Chromogranin A (CgA) is a glycoprotein stored in secretory granules of many endocrine and neuroendocrine cells. CgA undergoes tissue specific processing to release regulatory peptides. In the parathyroid, although processing is limited and variable, several CgA-derived peptides have been characterized including parastatin and betagranin. An early stage of CgA processing is the generation of a 64-kDa fragment (CgA64). In this study, we have purified CgA64 from porcine parathyroid glands by chromatographic separations. Edman degradation of this CgA64 yielded the N-terminal sequence NDQAELKEGTEEASSKEAAEKRGDXAVEKND corresponding to pCgA94-125. Amino acid composition suggests that CgA64 corresponds to CgA94-430 (i.e. the entire CgA molecule, less the N-terminal residues 1-93). To determine the origin of CgA64, we fractionated parathyroid membrane vesicles by sucrose gradient centrifugation. Intact CgA is predominantly located in dense sucrose fractions (secretory granules), whereas CgA64 is located near the top of the gradient (soluble protein fraction). In vitro incubation of these fractions revealed that the conversion of CgA did not occur in intact granules. These results indicate that CgA64 is not present in intact granules suggesting that it is not a naturally occurring secretory product in parathyroid cells.
Bibliographical noteFunding Information:
We thank Yancy R. Moore and Mary Beth Bowles for expert technical assistance, and we thank Fischer Packing Company and Swift (Louisville, KY) for allowing us to collect porcine parathyroid glands. This work was supported by Eli Lilly (DVC), PHS grant T32DE07254-08 (BHF) and grants from the American Heart Association, Kentucky Affiliate and NIH R01 DK53367 (SUG).
- Prohormone processing
- Secretory granules