Inactivation of transferrin iron binding capacity by the neutrophil myeloperoxidase system

Human serum apotransferrin was exposed to the isolated myeloperoxidase-H₂O₂-halide system or to phorbol ester-activated human neutrophils. Such treatment resulted in a marked loss in transferrin iron binding capacity as well as concomitant iodination of transferrin. Each component of the cell-free system (meyloperoxidase, H₂O₂, iodide) or neutrophil system (neutrophils, phorbol ester, iodide) was required in order to observe these changes. In the cell-free system, the H₂O₂ requirement was fulfilled by either reagent H₂O₂ or the peroxide-generating system glucose oxidase plus glucose. Both loss of iron binding capacity and transferrin iodination by either the myeloperoxidase system or activated neutrophils were blocked by azide or catalase. The isolated peroxidase system had an acidic pH optimum, whereas the intact cell system was more efficient at neutral pH. The kinetics of changes in iron binding capacity and iodination closely paralleled one another, exhibiting t( 1/2 ) values of <1 min for the myeloperoxidase-H₂O₂ system, 3-4 min for the myeloperoxidase-glucose oxidase system, and 8 min for the neutrophil system. That the occupied binding site is protected from the myeloperoxidase system was suggested by 1) a failure to mobilize iron from iron-loaded transferrin, 2) an inverse correlation between initial iron saturation and myeloperoxidase-mediated loss of iron binding capacity, and 3) decreased myeloperoxidase-mediated iodination of iron-loaded versus apotransferrin. Since as little as 1 atom of iodide bound per molecule of transferrin was associated with substantial losses in iron binding capacity, there appears to be a high specificity of myeloperoxidase-catalyzed iodination for residues at or near the iron binding sites. Amino acid analysis of iodinated transferrin (~2 atoms/molecule) demonstrated that iodotyrosine was the predominant iodinated species. These observations document the ability of neutrophils to inactivate transferrin iron binding capacity via the secretion of myeloperoxidase, formation of H₂O₂, and subsequent myeloperoxidase-catalyzed iodination. This sequence of events may help to explain the changes in iron metabolism associated with the in vivo inflammatory response.