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Pure monoiodo ACTH- 125 I was prepared that was biologically active and free of unlabeled ACTH. Extracts of adrenal cortex that contained ACTH-sensitive adenyl cyclase, bound ACTH- 125 I; extracts that lacked the ACTH-sensitive cyclase did not bind ACTH- 125 I. Unlabeled ACTH inhibited the binding of ACTH- 125 I. Five ACTH derivatives which varied widely in biological activity were tested. All inhibited the binding of ACTH- 125 I 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- 125 I. This study demonstrates directly the binding of ACTH to its biologically significant site.
Lefkowitz et al. (Sun,) studied this question.