Spin equilibria in human methemoglobin: effects of bezafibrate and inositol hexaphosphate as measured by susceptometry and visible spectroscopy.

The effects of inositol hexaphosphate (IHP) and a second allosteric effector, bezafibrate, on the spin-state equilibria of the mixed-spin derivatives of ferric human hemoglobin A are examined. Changes in spin-state equilibrium are monitored by measuring absorption spectra in the visible region (460-700 nm) as well as by direct measurements of magnetic susceptibility by means of a superconducting fluxmeter. The addition of IHP at pH 6.5 results in a measurable shift in the spin equilibria of these derivatives toward higher spin. However, the addition of bezafibrate in the presence of IHP results in still larger shifts toward the high-spin form. The changes in the free energies of the spin-state equilibria resulting from the combination of these two effectors are similar in magnitude to that which results from the R-state to T-state transition in carp hemoglobin.