CHARACTERISTICS OF A MEMBRANE-ASSOCIATED STEROID-BINDING SITE IN RAT-LIVER

The glucocorticoid dexamethasone binds a site in microsomes in a saturable manner which by competition studies also binds other classes of steroids. The characteristics of dexamethasone binding to microsomes is distinct from the cytosolic glucocorticoid receptor by virtue of a slower rate of association; a differential competition by the glucocorticoid receptor agonist triamcinolone acetonide and antagonist RU38486; and a lack of sensitivity to the reversible thiol reactive agent arsenite. However, both binding sites have a similar rate constant for complex dissociation; are sensitive to covalent thiol modification by N ethylmaleimide and iodoacetamide; and have a similar concentration-dependent sensitivity to the reversible thiol reactive agent methyl methanethiosulfonate. The binding of dexamethasone by microsomes therefore exhibits distinct properties from the soluble glucocorticoid receptor.