On the mechanism of hemoglobin-haptoglobin complex formation.

Abstract

In order to elucidate the mechanism of complex formation between human haptoglobin, Hp II, and bovine hemoglobin, Hb, measurements of the peroxidase activity and histidine determinations, as well as spectrophtometric and optical rotatroy dispersion titration experiments were performed.Following the raaction by slowly mixing the constituents of the complex in small portinos, the geometry of the complex turns out to depend on the sequence of mixing of the reactants. Adding Hb to Hp, a final complex Hb2→ Hp is formed. Teh inverse sequence of mixing leads to a complex of identical composition (Hp → Hb2), but different arrangement of the constituents, as shown by differences in the accessibility of histidine residues in both types of the complex. As suggested by the comparison of the histidine balance of both complexes, Hb2→ Hp may be described by the combination of αβ‐dimers of Hb with Hp (Hp (αβ)4), while Hp → Hb2 corresponds to the complex with two Hb tetramers (Hp(α2β2)2).Investigating the saturation process of complex formation in greater detail by spectrophtometric titration and optical rotatory dispersion, intermediary complexes are observed which prove the mechanism of complex formation to be a consecutive association process.Both series of complexes, Hp → Hb and Hb → Hp, seem to generate closely related configurations characterized by similar spectral and functional properties.