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

Wilson B.D. ; Gent W.L. (1985)


K'(A) (apparent association constant) and B(max.) (total receptor concentration) describing the interaction of tri-iodothyronine (T3) and it solubulized 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 strenght (I) and of pH on K'(A) and B(max.) appropriate measurements have been made simultaneously on single preparations. K'(A) and B(max.) were found to be effectively unchanged over the range I 0.05-0.60. Both parameters have been measured over the range pH 6.4-9.0 and the vlaues 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 α-NH3+ of T3 or with a group (RH+) in the receptor site. On the balance of evidence the first possibility is more likely, in which case the variation of B(max.) with pH is ascribed to conformational changes in the receptor protein.

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