Use of hemopexin domains and monoclonal antibodies to hemopexin to probe the molecular determinants of hemopexin-mediated heme transport

Plasmin cleaves rabbit serum apohemopexin (Mr = 60,000) at a single site producing a heme-binding domain (I, Mr = 35,000) and a second domain (II, Mr = 25,000) (W. T. Morgan and A. Smith (1984) J. Biol. Chem. 259, 12001-12005). The absorbance spectra of heme-domain I are indicative of a bis-histidyl coordination complex with the central heme iron atom. Chemical modification of the 5 histidine residues of apo-domain I with diethylpyrocarbonate abolished heme binding, supporting this assignment. Upon binding heme, domain I migrates more rapidly in sucrose gradients, and, in sedimentation velocity experiments, the s value of domain I increases from 3.17 +/- 0.04 to 3.71 +/- 0.09, a notably large increase which indicates that the domain becomes much more compact. This conformational change which plays a pivotal role in hemopexin function requires the bis-histidyl coordination with heme iron and leads to a tighter association between domain I and domain II shown by the co-migration of heme-domain I and domain II in sucrose gradients. In turn, the association of heme-domain I with domain II increases the thermal stability of the heme-domain I chromophore. Results of binding studies using mouse hepatoma cells and isolated domains indicate that domain I not only binds heme but also plays a vital part in the hemopexin-receptor interaction. The change in conformation of domain I upon heme binding and the association between domains I and II induced by heme are both notable determinants of the strength of the hemopexin-receptor interaction, but an intact "hinge region" between the domains is not necessary for receptor binding. The importance of both domains in bringing about the transport function of hemopexin is confirmed by the ability of three (two specific for domain I and one for domain II) of seven monoclonal antibodies raised against hemopexin to inhibit the hemopexin-receptor interaction.