Specificity of the interation of thyroxine with rat adenohypophyseal proteins

The proportion of thyroxine specifically bound to the proteins of the control and the oestrogenized rat adenohypophysis was estimated by determining the rate of ligand exchange, which was found to be higher in the oestrogenized adenohypophysis. The dissociation constants of this interaction were determined by analyzing saturation of the binding proteins with mounting labelled thyroxine concentrations and with a mixture of labelled and unlabelled thyroxine, using the equilibrium-maintaining batchwise Amberlite method. K(dis) for the interaction of thyroxine with the proteins of the control adenohypophysis was 6 X 10(-8) M and K(dis) for its interaction with the proteins of the oestrogenized adenohypophysis was 2.5 X 10(-8) M (i.e., 2.4 times lower). The maximum binding site concentrations were 1.05 X 10(-11) mol/mg. protein for the control adenohypophyses and 0.625 X 10(-11) mol/mg. protein for the oestrogenized adenohypophyses (i.e., 60% of the control value). Oestrogenization probably primarily stimulates the synthesis of specific thyroxine receptors in the rat adenohypophysis.