A Novel Possible Role for Met Hemoglobin as Carrier of Hydrogen Sulfide in the Blood.

Along with other gasotransmitters nitric oxide (NO) and carbon monoxide, hydrogen sulfide (HS) has recently emerged as an important signaling molecule with a particularly complex metabolism. Endogenous HS reacts with multiple cellular targets, including protein ferric heme groups, to elicit physiological responses, such as regulation of local blood flow. Recent evidence suggests that HS at low physiological concentrations is carried in the blood as bound to the small fraction of oxidized ferric hemoglobin (metHb). A relatively stable metHb-sulfide complex forms when HS and purified metHb react , with an affinity within the physiological range of sulfide in the blood. Formation and subsequent redox metabolism of metHb-sulfide complex have also been confirmed in isolated intact red blood cells (RBCs) containing enhanced metHb levels. Thus, HS may function as an endocrine signaling molecule and elicit responses at sites away from the site of production. In addition, metHb, considered as an inert or pathological hemoglobin derivative, may have a novel potential physiological role in the transport of HS in the blood. The transport of HS in the blood mediated by metHb would represent an O-independent pH-dependent mechanism for the blood-mediated control of blood flow and as such it is critical to understand the significance of this transport. Major challenges must be resolved to understand how metHb may carry HS in the RBCs, in particular determination of metHb-sulfide levels in the blood and identification of targets in the vasculature.