Palmer LA, Doctor A, Gaston B. SNO-hemoglobin and hypoxic vasodilation. Nat Med. 2008;14:1009. (Comment on Nat Med 2008;14:773-7)
Patel R, Townes T. SNO-hemoglobin and hypoxic vasodilation. Nat Med. 2008;14:1009-10. (Response to Comments on Nat Med 2008;14:773-7)
That increased blood flow correlates negatively with the fractional saturation of hemoglobin by oxygen rather than with dissolved oxygen tension suggested that the biochemical pathways that regulate blood flow are contained within the red cell itself and led to the hypothesis that oxygen delivery is matched to metabolic demand by allosteric coupling of hemoglobin deoxygenation to stimulation of vasodilation. It has been more than 12 years since Jia, et al. published their seminal paper suggesting a central role for S-nitrosylated hemoglobin (SNO-Hgb) in hypoxic vasodilation,1 and over the ensuing years, this concept has been extended such that SNO-Hgb is viewed by some as the principal mediator of the essential physiological coupling of increased blood flow to hypoxia. According to this model (see Figure), as red cells become oxygenated in the lungs, a nitric oxide (NO) group is covalently bound to the highly conserved Cys93 residue in the ß-chain of hemoglobin. In this high oxygen affinity state (the relaxed, R-state), SNO-Hgb is unreactive, but upon Hgb deoxygenation in the periphery and the consequent transition to the low oxygen affinity state (the tense, T-state), SNO-Hgb can react with red cell thiols (e.g., glutathione [GSH] or anion exchanger-1 [AE-1]) via transnitrosation and thereby transmit a vasodilatory signal (RSNO) out of the red cell. Over the past decade, experimental data have accumulated, suggesting that disturbances in the SNO-Hgb pathway lead to a variety of systemic vascular and pulmonary diseases and contribute to the pathobiology of diabetes, congestive heart failure, and the untoward consequences of transfusion of stored blood.2,3
Despite the elegance of the paradigm and an abundance of supporting experimental data, several key elements of the SNO-Hgb hypothesis have been challenged, including the allosteric nature of both SNO-Hgb dependent vasodilation and transnitrosation reactions, quantitation of in vivo SNO-Hgb concentration, the presence of physiologic arterial-venous SNO-Hgb gradients, and mechanisms of SNO-Hgb formation.
![As Red Cells Become Oxygenated in the Lungs, a Nitric Oxide (NO) Group is Covalently Bound to the Highly Conserved Cys93 Residue in the ß-Chain of Hemoglobin. In this high oxygen affinity state (the relaxed, R-state), SNO-Hgb is unreactive, but upon Hgb deoxygenation in the periphery and the consequent transition to the low oxygen affinity state (the tense, T-state), SNO-Hgb can react with red cell thiols (e.g., glutathione [GSH] or anion exchanger-1 [AE-1]) via transnitrosation and thereby transmit a vasodilatory signal (RSNO) out of the red cell.](https://ash.silverchair-cdn.com/ash/content_public/journal/thehematologist/6/1/10.1182_hem.v6.1.6175/2/m_6175a.jpeg?Expires=1724149421&Signature=zKYWuF8yON02apFuF4aNIN5R7~xa2GvB5ILo5E70M-WNidu0lnAK-YgF-OuDuKjoKUMgB~q6uKoKDtXCIPzv06A2aGJ1rIJs7iGW-QlESXQoM5ZrHsQUp7ZlTKqkSHFdX2ibg~2Wt953iD1FBb~b8YPEqV~9HLMAeJfOar-kxA3ezVTT33XDRYjFOdccKs8qjJnWiE0RC1xCaVPyGxLpU0V2HgWuA2KUCiGibyB-JfQ~1VorJk9-x-2nje8wzeu~WAwfFZytE6hgGHAZf8L7VguDyNRmY4d9MGf~dssB30IPI-rA~Hlfu2lKJfropca9cuhfAwui7v~dSwaA5Ld-Sw__&Key-Pair-Id=APKAIE5G5CRDK6RD3PGA)
As Red Cells Become Oxygenated in the Lungs, a Nitric Oxide (NO) Group is Covalently Bound to the Highly Conserved Cys93 Residue in the ß-Chain of Hemoglobin. In this high oxygen affinity state (the relaxed, R-state), SNO-Hgb is unreactive, but upon Hgb deoxygenation in the periphery and the consequent transition to the low oxygen affinity state (the tense, T-state), SNO-Hgb can react with red cell thiols (e.g., glutathione [GSH] or anion exchanger-1 [AE-1]) via transnitrosation and thereby transmit a vasodilatory signal (RSNO) out of the red cell.
As Red Cells Become Oxygenated in the Lungs, a Nitric Oxide (NO) Group is Covalently Bound to the Highly Conserved Cys93 Residue in the ß-Chain of Hemoglobin. In this high oxygen affinity state (the relaxed, R-state), SNO-Hgb is unreactive, but upon Hgb deoxygenation in the periphery and the consequent transition to the low oxygen affinity state (the tense, T-state), SNO-Hgb can react with red cell thiols (e.g., glutathione [GSH] or anion exchanger-1 [AE-1]) via transnitrosation and thereby transmit a vasodilatory signal (RSNO) out of the red cell.
In Brief
To investigate the importance of the conserved ßCys93 and specifically the role of SNO-Hgb in regulation of blood flow, Isbell, et al. generated two transgenic mouse strains in which the α and ß chains of murine hemoglobin were replaced with their human counterparts, either with or without an alanine substitution at the ßCys93 residue (HbC93A). Unexpectedly, only minor phenotypic differences were observed between the two strains under either basal physiological or exercise-stress conditions. Moreover, crucial ex vivo experiments indicated that SNO-Hgb was not required for red cells to simulate hypoxic vasodilation. The authors concluded that SNO-Hgb is not an essential physiological regulator, although they recognized the possibility that the humanized mouse model may have masked a pathophysiological effect of ßCys93 substitution.
As might be anticipated, these studies generated some lively correspondence from leaders of the SNO-Hgb field suggesting that the conclusions were erroneous because of flaws in experimental design, including the possibility that the presence of even small amounts of prenatal mouse hemoglobin could normalize responses in the HbC93A mice. While the criticisms appeared well reasoned and were thoughtfully presented, Patel and Townes, writing for the authors of the original study, provided equally cogent counter arguments.
Indirectly, all of the participants in the discussion suggested that a human model of ßCys93 deficiency might resolve the dilemma; however, humans who are homozygous mutant for ßCys93 have not been reported. Stamler and colleagues argued that the absence of documented homozygous human ßCys93 mutations suggests that humans cannot survive loss of the nitrosylation site at ßCys93. Patel and Townes countered with the argument that humans who are homozygous for ßCys93 mutations have not been identified because they have no clinical phenotype and, therefore, do not come to the attention of physicians. Where does this debate leave us? For my part, I am still unsure about the physiological relevance of SNO-Hgb, but the provocative studies of Isbell, et al. have enlivened further an already lively debate and provided a valuable model for further investigation of the functional significance of the highly conserved ßCys93.
Stay tuned.
