The gastrointestinal peptide hormones, gastrin and cholecystokinin (CCK), display four molecular characteristics. 1) Homology. Sequences of the primary structures are identical. Because the identity comprises the active site, the homology is functionally important. The homology reflects evolution from a common ancestor. 2) Heterogeneity. Each hormone exists in different molecular forms in any single species. The heterogeneity comprises both variations in lengths of the polypeptide backbone, "macroheterogeneity," and derivatizations of single amino acid residues, "microheterogeneity." Both types of modification govern the biological potency. The heterogeneity reflects enzymatic modifications of the primary ribosomal translation product. 3) Ubiquity. Each hormone is synthetized in different cell types, which are localized in gastrointestinal as well as extra-gastrointestinal tissues. The cell type determines the route by which the active peptide(s) reaches its target, either via blood (endocrine secretion) or by local release (neurocrine and paracrine secretion). Inasmuch as all cells contain gastrin and CCK genes, the variable expression probably reflects differentiation in development of the posttranscriptional biosynthetic machinery. 4) Differential principality. In different tissues and cells, different molecular forms may predominate. Moreover, one form is more potent with respect to one effect (e.g., CCK-8 in relation to pancreatic exocrine secretion), whereas another form (CCK-4) is more potent with respect to another effect (pancreatic endocrine secretion). Together the differential principality and secretory routes (blood borne or local) ensure that gastrin and CCK peptides regulate their targets with optimal effect in spite of the heterogeneity and wide distribution, which otherwise might cause disturbing interactions and subsequent inefficacy. The key to a better understanding of the basis characteristics is knowledge about the evolution and expression of the structural gene(s) for gastrin and CCK. Acquisition of such knowledge will be of considerable value, since available evidence suggests that the gastrin-CCK system is a good model for general features of regulatory peptides.
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