Bismuth, Interaction with Gastrin
The peptide hormone gastrin was originally identified as a stimulant of acid secretion, but is now known to also act as a growth factor in the gastrointestinal tract (Dockray et al. 2001). Gastrin is synthesized as a precursor of 101 amino acids (preprogastrin) which, on removal of the signal peptide of 21 amino acids, yields progastrin (80 amino acids). Proteolytic processing in antral G cells in the stomach generates a number of intermediate peptides, including glycine-extended gastrin 17 (Ggly), which has the sequence ZGPWLEEEEEAYGWMDFG. Transamidation of the C-terminal glycine yields the C-terminal amidated phenylalanine characteristic of amidated gastrin (Gamide). Both Ggly and Gamide are independently active, via different receptors, in the gastrointestinal tract.
Iron: Interaction with Gastrin
Fluorescence experiments have revealed that both Ggly and Gamide bind two ferric ions with high affinity (K d = 0.6 μM, pH 4.0) in aqueous solution (Baldwin et al. 2001), via the carboxylate groups in the side chains of Glutamates 7, 8, and 9 (Pannequin et al. 2002). Progastrin also binds two ferric ions, and the ferric ion–progastrin complex is very stable, with a half-life of 117 ± 8 days at pH 7.6 and 25°C (Baldwin 2004).
Binding of ferric ions is essential for the biological activities of Ggly in vitro. Mutation of Glutamate 7 of Ggly to Alanine reduced the stoichiometry of ferric ion binding from 2 to 1, and completely abolished biological activity in cell proliferation and migration assays (Pannequin et al. 2002). The iron chelator desferrioxamine also completely blocked Ggly activity in cell proliferation and migration assays in vitro (Pannequin et al. 2002) and in the colorectal mucosa in vivo (Ferrand et al. 2010). The minimum biologically active Ggly fragments are the heptapeptides LEEEEEA and EEEEEAY, and their activity is still dependent on ferric ions (He et al. 2004). Interestingly, mutation of Glutamate 7 of Gamide to Alanine had no effect on the biological activity of Gamide, even though the stoichiometry of ferric ion binding was again reduced from 2 to 1, presumably because the receptors for Ggly and Gamide are distinct (Pannequin et al. 2004a).
Bismuth: Interaction with Gastrin
Bi3+ ions also bind to Ggly, although with lower affinity than Fe3+ ions (K d = 5.8 ± 1.4 μM, pH 4.0) (Pannequin et al. 2004b). NMR spectroscopy indicated that, as with Fe3+ ions, binding was via the carboxylate groups in the side chains of Glutamates 7, 8 and 9. Because the Bi3+–Ggly complex is not recognized by the Ggly receptor, Bi3+ ions act as competitive inhibitors in both cell proliferation and migration assays in vitro (Pannequin et al. 2004b). In contrast Bi3+ ions did not reduce the binding of Gamide to its receptor, or inhibit the biological activity of Gamide.
Bismuth salts have been used for many years for the treatment of gastrointestinal disorders such as ulcers and diarrhea (Gorbach 1990). The direct anti-bacterial effect of Bi3+ ions on the gastric bacterium Helicobacter pylori provided the rationale for the use of bismuth salts, in combination with inhibitors of gastric acid production and with antibiotics, in the treatment of H. pylori–induced ulcers (Houben et al. 1999). The recognition of the ability of Bi3+ ions to inhibit the biological activity of Ggly suggests that bismuth salts may also interfere with the potentiation by Ggly of the Gamide-induced secretion of gastric acid (Chen et al. 2000). Since non-amidated gastrins like Ggly appear to act as growth factors for colorectal cancer (Aly et al. 2004), inhibition of Ggly activity by Bi3+ ions may offer a novel and selective therapy for this all too common disease.