Identification of dexamethasone-binding sites on male-rat liver plasma membranes by affinity labelling

Binding studies with [³H]dexamethasone identified two binding sites on plasma membranes prepared from the male rat liver, a low-capacity site with K(D) of 7.0 nM and a higher-capacity site with a K(D) of 90.1 nM. Both sites exhibited glucocorticoid responsiveness and specificity for glucocorticoids and progestins. Triamcinolone acetonide, which competes well for the binding of dexamethasone to the cytosolic glucocorticoid receptor, did not compete well for the binding of [³H]dexamethasone to the plasma-membrane binding sites. The binding sites were sensitive to protease and neuraminidase treatment, and resistant to extraction with NaCl, but were extracted with the detergent Triton X-100. As these experiments indicated the presence of plasma-membrane protein components which bind glucocorticoids at physiological concentrations, affinity-labelling experiments with dexamethasone mesylate were conducted. Two peptides were specifically labelled, one at approx. M(r) 66000 and one at M(r) 45000. The M(r)-66000 peptide was not sensitive to glucocorticoids, and was extracted by NaCl, and so did not correspond to either of the sites identified in the dexamethasone-binding studies. The M(r)-45000 entity, on the other hand, resembled the dexamethasone-binding sites in its response to glucocorticoid manipulation of the animal and in its resistance to salt extraction. This peptide was not present in rat serum. Thus we have identified a plasma-membrane peptide which binds dexamethasone. Whether this peptide is involved in transport of the glucocorticoid across the plasma membrane remains to be determined.