Glycine-extended progastrin processing intermediates: accumulation and cosecretion with gastrin.

Glycine-extended intermediates of peptide processing serve as substrates for carboxyl-terminal amidation, hence activation, of many brain-gut peptides. To explore the dynamics of accumulation and secretion of these important intermediates we utilized primary cultures of canine antral mucosal G-cells as a model system. Glycine-extended progastrin processing intermediates (G-Gly) accumulated rapidly in G-cells cultured in ascorbate-deficient media, exhibiting a fourfold increase over a 51-h culture period, while gastrin content fell to less than half of the initial level. In contrast, G-cells cultured in ascorbate-supplemented media accumulated G-Gly at a relatively low rate, while gastrin was preserved at a higher level. Under either condition, G-Gly and gastrin were progressively released into the culture media. The release of both immunoreactivities could be stimulated by bombesin and inhibited by somatostatin in similar fashion. By electron microscopy, the cultured G-cells exhibited no ultrastructural alterations. These data suggest that 1) the cellular homeostasis of G-Gly is regulated by the activity of an ascorbate-dependent amidation enzyme similar to one previously described in pituitary tissues, 2) carboxyl-terminal amidation is not an obligatory step for secretion of gastrin, and 3) the proportions of gastrin and G-Gly cosecreted from G-cells reflect their proportional accumulation within G-cell secretory granules. The physiological relevance of the released G-Gly has yet to be determined.