title: Peroxidase activity of hemoglobin-haptoglobin complexes: covalent aggregation and oxidative stress in plasma and macrophages.
abstract: As a hemoprotein, hemoglobin (Hb) can, in the presence of H(2)O(2), act as a peroxidase. In red blood cells, this activity is regulated by the reducing environment. For stroma-free Hb this regulation is lost, and the potential for Hb to become a peroxidase is high and further increased by inflammatory cells generating superoxide. The latter can be converted into H(2)O(2) and feed Hb peroxidase activity. Haptoglobins (Hp) bind with extracellular Hb and reportedly weaken Hb peroxidase activity. Here we demonstrate that: (i) Hb peroxidase activity is retained upon binding with Hp; (ii) in the presence of H(2)O(2), Hb-Hp peroxidase complexes undergo covalent cross-linking; (iii) peroxidase activity of Hb-Hp complexes and aggregates consumes reductants such as ascorbate and nitric oxide; (iv) cross-linked Hb-Hp aggregates are taken up by macrophages at rates exceeding those for noncovalently cross-linked Hb-Hp complexes; (v) the engulfed Hb-Hp aggregates activate superoxide production and induce intracellular oxidative stress (deplete endogenous glutathione and stimulate lipid peroxidation); (vi) Hb-Hp aggregates cause cytotoxicity to macrophages; and (vii) Hb-Hp aggregates are present in septic plasma. Overall, our data suggest that under conditions of severe inflammation and oxidative stress, peroxidase activity of Hb-Hp covalent aggregates may cause macrophage dysfunction and microvascular vasoconstriction, which are commonly seen in severe sepsis and hemolytic diseases.
authors.affiliation: Center for Free Radical and Antioxidant Health, University of Pittsburgh, Pittsburgh, Pennsylvania 15219, USA.
authors.forename: Alexandr
authors.initials: A
authors.lastname: Kapralov
authors.forename: Irina I
authors.initials: II
authors.lastname: Vlasova
authors.forename: Weihong
authors.initials: W
authors.lastname: Feng
authors.forename: Akihiro
authors.initials: A
authors.lastname: Maeda
authors.forename: Karen
authors.initials: K
authors.lastname: Walson
authors.forename: Vladimir A
authors.initials: VA
authors.lastname: Tyurin
authors.forename: Zhentai
authors.initials: Z
authors.lastname: Huang
authors.forename: Rajesh K
authors.initials: RK
authors.lastname: Aneja
authors.forename: Joseph
authors.initials: J
authors.lastname: Carcillo
authors.forename: Hülya
authors.initials: H
authors.lastname: Bayir
authors.forename: Valerian E
authors.initials: VE
authors.lastname: Kagan
mesh_terms: D006242:Haptoglobins / Q000378:metabolism*; D006454:Hemoglobins / Q000378:metabolism*; D006801:Humans; D006861:Hydrogen Peroxide / Q000378:metabolism; D007249:Inflammation / Q000378:metabolism; D008264:Macrophages / Q000378:metabolism* / Q000473:pathology; D018384:Oxidative Stress; D010544:Peroxidases / Q000378:metabolism*; D010949:Plasma / Q000378:metabolism*; D011485:Protein Binding; D055503:Protein Multimerization; D019163:Reducing Agents / Q000737:chemistry; D014661:Vasoconstriction
chemical_list: D006242:Haptoglobins; D006454:Hemoglobins; D019163:Reducing Agents; C035492:haptoglobin-hemoglobin complex; D006861:Hydrogen Peroxide; D010544:Peroxidases