Identification of NH2-terminal fragments of big gastrin in plasma.

Molecular forms of gastrin in human plasma were studied by radioimmunoassay using antisera specific for the NH2-terminus of big gastrin (gastrin 34). A plasma enzyme system that destroys immunoreactivity of NH2-terminal fragments of gastrin 34, but not of gastrin 34 itself, has been demonstrated and shown to be inhibited by phenanthroline. Endogenous immunoreactive human plasma gastrin that had been concentrated by immunoaffinity adsorption and separated by gel filtration was shown to consist of gastrin 34, together with peptides corresponding to its NH2-terminal tryptic peptide and a shorter NH2-terminal fragment. The latter material does not occur in antral extracts and cannot be generated by in vitro incubation of plasma and antral extracts, suggesting that it is produced by passage through a capillary bed of gastrin 34 or its NH2-terminal tryptic peptide or both. The results are of significance in understanding the mechanisms of secretion and metabolism of different forms of gastrin.