Thyroid hormone binding to isolated trout (Salmo gairdneri) liver nuclei in vitro: binding affinity, capacity, and chemical specificity.

Isolated nuclei from trout hepatocytes demonstrated in vitro high-affinity and low-capacity binding for 3,5,3'-tri-iodo-L-thyronine (T3). Binding was reversible; the dissociation rate was 5.7 X 10(-3) min-1 at 15 degrees C. Linear Scatchard plots suggested a single class of noncooperative sites (Kd = 1.4 +/- 0.10 X 10(-10) M; maximum binding capacity (MBC) = 62 +/- 10 fmol/mg DNA). After correction for site degradation, site occupancy by endogenous T3, and the dissociation rate of endogenous bound T3, the MBC was 106 +/- 16 fmol/mg DNA. The T3 affinity exceeded slightly that of the hepatocyte nucleus of the rat; the MBC was lower than for most other vertebrates. The relative binding affinities of thyroid hormone (TH) analogs for the T3 site were: TRIPROP greater than TRIAC greater than methyl-bridged T3 greater than TETRAPROP greater than T3 greater than TETRAC greater than TRIFORM greater than 3,5-dibromo, 3'-isopropyl thyronine greater than L-thyroxine (T4) greater than DL-T4 greater than 3'-isopropyl 3',5'-dimethyl thyronine greater than reverse T3 greater than 3,5-T2. MIT and DIT did not bind at all. This structure-affinity profile was similar but not identical to that of rat liver, indicating considerable but not complete evolutionary conservation of site structure. Parallel studies of T4 binding also indicate a single class of noncooperative sites (Kd = 7.2 +/- 2.4 X 10(-10) M). Both the MBC and the structure-affinity profile for T4 corresponded to those for T3. These observations, combined with the ability of excess T3 or T4 to completely displace both labeled T3 or T4, support a previous suggestion that in teleosts T3 and T4 bind to the same class of nuclear sites. These sites probably represent TH receptors.