Effects of p-hydroxymercuribenzoate binding on the visible absorption spectrum of methemoglobin

The binding of p-hydroxymercuribenzoate to human methemoglobin causes a perturbation of the visible heme abosrption spectrum which is expressed by an increase in absorbance in the high spin band regions, 480 to 510 nm and 590 to 640 nm, concomitant with a decrease in absorbance in the alpha- and beta-band absorption regions. The pH dependence of the p-hydroxymercuribenzoate-induced difference spectrum can be accounted for quantitatively by a 5% shift toward higher spin of the aquo form of methemoglobin, a 15% shift toward higher spin of the hydroxide form, and a shift in the apparent pKa for the water to hydroxide transition from 7.92 to 8.04 when mercurial is bound. The rate of these heme abosrbance changes is consistent with the rapid second order formation of the beta93 cysteine, mercury-mercaptide bond and does not represent a change due to the dissociation of methemoglobin tetramers into dimers, even though the latter, slow process does follow mercurial binding. The observation of an increase in spin produced by the binding of a reagent which also promotes dimer formation argues strongly against any direct correlation between an increase in spin and the appearance of deoxyhemoglobin-like conformations.