Iodinated Derivatives of Adrenocorticotropic Hormone

Iodinated derivatives of adrenocorticotropic hormone (ACTH) were prepared with the use of NaI and chloramine-T and isolated by isoelectric focusing. The monoiodinated derivative (pI 7.6) was resolved into [monoiodoTyr2]ACTH and [monoiodoTyr23]ACTH by chromatography on columns of carboxymethylcellulose. Both monoiodo-ACTH species exhibited only 30 % of the maximum hormonal activity when assayed in an ACTH-sensitive adenylate cyclase system from adrenal tumor cells. The loss of hormonal activity was a consequence of oxidation rather than iodination. Iodination with lactoperoxidase instead of chloramine-T did not preserve hormonal activity. The monoiodinated derivatives of ACTH prepared with chloramine-T as oxidant were restored to full hormonal activity following reduction with cysteine in acetic acid. Mild acid hydrolysis accompanying the reduction reaction resulted in the conversion of the iodinated derivatives to products identified as [monoiodoTyr2]α1–24-ACTH and [mono-iodoTyr23]α1–24-ACTH.