Characterization of gastrin amidation in the rat and porcine antrum: comparison with the pituitary.

The formation of biologically active gastrin from glycine-extended processing intermediates occurs via the action of a peptide alpha-amidating enzyme. The observation that gastrin exists primarily as unamidated precursors in the pituitary but as amidated gastrin in the antrum prompted these studies to examine whether the amidating enzymes in the two organs were different in their characteristics. Furthermore, the amidating enzyme in the stomach has not previously been characterized in extensive detail. Amidating activity was quantified by measuring the conversion of Tyr-Gly-Trp-Met-Asp-Phe-Gly (glycine-extended hexagastrin) to Tyr-Gly-Trp-Met-Asp-Phe-NH2 (amidated hexagastrin) by radioimmunoassay. The activity of the antral enzyme in both the rat and hog had a similar apparent molecular weight (45,000-60,000), cofactor requirements (copper, ascorbic acid, and catalase), pH optima (5.5-8.5), and Km (12 microM) as the pituitary enzyme. These data suggest that antral and pituitary peptide alpha-amidating enzymes are the same enzyme, thus it is unlikely that differences in amidating enzymes can account for the observed differences in the tissue specific processing of gastrin.