Altered amidation of pancreatic polypeptide in cultured dog islet tissue.

Three forms of immunoreactive pancreatic polypeptide (PPI) were detected in extracts of cultured dog pancreatic PP cells: PPI of (1) larger apparent molecular weight than PP, (2) similar apparent molecular weight but different isoelectric point than PP, and (3) identical apparent molecular weight and isoelectric point with PP. Dog pancreatic endocrine cells in culture were labeled biosynthetically with tritiated amino acids, and extracted proteins were fractionated by sodium dodecyl sulfate gel electrophoresis. A total of 97% of the PPI migrated like PP itself while about 3% of the PPI migrated like proteins up to 7200 molecular weight. PPI migrating like PP was analyzed further by isoelectric focusing and was found to occur in a neutral form like PP and a more acidic form. Peptide mapping of neutral and acidic PPI forms showed that both were like PP with the exception that the C-terminal [3H]tyrosine-containing peptide was a peptide with a net negative charge of 1 arising from a peptide extension of one or a few amino acids. The acidic form of PP was also shown to occur in pancreas extracts. However, neutral PPI was 90% of the total PPI in the pancreas extracts while the converse was true of culture extracts. We conclude that culturing the PP cell affects the efficiency of the process of amidation, that acidic PP could be either biosynthetic precursor or end product, and that the existence of the larger PP form(s) signals (signal) the possible production of yet other peptides by the PP cell.