Effect of inositol hexakisphosphate on the spectroscopic properties of the nitric oxide derivative of ferrous naturally glycated human hemoglobin HbA_1c

The effect of inositol hexakisphosphate (IHP) on the spectroscopic (EPR and absorbance) properties of the nitric oxide derivative of ferrous naturally glycated human hemoglobin HbA_1c (HbA_1cNO) has been investigated quantitatively. The results obtained show that 1) both in the absence and presence of IHP, the EPR and absorbance spectra of HbA_1cNO show the same basic characteristics described for the nitrosyl derivative of ferrous HbA₀, the nonglycated major component of human hemoglobin (HbA₀NO); and 2) HbA_1cNO binds IHP with an apparent dissociation equilibrium constant (θ{symbol} = 1.8 × 10^-2 M), which is at least four orders of magnitude higher than that estimated for the polyphosphate interaction with HbA₀NO (≤3 × 10^-6 M). These data provide further independent evidence that interaction(s) of polyphosphates at the specific cleft between β-chains along the dyad-axis is sterically hindered in HbA_1c by the presence of the two glucose residues covalently bound to the N-termini of β-chains, this finding being in agreement with the reduced effect of polyanions on HbA_1c spectral and ligand-binding properties.