Comparison of Thyroid Hormone Binding to Hepatic Nuclei of the Rat and a Teleost (Oncorhynchus kisutch)*

Abstract

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 free hormone by use of polyethylene glycol. Salmon liver nuclei specifically bound T³ with a high affinity [K_a = 1.031 ± 0.174 × 109 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.