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.
Edeker
B L
BL
Research Service, VA Medical Center, Iowa City, Iowa, USA.
Rasmussen
G T
GT
Britigan
B E
BE
D000838:Anions; D001059:Apoproteins / Q000737:chemistry; D001639:Bicarbonates / Q000737:chemistry*; D006801:Humans; D006997:Hypochlorous Acid / Q000737:chemistry; D066298:In Vitro Techniques; D007501:Iron / Q000737:chemistry; D009504:Neutrophils / Q000201:enzymology; D010084:Oxidation-Reduction; D009195:Peroxidase / Q000378:metabolism*; D010710:Phosphates / Q000737:chemistry*; D014168:Transferrin / Q000737:chemistry* / Q000378:metabolism
D000838:Anions; D001059:Apoproteins; D001639:Bicarbonates; D010710:Phosphates; D014168:Transferrin; C028284:apotransferrin; D006997:Hypochlorous Acid; D007501:Iron; D009195:Peroxidase