Solid-phase synthesis of cionin, a protochordate-derived octapeptide related to the gastrin/cholecystokinin family of peptides, and its mono-tyrosine-sulfate-containing derivatives.

Cionin, a protochordate-derived octapeptide amide related to the gastrin/cholecystokinin family of peptides, contains two consecutive tyrosine sulfate residues. In order to gain insight into the role of the respective tyrosine sulfate residue in biological activity, cionin and its derivatives in which one of the two tyrosine sulfate residues was replaced by tyrosine, were prepared by two Fmoc-based solid-phase approaches. In approach (1) Fmoc-Tyr(SO3Na)-OH was employed as a building block to assemble the Tyr(SO3Na)-containing peptide-resin, and a global deprotection/cleavage was conducted with 90% aqueous TFA in the presence of m-cresol and 2-methylindole at 4 degrees C. In approach (2) the Tyr(Msib) [Msib = p-(methylsulfinyl)benzyl] derivative was used for the peptide-chain assembly to achieve sulfation on the selective Tyr residue. Partially protected peptide with the Msib/Msz protecting groups [Msz = p-(methylsulfinyl)benzyloxycarbonyl], obtained after peptide-resin cleavage, was treated with DMF-SO3 complex in the presence of ethanedithiol to achieve the sulfation of free Tyr residue and the reduction of the Msib/Msz groups to TFA-labile Mtb/Mtz groups [Mtb = p-(methylthio)benzyl, Mtz = p-(methylthio)benzyloxycarbonyl]. Final deprotection of the Mtb/Mtz groups with 90% aqueous TFA in the presence of m-cresol and 2-methylindole gave the desired cionin derivative, which contains the tyrosine sulfate residue at the selective position. Yields obtained with approach (2) were considerably higher than those obtained with approach (1). Cionin and mono-Tyr(SO3H)-containing derivatives were assayed on exocrine pancreas in dogs.