Binding of thyroid hormone by erythrocyte cytoplasmic proteins.

Characteristics of iodothyronine-binding to dog erythrocyte cytosol proteins are described. Half-time of association of both thyroxine (T₄) and triiodothyronine (T₃) is 60 min and equilibrium is achieved at 120 min (20°C). Binding is enhanced at 37°C compared to 20°C. T₄ and T₃ binding capacities of the cytosol are 10 and 5 picomoles/mg cytosol protein, respectively. Gel filtration (G-100) reveals 3 protein fractions that dissociably bind both T₄ and T₃. Quantitative and qualitative differences distinguish the erythrocyte cytosol “receptor” proteins from those previously described in other dog tissues. The erythrocyte is a model for studying functions of cytosol “receptors” for iodothyronines.