Studies on the Oxidation-Reduction Potentials of Heme Proteins: VII. Oxidation-Reduction Equilibrium Of Hemoglobin Bound To Haptoglobin

Abstract

Abstract The oxidation-reduction equilibrium of human hemoglobin bound to haptoglobin has been investigated. The genetic type of haptoglobin (whether 1-1 or 2-2) has no effect on the oxidation-reduction properties of bound hemoglobin. The shape of the equilibrium curve changes with the molar ratio between haptoglobin and hemoglobin, being clearly asymmetrical when Hp:Hb = 1:1 and symmetrical when Hp:Hb = 2:1. The average value of n near 50% oxidation is less than 1 in either case. When Hp:Hb = 2:1, the oxidation-reduction equilibrium curve can be accounted for quantitatively on the hypothesis that there are two oxidation-reduction systems present in equimolar amounts, and each is characterized by n = 1 but different midpoint potentials (E1 and E2). The interpretation of these results is that when hemoglobin is bound to haptoglobin the stabilizing heme-heme interactions are suppressed with the result that the hemes of each type of chain (α and β) display their intrinsic oxidation-reduction properties. The proposition that the oxidation-reduction potentials of α and β chains are different (by 56 mv at pH 7) is in good agreement with independent experimental results on human hemoglobin and the isolated hemoglobin chains. The oxidation-reduction results taken alone indicate that when Hp:Hb = 2:1 only one type of complex is present. On the other hand, when Hp:Hb = 1:1 the equilibrium data suggest that more than one type of complex is formed, and that the two hemoglobin chains of either type (α or β) are not bound in the same way to the haptoglobin molecule.