ALL known gastrointestinal hormones are polypeptides rather than amines or steroids.1 They fall into two groups, gastrin and cholecystokinin-pancreozymin (CCK-PZ), according to their chemical structure. Both are straight-chain peptides that share the same four amino acids at the ends of amide groups (C-terminal tetrapeptide); probably as a consequence of their structure, they act (although often in opposite directions) on the same tissues. The amide end of the molecule has all the actions of the complete hormone.
The primary mission of gastrin is to stimulate gastric acid secretion, whereas CCK-PZ chiefly stimulates pancreatic secretion and contraction of the gallbladder. These hormones have a wide spectrum of additional actions. The CCK-PZ group is a series of polypeptides with similar amino acid sequences extending throughout the length of the peptide chain. Secretin, the prototype, has a helical structure; possibly as a result, virtually the entire sequence is necessary for biological activity. Secretin
Brooks FP. Gastrointestinal Hormones. JAMA. 1975;232(4):357–358. doi:10.1001/jama.1975.03250040011016