Interaction of human apohemoglobin with inositol hexaphosphate.

Experiments of sedimentation velocity and equilibrium indicate that in the presence of inositol hexaphosphate the degree of polymerization of apohemoglobin is shifted in favor of the formation of tetramers, with a maximum effect when the concentration of the polyphosphate is 1 mM. Above this concentration, a redissociation of the system into dimers is promoted. This phenomenon is probably due to the binding of inositol hexaphosphate to apohemoglobin with a stoichiometry higher than 1 mol of polyphosphate/4 subunits. The optical rotatory dispersion spectrum of apohemoglobin is also modified by its interaction with inositol hexaphosphate suggesting a small increase in the helical content of the protein. Measurements of circular dichroism in the near-UV region of the spectrum indicate that the environment of the aromatic chromophores of the protein such as tyrosine, phenyalanine, and tryptophan is not affected by the interaction. The presence of inositol hexaphosphate decreases the rate of reaction of the beta-93 cysteinyl residues of apohemoglobin with both p-mercuribenzoate and N-ethylmaleimide, suggesting a conformational change of the protein also at the level of its tertiary structure.