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FIGURE 8-7 Organization of preprosecretin. [The information was abstracted from Kopin, A. S., Wheeler, M. B., and Leiter, A. B. (1990). Secretin: Structure of the precursor and tissue distribution. Proc. Nat. Acad. Sei. USA 87, 2299-2303.]

Preprosecretin

Preprosecretin tide. GIP, which is a member of the secretin peptide family (Table 8-4), shares a high sequence homology with both glucagon (15 amino acids identical) and secretin (9 amino acids identical) (see Figure 8-6). GIP is biosynthesized largely by K cells present in the small intestine. Receptors for GIP have been identified in selected regions of the GI tract as well as the brain.

Little information is available concerning the mechanisms by which GIP elicits biological responses. Two categories of biological responses are known: (i) inhibition of gastric acid secretion and gastric motility and (ii) potentiation of pancreatic insulin secretion after oral as opposed to intravenous glucose administration.

H. Motilin

Motilin is a 22-amino-acid linear peptide (see Figure 8-8); the biological activity is believed to be associated with the nine N-terminal residues. The amino acid sequence is entirely different from that of all other known gastrointestinal hormones. The solution structure of motilin has been determined in detail via nuclear magnetic resonance (NMR) studies.

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