High Affinity Binding of Indium and Ruthenium Ions by Gastrins

The peptide hormone gastrin binds two ferric ions with high affinity, and iron binding is essential for the biological activity of non-amidated forms of the hormone. Since gastrins act as growth factors in gastrointestinal cancers, and as peptides labelled with Ga and In isotopes are increasingly used for cancer diagnosis, the ability of gastrins to bind other metal ions was investigated systematically by absorption spectroscopy. The coordination structures of the complexes were characterized by extended X-ray absorption fine structure (EXAFS) spectroscopy. Changes in the absorption of gastrin in the presence of increasing concentrations of Ga(3+) were fitted by a 2 site model with dissociation constants (K(d)) of 3.3 x 10(−7) and 1.1 x 10(−6) M. Although the absorption of gastrin did not change upon the addition of In(3+) ions, the changes in absorbance on Fe(3+) ion binding in the presence of indium ions were fitted by a 2 site model with K(d) values for In(3+) of 6.5 x 10(−15) and 1.7 x 10(−7) M. Similar results were obtained with Ru(3+) ions, although the K(d) values for Ru(3+) of 2.6 x 10(−13) and 1.2 x 10(−5) M were slightly larger than observed for In(3+). The structures determined by EXAFS all had metal:gastrin stoichiometries of 2:1 but, while the metal ions in the Fe, Ga and In complexes were bridged by a carboxylate and an oxygen with a metal-metal separation of 3.0–3.3 Å, the Ru complex clearly demonstrated a short range Ru—Ru separation, which was significantly shorter, at 2.4 Å, indicative of a metal-metal bond. We conclude that gastrin selectively binds two In(3+) or Ru(3+) ions, and that the affinity of the first site for In(3+) or Ru(3+) ions is higher than for ferric ions. Some of the metal ion-gastrin complexes may be useful for cancer diagnosis and therapy.