Modulation of nuclear receptor affinity for 3,5,3'-triiodothyronine by nucleotide in vitro

Nuclear non-histone protein fraction containing saturable receptors for thyroid hormone was prepared from rat liver by extraction with 0.3 mol l-1 NaCl. Inhibitory effect of ATP, UTP, GTP and CTP (1 mmol l-1 each) when co-incubated with the nuclear receptor and 3,5,3'-triiodothyronine was demonstrated. The experiments were conducted in the presence of 25 mmol l-1 Na2HPO4 in order to suppress the effect of the phosphate groups of nucleotide molecule by inorganic phosphate. All nucleotides inhibited (P less than 0.05 to 0.001) the specific binding of 3,5,3'-triiodothyronine to soluble receptors in the order ATP greater than CTP greater than GTP greater than UTP. In addition inhibitory effect of 0.5 mmol l-1 ATP on 3,5,3'-triiodothyronine binding to receptors was re-examined by Scatchard plot analysis. The data suggested that the diminution in the specific binding of receptors in the presence of ATP was due to an effect on the equilibrium association constant. The maximum binding capacity of receptors remained unchanged when the concentrations of ATP ranging from 0.1 to 1.0 mmol l-1 were used.