Abstract
We have investigated the thermodynamic properties of triiodothyronine (T3)-solubilized hepatic nuclear receptor interaction. These studies revealed that the equilibrium constant was markedly dependent on temperature and van't Hoff plots revealed a significant deviation from linearity. Both enthalpy and entropy changes decreased with increasing temperature such that at 37 degrees C both were negative. (delta H0 -27.1 kcal/mol; delta S0, -45.1 cal/mol.deg; and the heat capacity delta Cp0 (25 degrees C), -759 cal/mol.deg). Several T3 analogues revealed similar thermodynamic characteristics. Our finding that the T3-receptor interaction is characterized by a negative heat capacity is compatible with the previous proposal of hydrophobic bonding by Jorgensen. The large negative entropy change at 37 degrees C for T3-receptor reaction is in contrast to the more positive entropy values for several T3-plasma protein reactions. One possible explanation of this difference in entropy values between the nuclear receptor and plasma proteins is that T3 induces a conformational change in the receptor, a concept supported by previous data from our laboratory indicating that the occupation of the T3 receptor results in an alteration in the chromatographic migration of the receptor.
Highlights
The substitutionof iodine in the 3' and 5' position negative ASo than hormone agonist non-receptor protein inappears to have no majoefrfect on theACP'
Thisrathersmall rium techniques is complicated by uncertainty of free ligand change could not be detected by the approach taken in concentration [22].Our studies suggested that asignificant this study
Since the thermodyreceptor reaction with T3 non-receptor reactions reveals that namic characteristics of the reaction weresimilar by both the entropyof T:3-receptor reaction isconsiderably more neg- methods of determining theKAvalue, the presence of possible ative than the entropy values for Ta-non-receptor reactions inapparent binding sites appeared not to significantly alter (Table 111).One possible interpretation of this difference in the relationbetween KA andtemperature
Summary
(Received for publication, February 18, 1981,and in revised form, October 12, 1981). From theDivision of Endocrinology and Metabolism, Department of Medicine, University of Minnesota, Minneapolis, Minnesota 55455. We have investigated the thermodynamic properties thermodynamic considerations may be of value in assessing of triiodothyronine (Ta)-solubilized hepatic nuclear re-the effect of ligand binding on the conformation of the receptor ceptor interaction. We speculate that explanation of this differenceinentropyvaluesbethe difference in entropy change between receptor and nontween the nuclear receptoarnd plasma proteiniss that receptor Ts-binding proteins may reflect a specific Ts-induced. Concept supported by previous data from our laboratory indicating that the occupationof the TSreceptor
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