Blood Flow Regulation by S -Nitrosohemoglobin in the Physiological Oxygen Gradient

Abstract

The binding of oxygen to heme irons in hemoglobin promotes the binding of nitric oxide (NO) to cysteine␤93, forming S-nitrosohemoglobin. Deoxygenation is accompanied by an allosteric transition in S-nitrosohemoglobin [from the R (oxygenated) to the T (deoxygenated) structure] that releases the NO group. S-nitrosohemoglobin contracts blood vessels and decreases cerebral perfusion in the R structure and relaxes vessels to improve blood flow in the T structure. By thus sensing the physiological oxygen gradient in tissues, hemoglobin exploits conformation-associated changes in the position of cysteine␤93 SNO to bring local blood flow into line with oxygen requirements.Hemoglobin (Hb) is the tetrameric protein in red blood cells (RBCs) that transports oxygen (O 2 ) from the lung to the tissues (1). As RBCs saturated in O 2 migrate through small arteries and resistance arterioles, they are exposed to an O 2 gradient (2). By the time Hb reaches the capillaries, a large fraction (ϳ50 to 65%) of the O 2 has been lost to venous exchange (a functional shunt) (2). Only about 25 to 30% of the O 2 is extracted by the tissues to meet basal metabolic requirements (1-3). Exposed to increasing oxygen tension (PO 2 ) in postcapillary venules and veins (2), Hb is ϳ75% saturated in O 2 (1, 3) upon entering the lung. Thus, on average, only one of four O 2 molecules carried by Hb is used in the respiratory cycle, even though extensive deoxygenation occurs in the flowcontrolling resistance vessels.Hemoglobin exists in two alternative structures, named R (for relaxed, high O 2 affinity) and T (for tense, low O 2 affinity) (4). Hemoglobin assumes the T structure to efficiently release O 2 (4). The allosteric transition in Hb (from R to T) controls the reactivity of two highly conserved cysteines (Cys␤93) that can react with NO or SNO (S-nitrosothiol) (5). Thiol affinity for (S)NO is high in the R structure and low in the T structure. In other words, the NO group is released from thiols of Hb in low PO 2 (5). A major function of (S)NO in the vasculature is to regulate blood flow, which is controlled by the resistance arterioles (6). We therefore proposed that partial deoxygenation of SNO-Hb in these vessels might actually promote O 2 delivery by liberating (S)NO. That is, the allosteric transition in Hb would function to release (S)NO in order to increase blood flow.Hemoglobin is mainly in the R (oxy) structure in both 95% O 2 and 21% O 2 (room air) (4). Hb and SNO-Hb both contract blood vessels in bioassays (7) at these O 2 concentrations (Fig. 1A). That is, their hemes sequester NO from the endothelium. In hypoxia [Ͻ1% O 2 (at a simulated tissue PO 2 of ϳ6 mmHg)], which promotes the T structure (4), Hb strongly contracts blood vessels, whereas SNO-Hb does not (Fig. 1B). NO group release from SNO-Hb is accelerated in RBCs by glutathione (5), which enhances SNO-Hb relaxations through formation of S-nitrosoglutathione (GSNO) (Fig. 1C). The potentiation by glutathione is inversely related to the PO 2 (Fig. 1C), because NO group transfer fr...

The Oxygen Paradox of Neurovascular Coupling

“…Our data further show that the CBF response does not depend on deoxygenation of hemoglobin, which has been proposed as part of a universal mechanism for coupling of oxygen demand and supply. 65 Oxygen conc. of arterial blood 9,000 nmoL/mL Glucose conc.…”

Reassessing the risk of hemodilutional anemia: Some new pieces to an old puzzle

“…92 Although the mechanisms are disputed, NO may have profound effects on tissue perfusion through interactions with hemoglobin. 93 Evidence suggests that S-nitroso-hemoglobin (SNO-Hb) may be an important and regulated source of intravascular NO. 93 In addition, stable NO metabolites (nitrate-NO 3 -and nitrite-NO 2 -) may provide an additional source of biologically active NO in hypoxic vascular beds 94 (Figures 5, 6).…”

“…93 Evidence suggests that S-nitroso-hemoglobin (SNO-Hb) may be an important and regulated source of intravascular NO. 93 In addition, stable NO metabolites (nitrate-NO 3 -and nitrite-NO 2 -) may provide an additional source of biologically active NO in hypoxic vascular beds 94 (Figures 5, 6). In an experimental model of acute hemodilutional anemia, we have observed that whole blood methemoglobin (MetHb) levels are increased consistently by approximately 1-2%.…”

Interactions of nitrosylhemoglobin and carboxyhemoglobin with erythrocyte

“…It is therefore necessary to preserve NO bioactivity in the proximity of a pool of hemoglobin (∼10 mM) in the lumen. Several solutions to this paradox have been proposed, including i) the diffusion and transport limitation arises from the encapsulation of hemoglobin (Hb) in erythrocyte [8,9,10,11,12,13,14,5,15,16,17,18], ii) the formation of S-nitroso-hemoglobin (SNO-Hb) from the intramolecular transfer of nitrosylhemoglobin (HbFe II NO) [19,20,21,22,23,24,5,25], and iii) the NO bioactivity transduced from nitrite [26,27,28,29]. In the latter two cases, the formation of HbFe II NO can be argued to play a role for preserving NO bioactivity.…”

“…In early works, it has been suggested that the formation of metHbFe III from HbFe II NO is a direct reaction of O 2 with the bound NO. Stamler and coworkers utilized this idea of autooxidation reaction to describe denitrosylation [21]. However, later works began to consider that the autooxidation reaction may occur via three steps [36,37]: (1) the slow dissociation of NO from the iron center of hemoglobin or myoglobin, (2) the binding of oxygen to the newly formed deoxy-Fe II globin, and (3) the irreversible oxidation of HbFe II O 2 by NO referred to as NO dioxygenation (NOD).…”