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...

Recent studies have underscored questions about the balance of risk and benefit of RBC transfusion. A better understanding of the nature and timing of molecular and functional changes in stored RBCs may provide strategies to improve the balance of benefit and risk of RBC transfusion. We analyzed changes occurring during RBC storage focusing on RBC deformability, RBC-dependent vasoregulatory function, and S-nitrosohemoglobin (SNO-Hb), through which hemoglobin (Hb) O2 desaturation is coupled to regional increases in blood flow in vivo (hypoxic vasodilation). Five hundred ml of blood from each of 15 healthy volunteers was processed into leukofiltered, additive solution 3-exposed RBCs and stored at 1-6°C according to AABB standards. Blood was subjected to 26 assays at 0, 3, 8, 24 and 96 h, and at 1, 2, 3, 4, and 6 weeks. RBC SNO-Hb decreased rapidly (1.2 ؋ 10 ؊4 at 3 h vs. 6.5 ؋ 10 ؊4 (fresh) mol S-nitrosothiol (SNO)/mol Hb tetramer (P ‫؍‬ 0.032, mercuric-displaced photolysis-chemiluminescence assay), and remained low over the 42-day period. The decline was corroborated by using the carbon monoxide-saturated copper-cysteine assay [3.0 ؋ 10 ؊5 at 3 h vs. 9.0 ؋ 10 ؊5 (fresh) mol SNO/mol Hb]. In parallel, vasodilation by stored RBCs was significantly depressed. RBC deformability assayed at a physiological shear stress decreased gradually over the 42-day period (P < 0.001). Time courses vary for several storage-induced defects that might account for recent observations linking blood transfusion with adverse outcomes. Of clinical concern is that SNO levels, and their physiological correlate, RBC-dependent vasodilation, become depressed soon after collection, suggesting that even ''fresh'' blood may have developed adverse biological characteristics.adenosine triphosphate ͉ hemoglobin ͉ nitric oxide ͉ S-nitrosothiols ͉ transfusion

The current perspective of NO biology is formulated predominantly from studies of NO synthesis. The role of S-nitrosothiol (SNO) formation and turnover in governing NO-related bioactivity remains uncertain. We generated mice with a targeted gene deletion of S-nitrosoglutathione reductase (GSNOR), and show that they exhibit substantial increases in whole-cell S-nitrosylation, tissue damage, and mortality following endotoxic or bacterial challenge. Further, GSNOR(-/-) mice have increased basal levels of SNOs in red blood cells and are hypotensive under anesthesia. Thus, SNOs regulate innate immune and vascular function, and are cleared actively to ameliorate nitrosative stress. Nitrosylation of cysteine thiols is a critical mechanism of NO function in both health and disease.

Interactions of nitric oxide (NO) with hemoglobin (Hb) could regulate the uptake and delivery of oxygen (O(2)) by subserving the classical physiological responses of hypoxic vasodilation and hyperoxic vasconstriction in the human respiratory cycle. Here we show that in in vitro and ex vivo systems as well as healthy adults alternately exposed to hypoxia or hyperoxia (to dilate or constrict pulmonary and systemic arteries in vivo), binding of NO to hemes (FeNO) and thiols (SNO) of Hb varies as a function of HbO(2) saturation (FeO(2)). Moreover, we show that red blood cell (RBC)/SNO-mediated vasodilator activity is inversely proportional to FeO(2) over a wide range, whereas RBC-induced vasoconstriction correlates directly with FeO(2). Thus, native RBCs respond to changes in oxygen tension (pO2) with graded vasodilator and vasoconstrictor activity, which emulates the human physiological response subserving O(2) uptake and delivery. The ability to monitor and manipulate blood levels of NO, in conjunction with O(2) and carbon dioxide, may therefore prove useful in the diagnosis and treatment of many human conditions and in the development of new therapies. Our results also help elucidate the link between RBC dyscrasias and cardiovascular morbidity.

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It is proposed that the bond between nitric oxide (NO) and the Hb thiol Cys-␤ 93 (SNOHb) is favored when hemoglobin (Hb) is in the relaxed (R, oxygenated) conformation, and that deoxygenation to tense (T) state destabilizes the SNOHb bond, allowing transfer of NO from Hb to form other (vasoactive) S-nitrosothiols (SNOs). However, it has not previously been possible to measure SNOHb without extensive Hb preparation, altering its allostery and SNO distribution. Here, we have validated an assay for SNOHb that uses carbon monoxide (CO) and cuprous chloride (CuCl)-saturated Cys. This assay is specific for SNOs and sensitive to 2-5 pmol. Uniquely, it measures the total SNO content of unmodified erythrocytes (RBCs) (SNORBC), preserving Hb allostery. In room air, the ratio of SNORBC to Hb in intact RBCs is stable over time, but there is a logarithmic loss of SNORBC with oxyHb desaturation (slope, 0.043). This decay is accelerated by extraerythrocytic thiol (slope, 0.089; P < 0.001). SNORBC stability is uncoupled from O2 tension when Hb is locked in the R state by CO pretreatment. Also, SNORBC is increased Ϸ20-fold in human septic shock (P ‫؍‬ 0.002) and the O2-dependent vasoactivity of RBCs is affected profoundly by SNO content in a murine lung bioassay. These data demonstrate that SNO content and O2 saturation are tightly coupled in intact RBCs and that this coupling is likely to be of pathophysiological significance.sepsis ͉ nitric oxide ͉ vascular physiology E vidence has accumulated for an S-nitrosothiol (SNO)-based vascular signaling system in which hemoglobin (Hb) reactions with nitric oxide (NO) transduce redox gradients into bioactivities (1-6). There is agreement that human Hb undergoes Snitrosylation at Cys-␤ 93 (3,(7)(8)(9)(10)(11). Erythrocytes are proposed to couple O 2 tension to the distribution of NO activities (such as control of blood flow) by linking the allosteric transition of Hb (12, 13) to conformation-dependent changes in the redox activity of this Cys-␤ 93 (13-18) and the stereochemistry of this SNO bond at Cys-␤ 93 (6, 7). Indeed, Cys-␤ 93 SNO in human Hb (SNOHb) can be crystallized only with the Hb tetramer in the relaxed (R, oxygenated) conformation; the SNO bond is unstable with Hb in the tense (T, deoxygenated) conformation (7). These observations support a paradigm in which NO binding to Cys-␤ 93 is favored in the R state and NO binding to Fe(II) (and͞or transnitrosation to an alternate thiol) is favored in the T state (19-21). Thus, the change in stability of Cys-␤ 93 SNO during Hb transition between R and T states may serve to couple regional O 2 gradients to the deployment or quenching of NO bioactivities in the microcirculation (2, 6, 22).However, assaying SNOHb has been problematic. First, detection of the SNO bond has required dilution and͞or pretreatment of Hb to (i) control for artifactual identification of nitrite and Fenitrosyl species and (ii) prevent autocapture of NO on Fe during analysis (8,19,(23)(24)(25). As a result, attempts to quantify Cys-␤ 93 SNO density can be biased ...

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The mechanism by which hypoxia [low partial pressure of O2 (pO2)] elicits signaling to regulate pulmonary arterial pressure is incompletely understood. We considered the possibility that, in addition to its effects on smooth muscle, hypoxia may influence pulmonary vascular tone through an effect on RBCs. We report that exposure of native RBCs to sustained hypoxia is accompanied by a buildup of heme iron-nitrosyl (FeNO) species that are deficient in pO 2-governed intramolecular transfer of NO to cysteine thiol, yielding a deficiency in the vasodilator S-nitrosohemoglobin (SNO-Hb). hemoglobin ͉ red blood cell vasodilation ͉ S-nitrosylation I n the systemic microcirculation, blood flow is regulated by physiological O 2 gradients that couple the O 2 content of blood to regulated vasodilation and vasoconstriction (1-3). Blood flow is thereby matched to tissue O 2 demand. An analogous mechanism operates in the lungs, where O 2 uptake (ventilation) is optimized through regulated vasodilation and vasoconstriction (perfusion). Blood flow is thereby matched to alveolar ventilation (2). Because it is Hb O 2 saturation, not the partial pressure of O 2 (pO 2 ), that is coupled to blood flow in vivo (1, 3) it has been deduced that RBCs may serve as O 2 sensors within the integrated vascular system. In support of this idea, it has been shown recently that RBCs can act as O 2 -responsive transducers of vasodilator and vasoconstrictor activity (4-10), at least partly by modulating the availability of [6][7][8]10,11). According to these studies, RBCs release NO bioactivity under hypoxia and sequester it at hyperoxia. The release of NO bioactivity would facilitate hypoxic vasodilation in peripheral tissues and oppose hypoxic pulmonary vasoconstriction (HPV) in the lungs. S-nitrosothiol (SNO)-deficientThe mechanism by which NO bioactivity escapes from RBCs is incompletely understood. It is generally accepted that the rapid reaction of NO with the hemes of Hb produces a heme-iron nitrosyl adduct (Hb [FeNO]) that exhibits no vasodilator activity (4,7,12). Hb also sustains S-nitrosylation at two cysteine residues conserved in all mammals and birds. Biochemical and mutational analyses (␤93Cys3Ala) indicate that S-nitrosohemoglobin (SNO-Hb) is formed upon oxygenation of Hb [FeNO] by means of heme-to-Cys NO transfer (13-15) and by transnitrosylative transfer from low-mass S-nitrosothiols (SNOs) (16,17). SNO-Hb is very stable in the oxygenated (or R) structure and thus cannot effectively dilate blood vessels (5, 10, 18). However, upon deoxygenation [or with change in the spin state of the hemes (3)], the vasodilator potency of SNO-Hb is markedly potentiated (5,16,18). Crystal structures and molecular models show that the ␤-Cys NO gains solvent access in the deoxygenated (or T) state (3, 19). Solvent-exposed NO can exchange with acceptor thiols within the N-terminal cytoplasmic domain of the RBC membrane anion exchange protein (AE1; band 3) (4, 15). Transnitrosylation of AE1 by SNO-Hb involves a direct protein-protein interaction. The st...