Interaction of metal ions with gastrointestinal hormones: Binding studies of Mg⁺⁺ to biologically active analogs of little gastrin and minigastrin

The interaction of magnesium ions with Des‐Trp 1 ‐Nle 12 ‐minigastrin I (Nle 11 ‐HG‐13) and Nle 115 ‐little gastrin I (Nle 15 ‐HG‐17) has been studied by CD and spectrophotometric techniques in trifluoroethanol. Spectrophotometric titrations using murexide as a metallochromic indicator showed that there are three binding sites for magnesium ions in Nle 11 ‐HG‐13, with binding constants of the order of (6 ± 2) × 10 6 , (1.7 ± 0.5) × 10 6 , and (5.0 ± 0.5) × 10 5 M −1 . These figures have been independently confirmed by CD measurements in the far‐uv in the presence of increasing amounts of magnesium ions. Elongation of the peptide chain from Nle 11 ‐HG‐13 to Nle 15 ‐HG‐17 does not provide any additional binding site for the metal ions. In both hormones, we have observed different responses in the near‐ and fur‐uv CD properties with regard to added magnesium. The intensity of the CD bands in the aromatic region changes cooperatively with the ion/hormone molar ratio. These findings lead us to conclude that at the C‐terminal, the biologically important sequence, ‐Trp‐Nle‐Asp‐Phe‐Nh 2 , is directly involved in the interaction with magnesium.