title: Selective, high-affinity binding of ferric ions by glycine-extended gastrin(17).
abstract: 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 > or =2 equiv of ferric ions. Titration of glycine-extended gastrin(17) with ferric ions under stoichiometric conditions indicated that the stoichiometry of binding was 2.00 +/- 0.28 mol of Fe(3+)/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 Fe(3+)/mol and 0.62 +/- 0.19 microM, respectively. No interaction was detected with Co(2+), Cu(2+), Mn(2+), or Cr(3+). 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 gastrin(17) 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.
authors.affiliation: University of Melbourne Department of Surgery, Austin Hospital, Melbourne, Victoria, Australia. g.baldwin@surgeryaustin.unimelb.edu.au
authors.forename: G S
authors.initials: GS
authors.lastname: Baldwin
authors.forename: C C
authors.initials: CC
authors.lastname: Curtain
authors.forename: W H
authors.initials: WH
authors.lastname: Sawyer
mesh_terms: D001665:Binding Sites; D004578:Electron Spin Resonance Spectroscopy; D005290:Ferric Compounds / Q000737:chemistry* / Q000378:metabolism; D005753:Gastric Mucosa / Q000378:metabolism; D005755:Gastrins / Q000737:chemistry* / Q000378:metabolism*; D018698:Glutamic Acid; D006801:Humans; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D008024:Ligands; D008954:Models, Biological; D011487:Protein Conformation; D013050:Spectrometry, Fluorescence
chemical_list: D005290:Ferric Compounds; D005755:Gastrins; D008024:Ligands; C086854:glycine-extended gastrin 17; D018698:Glutamic Acid