Comparison of thyroid hormone binding to heptic nuclei of the rat and a teleost (Oncorhynchus kisutch).

A modified T3 binding assay was used with coho salmon liver nuclei. Purified nuclei were incubated with [125I]T3 (or [125I]T4), nonradioactive thyroid hormone, or thyroid hormone analog. Bound hormone was separated from hormone by use of polyethylene glycol. Salmon liver nuclei specifically bound T3 with a high affinity [Ka = 1.031 +/- 0.174 x 10(9) M-1 (mean +/- SE); n = 12] which was 50% of the affinity measured with rat liver nuclei using the same assay. Salmon nuclear T3-binding capacity was 0.127 +/- 0.035 pmol T3/mg DNA (n = 7). Rat liver nuclear capacity for T3 binding was 3.4-fold greater than in salmon liver nuclei. T3 binding to salmon liver nuclei was greater than to nuclei from salmon brain; T3 binding to salmon spleen nuclei was low. Binding competition studies with hormone analogs indicated that binding to the salmon nuclei was dependent on molecular structure. These data indicate that the T3 binding to salmon nuclei may be due to T3 receptor activity. Analysis of Scatchard plots showed that the affinity of salmon hepatic nuclei for T3 is 10-fold greater than the affinity for T4. Reciprocal binding competition experiments with T3 and T4 indicate that both hormones bind to the same site. This suggests that in salmon, as in mammals, the liver nuclei predominantly bind thyroid hormones at T3-binding sites. Hepatic nuclei of human, rat, chick embryo, amphibian larva, and salmon have similar affinities for thyroid hormones and similar relative affinities for several thyroid hormone analogs, suggesting that the receptor molecule has been highly conserved during evolution.