/2 hr after injection of T3 with increasing quantities of unlabeled T3. The nuclear binding capacity for T3 could be defined (0.52 ng/mg DNA). Analysis of these experiments also allowed an estimation of the association constant of nuclear sites for T3 (4.7 x 1011M-1). The affinity of these sites for T3 was estimated to be 20-40 fold greater than for thyroxine (T4). Chromatographic analysis of the nuclear radioactivity after injection of labeled T4 indicated that the binding of T4 by the nucleus could not be attributed to in vivo conversion of T4 to T3 but reflected intrinsic cross reactivity of the 2 iodothyronines at the nuclear binding sites.
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