The Preparation of Biologically Active125I-Labeled Adrenocorticotrophic Hormone by a Simple Enzymic Radioiodination Procedure Utilizing Lactoperoxidase

Adrenocorticotrophic hormone (ACTH) was radiolabeled by a method utilizing the enzyme lactoperoxidase. The 125I-labeled hormone so prepared retained between 50–55% of its biological activity as assayed using isolated adrenal cells, and had dose-response characteristics identical with native ACTH over the range 10−11−10−8g in this bioassay system. Specific activities of up to 600 μCi/μg have been attained using this method. The 125I-labeled hormone was found to be of high purity following CM-cellulose chromatography and gel-electrophoresis. Sequential Edman degradation of the labeled hormone showed that the position of the iodine atom was predominantly (>80%) on the tyrosine at position two in the ACTH molecule. (Endocrinology94: 1259, 1974)