Interaction of methemoglobin with inositol hexaphosphate. Presence of the T state in human adult methemoglobin in the low spin state.

The interaction of human methemoglobin with inositol hexaphosphate (IHP) was examined by proton NMR spectroscopy. Upon addition of IHP, new proton peaks which are distinguishable from those observed in the absence of IHP appeared irrespective of the spin state of the ferric heme iron. the IHP-induced NMR peaks of low spin azide methemoglobin were not observed when azide was titrated to stripped aquomethemoglobin. Therefore, it was indicated that the IHP-induced peaks are not due to the slight dissociation of azide. Azide titration to aquomethemoglobin in the presence of IHP further revealed that the IHP-induced peaks are not due to a localized structural change but rather due to the presence of an IHP-induced new conformer. The NMR spectral intensity calibration for the 5-methylimidazole complex of methemoglobin showed that the intensity of the spectrum of stripped methemoglobin decreased by about 80% upon addition of IHP, suggesting the structural alterations in both of the alpha and beta subunits. The intensity of the IHP-induced peak at 31.1 ppm in azide methemoglobin spectrum decreased at both higher and lower pD values with its optimal around pD 6.7. The pD dependence of this peak was closely similar to that of the exchangeable proton NMR peak which has been used as a measure of the T quaternary conformation for human aquomethemoglobin (Huang, T.-H. (1979) J. Biol. Chem. 254, 11467-11474). From all of the above results, it was concluded that human low spin methemoglobin can be switched from the R to T quaternary structures by the binding of IHP. The allosteric constant L4 = [T4]/[R4] for the fully ligated low spin methemoglobin increased with decreasing temperature and was estimated to be 1 to 5 at room temperature.