Structure-Function Studies on Red Pigment-Concentrating Hormone, II. The Significance of theC-Terminal Tryptophan Amide

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Abstract
The significance of the C-terminal tryptophan residue of the red pigment-concentrating hormone (RPCH: Glu-Leu-Asn-Phe-Ser-Pro-Gly-Trp-NH2) regulating the blanching of the crustacean chromatophores was investigated. RPCH and a number of analogs that differ only in the C-terminal part of the hormone, were synthesized and assayed for biological activity on the shrimp, Leander adspersus. The indole skeleton of tryptophan is an absolute requirement for the biological activity of the hormone. To provide maximum response the tryptophan must be blocked as the amide. The activity of synthetic [Tyr4]RPCH and adipokinetic hormone (AKH) purified from Schistocerca gregaria, was compared with the activity of synthetic RPCH.