[The characterization of nuclear triiodo-L-thyronine receptors of rabbit liver (author's transl)].

The binding characteristics and physical properties of the nuclear receptors of rabbit liver were studied. The receptors were extracted with 0.4M KCl from purified hepatic nuclei and were incubated with increasing doses of 125I-triiodo-L-thyronine (L-T3) and stable L-T3 for 2 hours at 20 degrees C. Scatchard analyses indicated that the association constant (Ka) of the nuclear receptors was 1.5 X 10(10)M-1, and that maximal binding capacity was about 0.12 pmol T3 per 1 ml of nuclear extract, equivalent to 1 g of liver or 0.2 pmol T3 per mg of protein of the nuclear extracts. The nuclear binding sites were specific for L-T3. When compared by the molar concentrations of hormone analogues required to produce 50% inhibition of L-T3 binding, the relative binding affinities of triiodothyroacetic acid (Triac), D-T3, L-T4 and D-T4 were, respectively, 1/2, 1/4, 1/50 and 1/170 of L-T3. The binding affinity of Triac for the isolated hepatic nuclei was also nearly half that of L-T3. Furthermore, to displace radioactive Triac from the binding sites, Triac was half as effective as L-T3. The molecular weight of the nuclear receptors was estimated to be about 40000 to 45000 by the elution profiles from a Sephadex G-100 column, and the sedimentation coefficient was slightly less than 4S on sucrose density gradient centrifugations. Both elution profiles from DEAE- and ECTEOLA- cellulose columns with a linear KCl gradient showed a sharp, narrow peak of radioactive T3 at about 0.15 to 0.2M KCl. The results obtained with rabbit liver were similar to those reported previously in rat liver except for the relative binding affinity for Triac. It is possible that this discrepancy may be due to a species-related difference.