Spin Equilibrium in Methemoglobin: Effects of Inositol Hexaphosphate and Bezafibrate as Measured by Resonance Raman Spectroscopy

The spin equilibria of several derivatives of human methemoglobin were probed by resonance Raman scattering. The intensity of lines in the Raman spectrum gives a measure of the high (S = 5/2) to low(S = 1/2) spin ratio which agrees well with the spin equilibria determined from direct magnetic susceptibility measurements. The addition of bezafibrate (BZF) to methemoglobin in the absence of organic phosphate, IHP, has very little effect on the spin equilibrium whereas in the presence of IHP, it augments the change in spin, significantly. When both IHP and BZF are added to the mixed spin derivatives (H sub 2 O, SCN sup -, OCN-, and NO sup - above 2) of human methemoglobin the spin equilibrium is shifted toward higher spin by about 700 cal/mole, similar to the spin change detected in derivatives of carp methemoglobin upon addition of IHP alone. These data support a general mechanism for the allosteric transition in which a constant fraction of the cooperative energy (~20%) is detected at the heme.