Structure-function studies on red pigment-concentrating hormone, II. The significance of the C-terminal tryptophan amide

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 has been investigated. RPCH and a number of analogues that differ only in the C-terminal part of the hormone, have been synthesized and assayed for biological activity on the shrimp Leander adspersus. It has been shown that 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 has been compared with the activity of synthetic RPCH.