Amidated and nonamidated
gastrins elicit different
biological effects via distinct receptors in different tissues. Amidated
gastrin 17 stimulates gastric acid secretion and the development of gastric
carcinoids, whereas
glycine-extended gastrin 17 stimulates proliferation of the colonic mucosa and the development of
colorectal cancers. Because
glycine-extended gastrin 17 binds two ferric
ions with high affinity (Baldwin, G. S., Curtain, C. C., and Sawyer, W. H. (2001) Biochemistry 40, 10741-10746), we have investigated the identity of the
iron ligands and the role of ferric
ions in
biological activity. Here we report the
solution structure of
glycine-extended gastrin 17, determined by NMR spectroscopy. The spectral changes observed upon the addition of ferric
ions revealed that Glu(7) acted as a
ligand at the first ferric binding site, and that Glu(8) and Glu(9) acted as
ligands at the second ferric ion binding site. Fluorescence quenching experiments confirmed that a GglyE7A mutant bound only one ferric ion. The inability of this mutant to stimulate proliferation or migration in the IMGE-5 cell line and the observation that the
iron chelator desferrioxamine selectively blocked the effects of
glycine-extended gastrin 17 indicated that binding of a ferric ion to Glu(7) was essential for
biological activity. This is the first report of an essential role for a
metal ion in the action of a
hormone.