Structure-Melanotropic Activity Relationship in Synthetic Polypeptides Related to ACTH

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Melanotropic (MSH) activities of synthetic octadeca and tetracosa-peptides were compared with those of α-MSH, β-human-MSH, and porcine ACTH. The MSH activity was determined by in vivo approach using a newly devised reflectance meter to measure the skin colour of Xenopus frog. From the shapes of their time-response curves, α-MSH, Gly1-ACTH (1-18) amide (I), and β-Ala1-ACTH (1-18) amide (II) werefound to be of short action, whereas two long chain peptides, porcine ACTH and ACTH (1-24)-OH were of comparatively long action.β-human-MSH, Orn15-ACTH (1-18) amide (III), and βA-Ala1, D-Phe7, Orn15-ACTH (1-18) amide (IV) possessed extremely prolonged activity. The MSH potencies of these peptides were calculated from the dose required to give a minimal melanotropic response and were ranked as IV>β-human-MSH=II=III>ACTH=ACTH (1-24) OH>I.Compound IV was found to be about 400 times as potent as native ACTH. This ranking differs slightly from that found in the in vitro MSH assay proposed by Inouye et al.(1970 b) and is markedly different from the ranking in steroidogenic and lipolytic activities proposed by Tanaka (1971). The structure-activity relation and possible mechanism for these properties are discussed.
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