Selective, high-affinity binding of ferric ions by glycine-extended gastrin17

Uptake of dietary iron is essential for replenishment of body stores. A role for the hormone gastrin in iron uptake as a chelator of ferric ions in the gastric lumen has been proposed previously [Baldwin, G.S. (1992) Med. Hypotheses 38, 70-74]. Here, spectroscopic evidence of selective, high-affinity binding of ferric ions to progastrin-derived peptides in aqueous solution at low pH is provided. The maximum at 281 nm in the absorption spectrum of glycine-extended gastrin(17) at pH 4.0 increased (2.07 +/-0.30)-fold in the presence of greater than or equal to2 equiv of ferric ions. Titration of glycine-extended gastrin17 with ferric ions under stoichiometric conditions indicated that the stoichiometry of binding was 2.00 +/-0.28 mol of Fe3+/mol of peptide. Fluorescence quenching experiments yielded values for the stoichiometry and apparent dissociation constant of the ferric ion-glycine-extended gastrin(17) complex at pH 4.0 of 2.39 +/-0.17 mol of Fe3+/mol and 0.62 +/-0.19 muM, respectively. No interaction was detected with Co2+, Cu2+, Mn2+, or Cr3+. Electron paramagnetic resonance spectroscopy suggested that the iron ligands were either oxygen or sulfur atoms. Fluorescence quenching experiments with peptides derived from the glycine-extended gastrin17 sequence indicated that one or more of the five glutamic acid residues were necessary for iron binding. The binding of ferric ions by glycine-extended gastrin(17) at low pH is consistent with a role for progastrin-derived peptides in iron uptake from the lumen of the gastrointestinal tract.