Isolated hepatic nuclei from euthyroid rats were incubated with tracer (125I)L-triiodothyronine (T3) and increasing doses of nonradioactive T3 for 30 min at 37degrees C. The T3 bound specifically to nuclear sites increased with increasing T3 doses to a plateau, which represented the nuclear binding capacity, M. Addition of 1 mM KCN, NaF, dinitrophenol, oriodoacetate did not affect nuclear binding, indicating that active metabolism was not required. Kinetic studies showed that the nuclear sites were equilibrated with T3 within 30 min of incubation (one-half maximal binding at 3 min) and that the rate of release of T3 in vitro (0.058 min-1) was the same for endogenous T3 or for T3 bound to nuclei in vitro. Nuclear T3 resisted extraction with 0.14 M NaC1 buffered at pH 7.5, but both endogenous hormone and T3 bound in vitro were readily extracted by 0.4 M KC1 at pH 8.0. The elution profiles of endogenous and in vitro-bound T3 from Sephadex G-100 columns showed a common protein peak with a molecular weight of 60-65,000, assuming a globular protein. Scatchard analysis of in vitro displacement studies showed a single class of binding sites. Mean M equals 0.23 times 10-9 M or 0.85 ng T3 for nuclei isolated from 1 g of liver. Mean M closely corresponded to that anticipated from reported in vivo studies. The apparent association constant Ka for the nuclear sites, 5.55 times 108 M-1, was lower than in studies in vivo, probably attributable to the different ionic milieu of nuclei in the incubation buffer and in the intact cell. Thus, the identity of the nuclear T3 binding sites studied in vitro to those reported for endogenous hormone is demonstrated by similar binding capacities, release rates, analogue binding affinities (previously reported), and localization to chromatin nonhistone proteins of comparable molecular weight. The role of cytosol protein in nuclear binding was assessed by comparing binding parameters for extensively washed nuclei and nuclei incubated either with contaminating or added cytosol. No difference in Ka or M was found. Moreover, it was unlikely that specific cytosol proteins were already present in nuclei and functioned during incubation as a shuttle for T3, since Ka and M for nuclei obtained from athyreotic rats were similar to Ka and M for nuclei from euthyroid animals. Thus, an initial interaction between T3 and specific cytosol proteins does not appear to be a prerequisite for translocation of T3 to nuclear sites.
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