Reactions of ferric hemoglobin and myoglobin with hydrogen sulfide under physiological conditions

Ferric hemoglobin (metHb) and myoglobin (metMb), present at low levels in vivo, have been recently found to oxidize hydrogen sulfide (H₂S) in excess, thus potentially contributing to removal of toxic H₂S in blood and heart, respectively. Here, we present a kinetic and thermodynamic study of the reaction of metHb and metMb with H₂S under physiological conditions, i.e. at low H₂S concentrations and with protein in excess of H₂S. We show here that both proteins react with sub-stoichiometric H₂S:heme ratios following two processes: a fast reversible binding of H₂S to ferric heme that prevails at high H₂S and a slow heme reduction to the ferrous state that prevails at low H₂S. While these two processes are fast for metMb, H₂S-induced heme reduction is slow for metHb and the metHb-H₂S complex once formed is therefore relatively stable. We find that metHb binds H₂S reversibly and cooperatively with a pH-dependent ligand affinity that is within the physiological range of H₂S concentrations found in blood. Stopped-flow kinetics show identical association rate constants for H₂S at varying pH, demonstrating that H₂S and not HS^- enters the ferric heme pocket. Dissociation rates of the metHb-H₂S complex increase when decreasing pH, consistent with the pH-dependent affinity. Taken together, these data are consistent with a novel biological role of metHb as a H₂S carrier in the blood, in parallel with the oxygen carrier function of the much more abundant ferrous Hb. In contrast, metMb in the heart could participate to redox-signaling involving H₂S.