Comparative Binding Studies of the Hemoglobin Haptoglobin and the Hemoglobin-Antihemoglobin Reactions

The species specificity of the reactions of human haptoglobin and antihuman hemoglobin antibodies with different animal hemoglobins was studied with respect to equilibrium behavior and reaction kinetics. These studies indicate that while the reaction of human haptoglobin with hemoglobin has a very broad species specificity, very distantly related hemoglobins (frog and carp) do exhibit weaker binding than those more closely related, most of which appear indistinguishable from the human protein in these tests. In contrast, antihuman hemoglobin antibodies show a very strict species specificity reacting most strongly with human hemoglobin. In addition, data are presented which demonstrate that the sites on hemoglobin where haptoglobin binds and the sites of antibody binding are completely independent. The proposed explanation is that antibodies are directed against regions on hemoglobin that have varied during evolution. In contrast, it is concluded that the site on hemoglobin for binding haptoglobin is a region of the hemoglobin molecule that has been conserved in evolution. On the basis of these studies and other known facts about the hemoglobin-haptoglobin reaction, it is suggested that the binding site on hemoglobin for haptoglobin is situated in the α1β2 contact region of the hemoglobin molecule.