Thyroid hormone receptors. Alteration of hormone-binding specificity

In previous studies of nuclear extracts containing solubilized receptors for thyroid hormones, triiodothyronine (T₃) but not thyroxine (T₄) binding activity was lost, even though in the starting material T₃ and T₄ appeared to bind to the same proteins. In the current studies this paradoxical loss of T₃ but not T₄ binding activity was studied in greater detail. Competition, iodoacetamide inhibition and binding site concentration studies with [¹²⁵I]T₃ and [¹²⁵I]T₄ were consistent with the idea that T₃ and T₄ are bound by the same protein(s) in the initial extracts. When such preparations were heated at 50°C, [¹²⁵I]T₃ but not [¹²⁵I)T₄ binding activity was rapidly lost. There was no change in the T₄ binding site concentration and no change or a modest reduction in affinity. By contrast, heating markedly reduced the concentration of high affinity T₃ binding sites; the affinity of the remaining sites for T₃ was 0.1% that of the starting material. When the pH of the extract was lowered from 7.6 to 6.0 the high affinity T₃ binding site concentration was markedly reduced but the T₄ binding site concentration and affinity were minimally affected. T₃ was a more avid competitor than T₄ for [¹²⁵I]T₄ binding at pH 7.6 but had weak competitor activity at pH 6.0. These heat- and pH-induced influences could occur if the receptors are destroyed and a T₄-binding species is generated or exposed. Alternatively, the data can be explained by a model in which the thyroid hormone receptor can be converted to a form which retains binding activity for T₄, but which has reduced affinity for T₃; in this case a fundamental unit of the receptor could be more similar than previously apparent to certain cytosol or plasma proteins than bind T₄ more avidly than T₃.