Unraveling the Reactions of Nitric Oxide, Nitrite, and Hemoglobin in Physiology and Therapeutics

Abstract

Abstract-The ability of oxyhemoglobin to inhibit nitric oxide (NO)-dependent activation of soluble guanylate cyclase and vasodilation provided some of the earliest experimental evidence that NO was the endothelium-derived relaxing factor (EDRF). The chemical behavior of this dioxygenation reaction, producing nearly diffusion limited and irreversible NO scavenging, presents a major paradox in vascular biology: The proximity of large amounts of oxyhemoglobin (10 mmol/L) to the endothelium should severely limit paracrine NO diffusion from endothelium to smooth muscle. However, several physical factors are now known to mitigate NO scavenging by red blood cell encapsulated hemoglobin. These include diffusional boundaries around the erythrocyte and a red blood cell free zone along the endothelium in laminar flowing blood, which reduce reaction rates between NO and red cell hemoglobin by 100-to 600-fold. Beyond these mechanisms that reduce NO scavenging by hemoglobin within the red cell, 2 additional mechanisms have been proposed suggesting that NO can be stored in the red blood cell either as nitrite or as an S-nitrosothiol (S-nitroso-hemoglobin). The latter controversial hypothesis contends that NO is stabilized, transported, and delivered by intra-molecular NO group transfers between the heme iron and ␤-93 cysteine to form S-nitrosohemoglobin (SNO-Hb), followed by hypoxia-dependent delivery of the S-nitrosothiol in a process that links regional oxygen deficits with S-nitrosothiol-mediated vasodilation. Although this model has generated a field of research examining the potential endocrine properties of intravascular NO molecules, including S-nitrosothiols, nitrite, and nitrated lipids, a number of mechanistic elements of the theory have been challenged. Recent data from several groups suggest that the nitrite anion (NO 2 Ϫ ) may represent the major intravascular NO storage molecule whose transduction to NO is made possible through an allosterically controlled nitrite reductase reaction with the heme moiety of hemoglobin. As subsequently understood, the hypoxic generation of NO from nitrite is likely to prove important in many aspects of physiology, pathophysiology, and therapeutics. Key Words: nitric oxide Ⅲ nitrite Ⅲ hemoglobin Ⅲ vasodilation Ⅲ red blood cell N itric oxide (NO) is the endothelium-derived relaxing factor that modulates vascular tone by activating soluble guanylyl cyclase (sGC) in smooth muscle. [1][2][3][4] It is now appreciated that NO is produced in endothelial cells by the endothelial NO synthase enzyme and participates in many aspects of normal vascular physiology, including tonic vasodilation and inhibition of platelet activation and endothelial adhesion molecule expression. Endothelial dysfunction, characterized by reduced bioavailability of endothelial derived NO, is a central mechanistic feature of coronary artery disease and its risk factors, including diabetes, hypertension, smoking, and obesity. Excessive NO production from inducible NO synthase is a central mechanism of septic shock. Nov...

Development of Recombinant Hemoglobin-Based Oxygen Carriers

“…The more difficult challenge has been to inhibit the rate of NO scavenging without markedly altering the rates of O 2 uptake and release. The key reactions of NO with reduced Hb are shown in Figure 3A (69,99). Even if some free NO binds to deoxyHb, its ultimate fate will be irreversible dioxygenation to nitrate if any O 2 is present (45).…”

Human red blood cells at work: identification and visualization of erythrocytic eNOS activity in health and disease

“…Our results invite a rethink about the role of hemoglobin in NO biology, because its oxygenated form is considered an NO scavenger under any condition. Several factors have been identified that explain why endothelial NO escapes scavenging by hemoglobin, 8,11,45,48,49 but those discussions did not consider the possibility that NO might be produced in RBCs themselves. The observation that NO formation can be detected in the vicinity of abundant oxyhemoglobin would seem to warrant a careful reassessment of this universally accepted paradigm.…”

The potential of Angeli's salt to decrease nitric oxide scavenging by plasma hemoglobin

“…In normal physiology, the reason that endothelial-derived NO is not scavenged to the extent predicted, based purely on kinetic calculations, is that red blood cell (RBC) encapsulated Hb in the blood reacts with NO much more slowly than does cell-free Hb [35][36][37][38][39][40][41][42][43][44][45]. Three mechanisms contribute to reduced NO scavenging by RBCs [46]. : (1) the rate of the reaction is largely limited by external diffusion of NO through the plasma to the surface of the RBC [44], especially due to the presence of a red cell free zone adjacent to the vessel walls where NO is made [37][38][39]; (2) NO diffusion is partially blocked by a physical barrier across the RBC membrane [40,43,47]; and (3) RBC-encapsulated Hb is efficiently compartmentalized in the lumen; it does not extravasate into the endothelium and interstitium [44,[48][49][50][51][52].…”

“…All three mechanisms are disrupted during intravascular hemolysis [46]. The increased ability of cell-free Hb to scavenge NO has been widely attributed to the hypertension, increased systemic and pulmonary vascular resistance, and morbidity and mortality associated with administration of hemoglobin-based oxygen carriers (HBOCs or "blood substitutes") [53][54][55][56][57][58][59][60][61][62][63][64][65][66][67].…”