Bicarbonate and phosphate ions protect transferrin from myeloperoxidase-mediated damage

Exposure to hypochlorous acid (HOCl), the main product of the reaction of neutrophil myeloperoxidase (MPO), H2O2, and Cl-, reportedly decreases apotransferrin's iron binding capacity. Optimal transferrin iron binding requires the coexistent binding of anions such as bicarbonate (HCO3-) near the protein's two iron binding sites. Recently, we found that if HCO3- was also present during HOCl exposure, apotransferrin retained its ability to inhibit iron-catalyzed hydroxyl radical generation. Therefore, we examined apotransferrin iron binding capacity after exposure to the MPO/H2O2/I- system in the presence and absence of several anions (HCO3-, H2PO4, SO4(2-), and ClO4-) known to bind to apotransferrin. Although the MPO system decreased apotransferrin iron uptake to only 46% of the untreated apotransferrin control, apotransferrin treated in the presence of 1 mM HCO3- or H2PO4- retained 84 and 74%, respectively, of its iron binding capacity. Similar results were seen when apotransferrin was treated with NaOCl. These results could not be explained on the basis of a loss of MPO activity or scavenging of HOCl. In contrast, SO4(2-) and ClO4- were unable to prevent the MPO-mediated loss of apotransferrin iron binding capacity. NaOCl had no effect on the ability of transferrin to bind any of these anions, as assessed by the anion-induced change in apotransferrin absorbance spectrum. HCO3- but not H2PO4-, SO4(2-), or ClO4- decreased MPO-mediated oxidation (iodination) of apotransferrin. Under some conditions H2PO4- actually increased apotransferrin iodination. HCO3- and H2PO4- may protect apotransferrin from MPO-mediated oxidative damage by preventing selective oxidation of one or both iron binding sites. This process may allow transferrin to retain its iron binding function during MPO exposure in vivo.