Identification and characterization of corticotropin receptors in bovine and human adrenals

Corticotropin (ACTH) receptors have been characterized by covalent cross-linking of radiolabeled ACTH ([125I]ACTH) with the bifunctional cross-linking reagent disuccinimidyl suberate to cultured bovine adrenal fasciculata reticularis cells (BAC), and to crude plasma membrane fractions prepared from both human and bovine adrenals. Incubation of BAC with [125I]ACTH at 20 degrees C followed by cross-linking resulted in the specific labeling of two binding proteins with apparent M(r) of 154,000 and 43,000 as measured by SDS-PAGE under reducing and non-reducing conditions. In addition, in some experiments another band with an apparent M(r) of 124,000 was observed. All of these bands disappeared when the incubation was performed in the presence of an excess of unlabeled ACTH. When BAC were incubated with [125I]ACTH in the presence of 100 microM phenylarsine oxide at 20 degrees C, a condition which prevents the internalization of the ACTH-receptor complex, the bulk of the radioactivity was present in the 43,000 band. After [125I]ACTH cross-linking to BAC, subcellular preparations followed by SDS-PAGE revealed that the 20,000 g fraction contained mainly the 43,000 M(r) form. Cross-linking of [125I]ACTH to plasma membrane-enriched fractions prepared from human and bovine adrenals resulted only in the labeling of the 43,000 protein. These results indicate that the ACTH receptor present at the cell surface is a macromolecule of 43,000, and suggest that the 154,000 form probably represents association of the ACTH-receptor complex to another macromolecule. The 154,000 protein would be formed during or after internalization of the ACTH-receptor complex.