Incubation of ferrihemoglobin as well as of hemoglobin isolated chains in ferri-form with hemopexin at pH 7.0 leads to the gradual transfer of heme to the hemopexin molecules until the equilibrium is attained.
The reaction of ferrihemoglobin with hemopexin can be described as a four-stage process involving successive transfer of the four heme groups from hemoglobin tetramer to the four molecules of hemopexin. The heme binding sites of the hemoglobin molecule are nonequivalent and cooperative.
The transfer of heme from the isolated hemoglobin chains in ferri-form occurs as the simple competitive reaction which can be characterized by a single equilibrium constant.
The affinity constant value for the binding of heme to the single binding center of hemopexin molecule was estimated as 1.9 × 1014× M−1.
The direct transfer of heme from hemoglobin to hemopexin may occur in vivo as a part of hemoglobin degradation process.