Physiological Laboratory, University of Liverpool, U.K.
Chromogranin A is a secretory protein expressed widely in neuroendocrine cells. It is known to be phosphorylated but the precise sites of phosphorylation are not known. We have isolated, from bovine pancreas and ileum, chromogranin A fragments corresponding to a region giving rise to a biologically active product, pancreastatin. Phosphorylation patterns were determined by fast atom bombardment mass spectrometry and alkaline phosphatase digestion followed by ion-exchange chromatography and radioimmunoassay. In the pancreas, there were unmodified, mono- and di-phosphorylated forms of the fragment chromogranin A(248-313) with Arg and Glu at positions 293 and 301 respectively; in addition, there were small amounts of monophosphorylated peptide with an alternative primary sequence of His and Lys at 293 and 301 respectively. Two products of cleavage, pancreastatin and the fragment 297-313, were also found in unmodified and monophosphorylated forms. In the ileum, peptides with both alternative primary sequences were found, pancreastatin was absent, and phosphorylation was generally less than in the pancreas. Chromogranin A-derived peptides therefore exhibit tissue-specific patterns of phosphorylation and cleavage, and at least two phosphorylation sites occur in the region giving rise to a biologically active product.