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 decrea ses apotransferrin's iron binding capacity, Optimal transferrin iron b inding requires the coexistent binding of anions such as bicarbonate ( HCO3-) near the protein's two iron binding sites, Recently, we found t hat if HCO3- was also present during HOCl exposure, apotransferrin ret ained its ability to inhibit iron-catalyzed hydroxyl radical generatio n, Therefore, we examined apotransferrin iron binding capacity after e xposure to the MPO/H2O2/I- system in the presence and absence of sever al anions (HCO3-, H2PO4-, SO42-, and ClO4-) known to bind to apotransf errin, Although the MPO system decreased apotransferrin iron uptake to only 46% of the untreated apotransferrin control, apotransferrin trea ted in the presence of 1 mM HCO3- or H2PO4- retained 84 and 74%, respe ctively, of its iron binding capacity, Similar results were seen when apotransferrin was treated with NaOCl. These results could not be expl ained on the basis of a loss of MPO activity or scavenging of HOCl. In contrast, SG(4)(2-). and CLO(4)(-) were unable to prevent the MPG-med iated loss of apotransferrin iron binding capacity, NaOCl had no effec t on the ability of transferrin to bind any of these anions, as assess ed by the anion-induced change in apotransferrin absorbance spectrum, HCO3- but not H2PO4-, SO42-, Or ClO4- decreased MPG-mediated oxidation (iodination) of apotransferrin, Under some conditions H2PO4- actually increased apotransferrin iodination, HCO3- and H2PO4- may protect apo transferrin from MPG-mediated oxidative damage by preventing selective oxidation of one or both iron binding sites, This process may allow t ransferrin to retain its iron binding function during MPO exposure in vivo.