Dexamethasone 21-(beta-isothiocyanatoethyl) thioether: a new affinity label for glucocorticoid receptors

The C-21 methanesulfonate ester of the synthetic glucocorticoid dexamethasone (Dex) is an efficient electrophilic affinity label of glucocorticoid receptors and exhibits irreversible antiglucocorticoid activity. In an effort to obtain other affinity labeling steroids with differing biological activities, several new derivatives of Dex were prepared which contained a reactive electrophilic substituent at various distances from the C-21 position. All compounds displayed relatively low affinity for rat glucocorticoid receptors (less than or equal to 8% of that of Dex) in a cell-free competition assay. Nevertheless, one compound, dexamethasone 21-(beta-isothiocyanatoethyl) thioether (Dex-NCS), appeared to be an affinity label by virtue of its ability to block the cell-free exchange binding of [3H]Dex. [3H]Dex-NCS was thus synthesized and reacted with cell-free receptors to give, after analysis on denaturing SDS-polyacrylamide gels, only one specifically labeled species at 98 kDa, which is the molecular weight of authentic rat glucocorticoid receptor. These data directly establish Dex-NCS as a new affinity label for glucocorticoid receptors. Data on the reactivity of Dex-NCS and the stability of [3H]Dex-NCS-labeled receptors suggest that a cysteine SH group has been labeled.