Stability of nuclear thyroid-hormone receptor and its behavior on hydroxylapatite

Rat-liver chromatin was kept under several conditions. The maximum binding capacity (Bmax) and the apparent association constant (Ka) for triiodothyronine (T3)-binding were determined. Binding activity (Bmax x Ka) was unstable at 0 and -20 degrees C in the reaction buffer. Maximum binding activity was found upon storage at -70 degrees C or in liquid nitrogen where about 70% of the original activity remained after 3 weeks. With storage, Ka decreased more rapidly than Bmax which showed little change. These temperature-dependent changes in the stored chromatin were specific for the receptor and not observed for other characteristics of the chromatin, i.e., template activity and the electrophoretic separation pattern of chromatin proteins. The elution profile of the receptor on a hydroxylapatite column was unchanged during storage of the chromatin, suggesting that the chemical nature of the binding protein(s) may not change during storage.