IDENTIFICATION OF GASTRIN COMPONENT-I AS GASTRIN-71 - THE LARGEST POSSIBLE BIOACTIVE PROGASTRIN PRODUCT

Gastrin component I is the largest hormonally active form of gastrin. In order to determine its structure, we isolated progastrin-derived pe ptides from normal human antral tissue. A radioimmunoassay specific fo r sequence 20-25 of human progastrin was developed to monitor the puri fications. After four or five steps of reverse-phase chromatography, t he peptides were pure and could be identified by a combination of micr osequence, amino acid and mass spectral analysis as well as by a libra ry of sequence-specific immunoassays. In addition to intact progastrin 1-80, fragments 1-71, 1-35, 6-35, 20-35, and 20-36 of progastrin were identified. Only the 71 amino-acid peptide contained at its C-terminu s the cr-amidated bioactive site (Trp-Met-Asp-Phe-NH2). This unoheptac ontapeptide amide (gastrin-71) corresponds to component I and is the l argest possible bioactive product of progastrin. Its structure shows t hat progastrin is used in its entirety for biosynthesis of active pept ides. The occurrence of fragments 6-35, 20-35, and 20-36 demonstrate t hat antral progastrin is partially cleaved at two monobasic sites (Arg 5 and Arg19) in addition to processing at the three C-terminal dibasic sites. The results show that both the N- and C-terminal parts of antr al progastrin undergo extensive processing, The results also suggest t hat progastrin may follow two different processing pathways of which t he less trafficked releases gastrin-71.