Human pancreatic icosapeptide: isolation, sequence, and immunocytochemical localization of the COOH-terminal fragment of the pancreatic polypeptide precursor.

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Abstract
In dogs, the COOH-terminal part of the pancreatic polypeptide [PP] precursor gives rise to a stable icosapeptide product against which an antiserum has been raised. By immunohistochemistry, icosapeptide immunoreactivity was localized in human pancreas exclusively to cells that also stored PP. Analytical peptide chemistry demonstrated that a peptide corresponding to the canine icosapeptide could be extracted from the PP-rich duodenal part of the human pancreas. The human pancreatic icosapeptide was isolated by acid ethanol extraction, gel filtration, anion-exchange chromatography and reverse-phase high-performance liquid chromatography. The COOH-terminal sequence of the human icosapeptide is very similar to that of the canine icosapeptide, whereas none of the first 9 amino acid residues are identical. When the primary structure of peptides from 3 different species are compared, it is apparent that the PP part of the common precursor is a well-conserved sequence as compared to the icosapeptide part, although 8 of 11 residues in the COOH-terminal sequence of the icosapeptide are identical in all 3 species.