Chromogranin A (CgA), previously referred to as secretory protein-1, is a 50-kilodaiton protein present in secretory granules of many endocrine and neuroendocrine cells. In the parathyroid it is present and cosecreted with PTH in response to hypocalcemia. CgA appears to be a precursor of bioactive peptides including pancreastatin, β-granin, vasostatin, and chromostatin. The presence of several highly conserved pairs of basic amino acids, putative cleavage sites, in the CgA molecule suggests that other yet unidentified bioactive peptides might exist within the molecule. We tested this speculation by subjecting porcine parathyroid CgA to digestion by endoproteinase Lys-C. Resulting CgA-derived peptides were isolated by reverse-phase C18 HPLC and tested for their ability to affect low-Ca2+ stimulated secretion by porcine parathyroid cells. We characterized one peptide, which we named parastatin, that inhibited secretion of both PTH and CgA in a dose-dependent fashion over the range of 0.2-0.6 μM. Parastatin migrated as a single band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis with an apparent mol wt of 11, 000. Edman degradation yielded the sequence L-S-F-R-A-P-A-Y-G-F-R-G-P-G-L corresponding to residues 347-361 of porcine CgA. Amino acid analysis of endoproteinase Lys-C and endoproteinase Asp-lV-generated fragments indicated that parastatin corresponds to residues 347-419 of CgA. A synthetic NH2-terminal fragment of rat parastatin corresponding to residues 1-19 was as inhibitory as intact porcine parastatin on parathyroid gland secretion. These results extend the concept that CgA is a precursor of biologically active peptides.
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