A Proton Magnetic Relaxation Study of tpe Interaction between Methaemoglobin and Inositol Hexaphosphate

Inositol hexaphosphate is the strongest allosteric effector even for the metform of haemoglobin. Its effects upon the quaternary structure of the tetramer have been studied in relation to the overall conformational state(s) of the haem-pockets in aqueous solutions of human haemoglobins. The method useci, proton magnetic relaxation, yields information about the accessibility of solvent pl.\u27otons towards the haem-iron. No differences in the relaxation rates were detected by this method between the unstripped carbonmonoxyhaemoglobin and the phosphate-stripped sample in the presence and absence of IHP. There are considerable changes in those relaxation rate·s due to the paramagnetic haem-iron of aquomethaemoglobin when IHP is added to the stripped adult haemoglobin, but none is observed for the foetal haemoglobin, although a similar shift in the spin-state equilibrium ts expected for both haemoglobins on addition of ,!HP. Neither was there any change with IHP in solutions of adult fluoromethaemoglobin. It is concluded thart there is no tightening of the haem-pockets upon addition of IHP to solutions of any of the three haemoglobin samples. An increase in the accessibility of the haem-pockets is probable only for the aquometfom1 of the adult haemoglobin. It is suggested that the structural aspect of ligand affinity, i.e. the haem-pocket conformation, is not as decisive in altering the affinity by IHP as is possibly the change in the haem-iron spin-state induced by !HP-binding