Triiodothyronine Nuclear Receptor in Chick Embryo Nature and Properties of Hepatic Receptor*

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Abstract
Livers uclei from chick embryos of 17 day incubation are capable of binding T3. This binding was optimal at 37 C and was abolished by the addition of proteolytic enzymes to the incubation medium. Approximately 70% of the nuclear bound [125I]T3 was removed with 0.4 M KC1 at pH 8.0 and was bound to a macromolecule, since 40-60% of the radioactivity in these extracts was excluded from small Sephadex G-50 columns. Glycerol density gradient analysis showed that this macromolecule had a sedimentation coefficient of 4.3S. From these data and the results of Sephadex G-100 gel filtration, the molecular weight of the [125I]T3 protein complex was calculated as 60,000. Binding studies performed on two preparations of purified nuclei and on nuclear extracts revealed a single class of binding sites with the properties of a receptor. In the experiments with purified nuclei, the association constant varied between 1.04– 1.07 × 109 M-1 whereas with nuclear extracts it was 2.17 × 109 M-1 The binding capacity was 363 fmol (0.23 ng) T3 and 836 fmol (0.54 ng) T3 per mg protein in the two preparations of purified nuclei. The nuclear extracts had a capacity of 466 fmol (0.31 ng) T3/mg protein. Expressed as femtomoles per ng DNA, the binding capacities were, respectively, 1.6, 2.61, and 1.06. Serial studies at different periods of embryogenesis showed that the binding capacity and the association constant rose progressively during embryogenesis (binding capacity: 9 days, 0.45 ± 0.08; 19 days, 0.73 ± 0.35 ng T3/mg DNA; Ka: 9 days, 1.04 ± 0.21; 19 days, 3.31 ± 0.32 x 109 M-1. T3 analogs were not very effective in inhibiting [125I]T3 nuclear binding, with the exception of Triac (3,5,3′-triiodothyroacetic acid) and 3,5,3′-triiodo-D-thyronine, but they all displaced the labeled T4. These findings indicate that early in embryogenesis the chick embryo liver possesses a putative T3 receptor similar to that observed in other experimental animals.