Biosynthesis of pancreatic polypeptide. Identification of a precursor and a co-synthesized product

Pseudoislets obtained by gyrotational incubation of purified endocrine cells from canine duodenal pancreas were incubated with [3H]leucine and [35S]methionine to study the biosynthesis of pancreatic polypeptide. Gel filtration of islet extracts demonstrated two major radiolabeled peptides which were immunoprecipitable with antisera against pancreatic polypeptide. One had a molecular weight similar to that of pancreatic polypeptide (4300), whereas the other had an apparent molecular weight of about 9000. On polyacrylamide gel electrophoresis, the larger labeled peptide co-migrated with a large immunoreactive form of pancreatic polypeptide which reacts in immunoassays directed towards the NH2-terminal region of pancreatic polypeptide, but not in an assay depending on the COOH-terminal tyrosine amide. Sequential Edman degradation of both immunoprecipitable labeled peptides identified [3H]leucine at position 3 and [35S]methionine at position 17, positions which align with the leucine and methionine residues in the NH2-terminal sequence of pancreatic polypeptide. In pulse-chase labeling experiments, the larger peptide appeared early and disappeared rapidly, whereas pancreatic polypeptide and an additional smaller peptide appeared later and reached a plateau as th precursor was processed. That both pancreatic polypeptide and the smaller peptide arise from the same precursor was further supported by electrophoretic mapping of the tryptic fragments from the three peptides. We conclude that pancreatic polypeptide and a peptide with a molecular weight of 2500 to 3000 are co-synthesized from a larger precursor and that the pancreatic polypeptide sequence is located at the far NH2-terminal end of the precursor form.