'Salivary peptide P-C' of human pancreatic B-cells shares only partly immunoreactivity with salivary peptide P-C indicating a new B-cell protein which is different from insulin

Abstract

Abstract. Salivary peptide P-C like immunoreactivity, originally isolated from human whole saliva has later been found in the human pancreatic B-cells. In the present work an indirect immunofluorescence technique using monoclonal antibodies against isolated salivary peptide P-C was applied to Bouin fixed pancreas and parotid glands to study the possible identity of the two substances. Positive P-C immunofluroescence was found in the serous cells of parotid glands but not in pancreatic B-cells, suggesting that pancreatic P-C substance is not salivary peptide P-C itself, but a substance sharing the common antigenic site with salivary peptide P-C. To examine this, an indirect immunofluorescence technique using polyclonal P-C antisera pre-absorbed with six kinds of synthetic fragments (1–22, 23–44, 23–29, 30–44, 30–38 and 38–44) of salivary peptide P-C was applied to the human pancreas. The result showed that pancreatic P-C substance was a substance which shares the common antigenic site with the 38–44 amino acid residue of salivary peptide P-C. Western blot analysis using extracts of human pancreata further showed that pancreatic P-C substance is not a precursor of insulin but a protein with molecular weight of 11 500 dalton, indicating the presence of a new protein in the insulin secretory granules of human pancreatic B-cells.