ACTH Receptors in the Adrenal: Specific Binding of ACTH- 125 I and Its Relation to Adenyl Cyclase

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Abstract
Pure monoiodo ACTH-125I was prepared that was biologically active and free of unlabeled ACTH. Extracts of adrenal cortex that contained ACTH-sensitive adenyl cyclase, bound ACTH-125I; extracts that lacked the ACTH-sensitive cyclase did not bind ACTH-125I. Unlabeled ACTH inhibited the binding of ACTH-125I. Five ACTH derivatives which varied widely in biological activity were tested. All inhibited the binding of ACTH-125I in direct proportion to their biological activity. Albumin, insulin, and four unrelated iodinated hormones were inert. The addition of excess hormone or acetic acid produced rapid dissociation of bound ACTH-125I. This study demonstrates directly the binding of ACTH to its biologically significant site.