Effects of pH and ionic strength on the binding of L-tri-iodothyronine to the solubilized nuclear receptor

K'A (apparent association constant) and Bmax. (total receptor concentration) describing the interaction of tri-iodothyronine (T3) and its solubilized rat liver nuclear receptor (R) are found to be moderately consistent in successive preparations, but both quantities diminished after a few days. To achieve comparability in the effects of ionic strength (I) and of pH on K'A and Bmax, appropriate measurements have been made simultaneously on single preparations. K'A and Bmax. were found to be effectively unchanged over the range I0.05-0.60. Both parameters have been measured over the range pH 6.4-9.0 and the values of K'A analysed in terms of the 4'-OH ionization of T3 and that of a cationic acidic group, shown to require pK' = 7.6. This group could be identified either with the terminal alpha-NH3+ of T3 or with a group (RH+) in the receptor site. On the balance of evidence the first possibility is the more likely, in which case the variation of Bmax. with pH is ascribed to conformational changes in the receptor protein.