Binding of RU486 and deacylcortivazol to the glucocorticoid receptor is insensitive to sulfhydryl-modifying agents

The differential sensitivity of the rat liver glucocorticoid receptor (GR) to sulfhydryl group modifying agents when bound to various agonist and antagonist ligands was studied. [3H]Triamcinolone acetonide (TA) binding was completely abolished by previous treatment of the unbound receptor with various N-alkylmaleimides. On the contrary, [3H]RU486 binding was only slightly affected by treatment with N-ethylmaleimide (NEM) and more significantly decreased with maleimides bearing bulky substituents. Ligand exchange experiments demonstrated that, unlike the agonist TA, the antiglucocorticoid RU486 was unable to protect the GR binding site from the effect of NEM. This lack of protection would seem to be due to the presence of the bulky 11 beta-substituent in RU486 since RU26988 and RU28362, two 11 beta hydroxylated glucocorticoids bearing the same 17 alpha-propynyl side chain as RU486 but lacking the 11 beta-substituent could protect GR against NEM. The ability of a GR ligand to prevent NEM inactivation of TA binding appeared unrelated to its agonist or antagonist nature: deacylcortivazol, a potent agonist, afforded no protection whereas antagonists of the 17 beta-carboxamide series did. These data strongly suggest that compounds bearing bulky substituents on the steroid A and/or C rings, like deacylcortivazol and RU486, are positioned differently from canonical glucocorticoids in the steroid binding groove of the GR.