Identification and characterization of glycine-extended post-translational processing intermediates of progastrin in porcine stomach.

We developed a radioimmunoassay specific for glycine-extended progastrin processing intermediates (G-Gly) using antisera generated against the synthetic peptide Tyr-Gly-Trp-Met-Asp-Phe-Gly. Distribution of immunoreactivity in the porcine gastrointestinal tract obtained with this antibody paralleled that of gastrin with the mucosa containing the highest quantity, 116 +/- 22 pmol/g, wet weight (mean +/- S.E., n = 5), or roughly 4% of gastrin concentration. This immunoreactivity was localized specifically to antral mucosal G-cells by immunohistochemistry. On Sephadex G-50 column chromatography of porcine antral mucosal extracts glycine-extended progastrin processing intermediates were separated into three principal molecular forms, each corresponding to known molecular forms of gastrin, component I, tetratriacontagastrin (G34) and heptadecagastrin (G17). Following purification by antibody-coupled affinity chromatography, one molecular form corresponding to G17 in size was shown to have an amino terminus identical to that of G17. Another molecular form corresponding to G34 in size could be converted to the molecular form corresponding to G17 by tryptic digestion. Our findings indicate that glycine-extended progastrin processing intermediates may serve as immediate precursors for each molecular form of gastrin, thus suggesting an alternative pathway for gastrin biosynthesis more complex than that previously conceived.