Development and Validation of a Radioimmunoassay for Peptides Related to β-Melanocyte-Stimulating Hormone in Human Plasma: The Lipotropins

Abstract

A radioimmunoassay is described for the measurement of human “β-melanocyte-stimulating hormone” (“β_h-MSH”). Two antisera have been used, one of which cross-reacts with synthetic β_h-MSH as well as with the two larger pituitary peptides β_h- and γ_h-lipotropin (β_h- and γ_h-LPH) and the other mainly with β_h-MSH and γ_h-LPH. The sensitivity and reliability of the assay have been improved by employing a simple plasma extraction procedure, and the shelf-life of the iodinated β_h-MSH tracer has been increased more than five-fold by storage in a concentrated human serum albumin solution. Using a 5 ml plasma sample the detection limit is 6 pg/ml. The mean resting “β_h-MSH” level in normal subjects is 21 pg/ml (range 13–38 pg/ml) at 9 am and 12 pg/ml (range 6–20 pg/ml) at 9 pm. Levels are considerably elevated (51–12,000 pg/ml) in patients with Addison's disease, Nelson's syndrome, Cushing's disease and the “ectopic” ACTH syndrome. After administration of insulin or pyrogen, the concentration of plasma “β_h-MSH” increases in parallel with that of ACTH and they are approximately equivalent on a molar basis. The stability of purified β_h- and γ_h-LPH, and endogenous “β_h-MSH” when incubated in vitro in fresh blood or plasma are similar, in contrast to the less stable peptide synthetic β_h-MSH. It is suggested that “β_h-MSH” immunoreactivity in human plasma is due to β_h- and γ_h-LPH rather than β_h-MSH.