Uptake of Fe III influences the topographical arrangement of N‐glycan(s) in human transferrin

The relative prominence of sialyl and α‐mannosyl residues of apo‐ and metal‐bound transferrin were compared. Although the presence or absence of Fe III did not significantly affect the rate of desialylation of the glycoprotein, equilibrium dialysis with serotonin revealed substantial differences between apo‐ and metalbound transferrins: binding of serotonin by Fe III ‐transferrin (2.3 residues/mol) exceeded that of Cu II ‐ or Mn II ‐transferrin (each 1.3 residues/mol); in comparison apo‐transferrin (0.6 residues/mol) and Fe III ‐asialo‐transferrin (0.5 residues/mol) bound significantly less serotonin. In the case of α‐mannosyl residues, differences between certain metal‐bound transferrins were observed from Sepharose‐concanavalin A chromatography. Thus 20–24% of Fe III ‐transferrin passed through the lectin column compared to 12–16% for Mn II ‐and Cu II ‐transferrins. We conclude that incorporation of metal ions, particularly Fe III , which is known to change the tertiary structure of transferrin, alters the position(s) of the N ‐glycan(s) relative to the polypeptide. Such a change may govern the destiny of apo‐ and metal‐bound transferrin in the circulation.