Primary Structures of Somatostatins from the Islet Organ of the Hagfish Suggest an Anomalous Pathway of Posttranslational Processing of Prosomatostatin-1*

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Abstract
The cyclostomes represents the first class of vertebrate in evolution to develop an endocrine pancreas. Two peptides with somatostatin-like immunoreactivity were isolated from the islet organ of one such cyclostome, the Atlantic hagfish (Myxine glutinosa). The primary structure of the more abundant peptide was established as: Ala-Val-Glu-Arg-Pro5-Arg-Gln-Asp-Gly-Gln10-Val-His-Glu-Pro15-Gly-Arg-Glu-Arg-Lys20-Ala-Gly-Cys-Lys-Asn25-Phe-Phe-Trp-Lys-Thr30-Phe-Thr-Ser-Cys. The second peptide, comprising 27% ofthe total immunoreactivity in the islet extract, was identical to mammalian somatostatin-14. The pathway of posttranslational processing of prosomatostatin in the hagfish islet differs markedly from the pathway in the higher vertebrates. In the mammalian pancreas, prosomatostatin is cleaved at the site of the single arginyl residue (corresponding to position 6 in hagfish somatostatin-34) and at the arginine-lysine site (corresponding to positions 19 and 20 in the hagfish peptide) to generate somatostatin-14, and somatostatin-28(1-12)-peptide. In the hagfish islet, Arg6 is not used as a cleavage site and cleavage at Arg19-Lys20 represents only a minor pathway of processing. The data provide further evidence of the strong evolutionary pressure to conserve the complete amino acid sequence of somatostatin-14.

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