Atger M, Milgrom E
J Biol Chem. 1976 Aug 10;251(15):4758-62.
Steroid-receptor complexes formed at low temperature and ionic strength do not bind to nuclei or chromatin. After a temporary exposure to high temperature, or ionic strength, or both, a fraction of them becomes activated (able to bind to nuclei). An assay of the activated form of the complex based upon titration with nuclei in excess was established. This assay was used to perform kinetic and equilibrium studies of the thermal activation of glucocorticoid-receptor complex in order to elucidate its mechanism. It was found that the reaction is of apparent first order and yields a monomolecular product. It thus probably consists of a conformational change in the steroid-receptor complex. The rate of activation is 1.37 +/- 0.06 X 10(-3) S-1 at 25 degrees. The free energy of thermodynamic activation (The word activation is used here in its usual thermodynamic meaning and not in the sense of receptor modification) of this reaction is greater than G = 21.3 Kcal. The corresponding enthalpy and entropy are respectively greater than H = 31.4 kcal and greater than S = 4 cal/degree. These positive and high values of greater than H and greater than S are very similar to those described for denaturation reactions of proteins suggesting that breakage of some noncovalent bonds could take place during activation. The reaction proceeds until approximately 60% of the complexes are activated. It was shown that this corresponds to an equilibrium between activated and nonactivated forms and not to the presence of a population of complexes unable to undergo activation. This equilibrium is not modified by temperature variations between 10 degrees and 30 degrees. It is possible to activate over 80% of the complexes when the activation is performed in the presence of excess acceptor, thus shifting the equilibrium. A similar situation is probably observed in situ in cells since 90% of the complexes are found in the nuclei when liver slices are incubated with hormone.
在低温和低离子强度下形成的类固醇 - 受体复合物不会与细胞核或染色质结合。在短暂暴露于高温、或高离子强度、或两者之后，其中一部分会被激活（能够与细胞核结合）。基于用过量细胞核进行滴定建立了复合物活化形式的测定方法。该测定方法用于对糖皮质激素 - 受体复合物的热活化进行动力学和平衡研究，以阐明其机制。发现该反应为表观一级反应，并产生单分子产物。因此它可能由类固醇 - 受体复合物的构象变化组成。在25℃时活化速率为1.37±0.06×10⁻³ s⁻¹。该反应的热力学活化自由能（此处“活化”一词是其通常的热力学意义，而非受体修饰的意义）大于G = 21.3千卡。相应的焓和熵分别大于H = 31.4千卡和大于S = 4卡/度。这些大于H和大于S的正值且较高的值与蛋白质变性反应所描述的值非常相似，表明在活化过程中可能发生了一些非共价键的断裂。反应进行到约60%的复合物被激活。结果表明，这对应于活化形式和未活化形式之间的平衡，而不是存在一群无法活化的复合物。该平衡在10℃至30℃之间的温度变化下不会改变。当在过量受体存在下进行活化时，可以激活超过80%的复合物，从而使平衡发生移动。在细胞原位可能也观察到类似情况，因为当肝切片与激素一起孵育时，90%的复合物存在于细胞核中。
