Photoaffinity labeling of corticotropin receptors

A photoaffinity label for corticotropin (ACTH) receptors was prepared by selective chemical modification of the single tryptophan residue in the hormone by reaction with 2-nitro-5-azidophenylsulfenyl chloride. The photoreactive derivative. [(2-nitro-5-azidophenylsulfenyl)-Trp⁹]ACTH (2,5-NAPS-ACTH), stimulated corticosterone synthesis to 60% of the maximal rate induced by ACTH in isolated rat adrenocortical cells. 2,5-NAPS-ACTH caused only a marginal stimulation of cyclic AMP production compared to the unmodified hormone. Stimulation of corticosterone production and cyclic AMP accumulation induced by ACTH were both inhibited in a competitive manner by 2,5-NAPS-ACTH. Photolysis of adrenocortical cells in the presence of 2,5-NAPS-ACTH resulted in a 40% inactivation of ACTH receptors mediating steroidogenesis, as shown by the decrease in response to subsequent stimulation with ACTH. No loss of function was observed when photolysis was conducted in the presence of the photoresistant analog [(2,4-dinitrophenylsulfenyl)-Trp⁹]ACTH. Covalent attachment of the hormone to the receptors was also demonstrated by photolyzing adrenocortical cells in the presence of tritiated 2,5-NAPS-ACTH of high specific radioactivity (90 Ci/mmol) and analyzing the cell proteins by sodium dodecyl sulfate/polyacrylamide gel electrophoresis. A protein with an approximate molecular weight of 100,000 was specifically labeled by this procedure. The unique labeling of an adrenocortical cell protein and the concomitant loss of ACTH responsiveness suggest that physiologically relevant receptors are photolabeled by this method.