The influence of inorganic anions on the formation and stability of Fe 3+-transferrin-anion complexes

Abstract

Harris (Biochemistry 24 (1985) 7412) reports that inorganic anions bind to human apotransferrin in such a way as to perturb the ultraviolet spectrum. The locus of binding is thought to invole the specific metal/anion-binding sites since no perturbation is observed with Fe 3+-transferrin-CO 3 2−. Paradoxically, we were unable to demonstrate the formation of Fe 3+-transferrin-inorganic anion complexes despite the presence of high concentrations of SO 4 2−, H 2PO 4 −, Cl −, ClO 4 − or NO 3 −. Similar results were found for human lactoferrin. Electron paramagnetic resonance spectroscopy and visible spectrophotometry were used to monitor the results. An attempt to form the H 2PO 4 − complex by displacement of glycine from Fe 3+-transferrin-glycine resulted only in the disruption of the ternary complex. A series of inorganic anions varied in their ability to release iron from Fe 3+-tranferrin-CO 3Psu2− at pH 5.5, the approximate pH of endosomes where iron release takes place within cells. The order of effectiveness was H 2P 2O 7 2−⪢H 2PO 4 −>SO 4 2−>NO 3 −>Cl −>ClO 4 −. The rate of iron removal from Fe 3+-transferrin-CO 3 2− at pH 5.5 by a 4-fold excess of pyrophosphate was greatly enhanced by physiological NaCl concentration. Iron removal was complete within 10 min, the approximate time for iron release from Fe 3+-transferrin-CO 3 2− in developing erythroid cells. Thus, inorganic anions may have a significant effect on the release of iron under physiological conditions despite the fact that such inorganic anins cannot act as synergistic anins. The results are discussed in relation to a special role for the carboxylate group in allowing ternary complex formation.