KINETICS OF HEME INTERACTION WITH HEME-BINDING PROTEINS - THE EFFECT OF HEME AGGREGATION STATE

The kinetics of the interaction of heme with hemopexin and albumin was monitored by measuring the time dependence of changes in the Sorer ab sorption spectra. Since the protein binding sites can only bind heme m onomers, the binding kinetics apparently reflected the slow dissociati on of heme dimers, resulting from dimer/monomer equilibria in aqueous heme solutions. The dissociation of heme dimers is characterized by th e rate constant of (3-4) x 10(-3) s(-1). The measurements further reve aled significant differences in the kinetic profiles (slowing down the binding interaction) that were dependent on the storage time of heme solutions at room temperature. These presumably responded to the gradu al formation of higher aggregates of heme, which cannot dissociate int o dimers/monomers. Alternatively, partial autooxidation of heme molecu les could increase the stability of heme dimers and obstruct specific binding of heme to the proteins. (C) 1997 Elsevier Science B.V.