In previous studies of nuclear extracts containing solubilized receptors for thyroid hormones, triiodothyronine (T 3) but not thyroxine (T 4) binding activity was lost, even though in the starting material T 3 and T 4 appeared to bind to the same proteins. In the current studies this paradoxical loss of T 3 but not T 4 binding activity was studied in greater detail. Competition, iodoacetamide inhibition and binding site concentration studies with [ 125I]T 3 and [ 125I]T 4 were consistent with the idea that T 3 and T 4 are bound by the same protein(s) in the initial extracts. When such preparations were heated at 50°C, [ 125I]T 3 but not [ 125I)T 4 binding activity was rapidly lost. There was no change in the T 4 binding site concentration and no change or a modest reduction in affinity. By contrast, heating markedly reduced the concentration of high affinity T 3 binding sites; the affinity of the remaining sites for T 3 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 T 3 binding site concentration was markedly reduced but the T 4 binding site concentration and affinity were minimally affected. T 3 was a more avid competitor than T 4 for [ 125I]T 4 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 T 4-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 T 4, but which has reduced affinity for T 3; in this case a fundamental unit of the receptor could be more similar than previously apparent to certain cytosol or plasma proteins than bind T 4 more avidly than T 3.
|Original language||English (US)|
|Number of pages||6|
|Journal||Journal of Biological Chemistry|
|State||Published - 1979|
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