UNEXPECTEDLY STRONG BINDING OF A LARGE METAL-ION (BI3+) TO HUMAN SERUM TRANSFERRIN

Large metal ions (>0.9 Angstrom ionic radius) have previously been fou nd to bind only weakly to human serum transferrin (hTF, 80 kDa), presu mably because the interdomain cleft cannot close around the metal and synergistic anion. Surprisingly, therefore, we report that Bi3+ (ionic radius 1.03 Angstrom), a metal ion widely used in anti-ulcer drugs, b inds strongly to both the N- and C-lobes with log K-1 = 19.42 and log K-2 = 18.58 (10 mM Hepes, 5 mM bicarbonate, 310 K). The uptake of Bi 3+ by apo hTF from bismuth citrate complexes is very slow (hours), whe reas that from bismuth nitrilotriacetate is rapid (minutes), Evidence from absorption and NMR spectroscopy is presented to show that Bi3+ bi nds to the specific Fe3+ binding sites along with carbonate as the syn ergistic anion. Under the conditions used, preferential binding of Bi3 + to the C-lobe of hTF is observed. Linear free energy relationships s how that there is a strong correlation between the strength of binding of Bi3+ and Fe3+ to a wide variety of ligands which include transferr in. Therefore we conclude that the strength of metal ion binding to tr ansferrin is determined more by the ligand donor set than by the size of the ion.