Structural studies of haptoglobins: II. Reversible dissociation into subunits of Hp 1-1 and Hp 2-2

Abstract Chromatographic, sedimentation velocity, and equilibrium experiments have been carried out to investigate the subunit structures of haptoglobins (Hp 1-1 and Hp 2-2) in relation to their hemoglobin binding and activating properties. DEAE-cellulose chromatography showed anomalous elution profiles. Bio-Gel P 150 chromatography revealed, at pH 5.50, near the isoionic point, that haptoglobins dissociate reversibly in the concentration range 0.25–2.0 mg/ml, but, they remain stable at higher protein concentration. Sedimentation and equilibrium ultracentrifugation indicated that, at pH 11.50, Hp 1-1 and Hp 2-2 are homogenous and dissociated into subunits of molecular weight 40,000 ± 2500. This dissociation is reversible. It is suggested that Hp 1-1 possesses an oligomeric structure with a monomer-dimer equilibrium and that Hp 2-2 is made of a subunit of identical molecular weight in equilibrium with a series of higher order polymeric species.