Processing of chromogranin A by bovine parathyroid secretory granules: production and secretion of N-terminal fragments

Chromogranin A (CgA) is a glycoprotein located in the secretory granules of multiple neuroendocrine tissues, including the parathyroid gland. Although the function of CgA is not known, a role has been proposed for CgA as a prohormone for biologically active peptides. Using 3H-CgA as a substrate for bovine parathyroid secretory granule extracts, we demonstrate a precursor product relationship between intact CgA and multiple N-terminal fragments of CgA. N-terminal CgA fragments of mol wt 24, 26, and 33 k are generated in a time dependent manner in the presence of bovine parathyroid secretory granule enzymes. The generation of the 33 kilodalton (kDa) N-terminal CgA fragment is calcium dependent. In the presence of EDTA, intermediate CgA fragments of mol wt 36 and 45 are generated. The effect of EDTA is reversible with added calcium. Based on immunodetection on Western blots, the 26 kDa N-terminal fragment of CgA is secreted by bovine parathyroid cells in a time and calcium-dependent manner in parallel with P...