Immunologic Studies of Adrenocorticotropic Hormone (ACTH): Dissociation of Biologic and Immunologic Activities

Abstract

The relationship between the immunologic and biologic activities of ACTH was studied in a series of natural and synthetic as well as chemically modified ACTH preparations. Three natural ACTH preparations, sheep, human and pork (αs-, αh- and αp-ACTH) and synthetic 39 amino acid ACTH (αp1–39-ACTH) had similar immunologic activity when tested by radioimmunoassay. In contrast, synthetic 26 and 24 amino acid ACTH (αb1–26-ACTH and α1–24-ACTH), although fully active biologically, had very little immunologic activity. Synthetic 1–19 and 1–17 amino acid ACTH (α1–19-ACTH and α1–17-ACTH) had no detectable immunologic activity, although they have partial steroidogenic activity. Periodate oxidation completely destroyed steroidogenic activity but had no effect on immunologic activity of natural ACTH. Fibrinolysin digestion abolished the steroidogenic activity of ACTH but increased the immunologic activity. A biologically inert fraction corresponding to the 22nd to 39th amino acid of sheep ACTH (αs22–39-ACTH)was immunologically more active on a molar basis than the whole molecule. These results lead to the conclusion that the immunologic activity resides in the C-terminal portion of the molecule while the biologically active site is in the N-terminal portion, and that immunologic and biologic activities can be dissociated.