Structural studies of haptoglobins.3. Interactions with hemoglobin.

Abstract The interactions between human haptoglobin and horse hemoglobin have been investigated by polyacrylamide gel electrophoresis, potentiometry, X-ray smallangle scattering, and kinetics of the peroxidase activity. Two different complexes of identical stoichiometry (1:1) have been isolated and characterized: Hp · Hb and Cx. A third complex, Cd, of stoichiometry 2:1 has also been described. The nature of the complexes formed by Hp and Hb depends on the mole ratio of the two proteins present in the initial mixture. Hp · Hb is made up of one molecule of haptoglobin bound to two dimers of hemoglobin: one of them can be displaced by addition of haptoglobin. Cx is formed by 1 mole of haptoglobin bound to 3 or 4 hemoglobin chains; in Cd 1 mole of haptoglobin is bound to 1 hemoglobin dimer. In the last two complexes the binding of hemoglobin is irreversible. The difference in the nature of the binding of hemoglobin to haptoglobin, which leads to a rearrangement of the heme-iron environment, accounts for the functional properties of these complexes.