title: Haptoglobin alters oxygenation and oxidation of hemoglobin and decreases propagation of peroxide-induced oxidative reactions.
abstract: We compared oxygenation and anaerobic oxidation reactions of a purified complex of human hemoglobin (Hb) and haptoglobin (Hb-Hp) to those of uncomplexed Hb. Under equilibrium conditions, Hb-Hp exhibited active-site heterogeneity and noncooperative, high-affinity O(2) binding (n(1/2)=0.88, P(1/2)=0.33 mm Hg in inorganic phosphate buffer at pH 7 and 25 °C). Rapid-reaction kinetics also exhibited active-site heterogeneity, with a slower process of O(2) dissociation and a faster process of CO binding relative to uncomplexed Hb. Deoxygenated Hb-Hp had significantly reduced absorption at the λ(max) of 430 nm relative to uncomplexed Hb, as occurs for isolated Hb subunits that lack T-state stabilization. Under comparable experimental conditions, the redox potential (E(1/2)) of Hb-Hp was found to be +54 mV, showing that it is much more easily oxidized than uncomplexed Hb (E(1/2)=+125 mV). The Nernst plots for Hb-Hp oxidation showed no cooperativity and slopes less than unity indicated active-site heterogeneity. The redox potential of Hb-Hp was unchanged by pH over the range of 6.4-8.3. Exposure of Hb-Hp to excess hydrogen peroxide (H(2)O(2)) produced ferryl heme, which was found to be more kinetically inert in the Hb-Hp complex than in uncomplexed Hb. The negative shift in the redox potential of Hb-Hp and its stabilized ferryl state may be central elements in the protection against Hb-induced oxidative damage afforded by formation of the Hb-Hp complex.
authors: Chemistry Department, French Family Science Center, Box 90346, Duke University, Durham, NC 27708-0346, USA.
authors: Sambuddha
authors: S
authors: Banerjee
authors: Yiping
authors: Y
authors: Jia
authors: Claire J Parker
authors: CJ
authors: Siburt
authors: Bindu
authors: B
authors: Abraham
authors: Francine
authors: F
authors: Wood
authors: Celia
authors: C
authors: Bonaventura
authors: Robert
authors: R
authors: Henkens
authors: Alvin L
authors: AL
authors: Crumbliss
authors: Abdu I
authors: AI
authors: Alayash
mesh_terms: D002248:Carbon Monoxide / Q000737:chemistry; D003497:Cyclic N-Oxides / Q000737:chemistry; D016166:Free Radical Scavengers / Q000737:chemistry; D006242:Haptoglobins / Q000737:chemistry*; D006454:Hemoglobins / Q000737:chemistry*; D006801:Humans; D006861:Hydrogen Peroxide / Q000737:chemistry*; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D016877:Oxidants / Q000737:chemistry*; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry; D011485:Protein Binding; D055503:Protein Multimerization; D055550:Protein Stability; D021122:Protein Subunits / Q000737:chemistry
chemical_list: D003497:Cyclic N-Oxides; D016166:Free Radical Scavengers; C485359:HP protein, human; D006242:Haptoglobins; D006454:Hemoglobins; D016877:Oxidants; D021122:Protein Subunits; C017245:5,5-dimethyl-1-pyrroline-1-oxide; D002248:Carbon Monoxide; D006861:Hydrogen Peroxide; D010100:Oxygen