Radioimmunologic Evidence for α-MSH (Melanocyte Stimulating Hormone) in Human Pituitary and Tumor Tissues

Abstract

Immunization of a rabbit with synthetic α-MSH resulted in an antiserum that neutralized the biologic activity of α-MSH but not β-MSH or ACTH. This antiserum was used in the development of a radioimmunoassay method for α-MSH. Binding of ¹³¹I-α-MSH to antibodies could be demonstrated by chromatoelectrophoresis, and displacement of ¹³¹I-α-MSH from the antibodies was observed with 0.1–5.0 nanograms (ng) of unlabeled synthetic α-MSH. Several analogues of α-MSH, including highly purified human ACTH, α_P-ACTH, α^1–10ACTH, α^1–24ACTH, α^11–24ACTH, and 3 kinds of β-MSH (human, porcine and bovine) all failed to bring about comparable displacement of ¹³¹I-α-MSH from the antibodies, even in quantities as large as 10 μg-Two analogues, ACTH and Nacetyl-α^1–24ACTH, showed some cross-reactivity with the α-MSH antiserum, but the quantities required for displacement of ¹³¹I-α-MSH were 2000 times greater than those of α-MSH itself. Extracts of human pituitaries and extracts of tumors from patients with the “ectopic ACTH syndrome” displaced ¹³¹I-α-MSH from the antibodies with dissociation curves identical in slope with that of α-MSH. The estimated quantity of α-MSH in human pituitaries was about 1 μg/g of wet tissue and represented about 2% of the total MSH as estimated by bio-assay. The concentrations of α-MSH in 2 tumors were 0.07 and 0.12 μg/g of wet tissue, representing about 15 % of the total MSH as estimated by bio-assay.