Hemoglobin βCys93 is essential for cardiovascular function and integrated response to hypoxia

Abstract

Oxygen delivery by Hb is essential for vertebrate life. Three amino acids in Hb are strictly conserved in all mammals and birds, but only two of those, a His and a Phe that stabilize the heme moiety, are needed to carry O2. The third conserved residue is a Cys within the β-chain (βCys93) that has been assigned a role in S-nitrosothiol (SNO)-based hypoxic vasodilation by RBCs. Under this model, the delivery of SNO-based NO bioactivity by Hb redefines the respiratory cycle as a triune system (NO/O2/CO2). However, the physiological ramifications of RBC-mediated vasodilation are unknown, and the apparently essential nature of βCys93 remains unclear. Here we report that mice with a βCys93Ala mutation are deficient in hypoxic vasodilation that governs blood flow autoregulation, the classic physiological mechanism that controls tissue oxygenation but whose molecular basis has been a longstanding mystery. Peripheral blood flow and tissue oxygenation are decreased at baseline in mutant animals and decline excessively during hypoxia. In addition, βCys93Ala mutation results in myocardial ischemia under basal normoxic conditions and in acute cardiac decompensation and enhanced mortality during transient hypoxia. Fetal viability is diminished also. Thus, βCys93-derived SNO bioactivity is essential for tissue oxygenation by RBCs within the respiratory cycle that is required for both normal cardiovascular function and circulatory adaptation to hypoxia.

HbE/β-Thalassemia and Oxidative Stress: The Key to Pathophysiological Mechanisms and Novel Therapeutics

“…SNO Hb formation has been shown (198) to play a role in limiting oxidative stress and erythrocyte membrane damage by further supporting the concept that NO in many or all of its bioactive forms can function to maintain RBC homeostasis in the presence of oxidative stress. Furthermore, SNO Hb formation has also been shown to be a significant contributor to the mechanism for hypoxic vasodilation (212). It would therefore appear that SNOHb formation is significant for both Band 3 functionality and stability and for RBC functionality with respect to hypoxic vasodilation.…”

Regulation of oxygen delivery to the body via hypoxic vasodilation

“…βCys93 mutant mice had litters with approximately half as many pups as WT mice (11), suggesting a role for SNO-Hb in the regulation of fetal blood flow. Furthermore, βCys93 mutant mice exhibited myocardial infarction and cardiogenic shock under hypoxic conditions, suggesting that alterations in SNO-Hb and RBC play a role in ischemic coronary syndromes and heart failure (11). Thus, by understanding the mechanisms by which SNO-Hb mediates hypoxic vasodilation, better diagnostic strategies and therapeutic interventions for these pathological conditions can be gained.…”

“…1). In PNAS, Zhang et al (11) present novel information adding a definitive genetic layer of proof supporting the third mechanism, the SNO-Hb pathway in RBC-dependent hypoxic vasodilation. Importantly, the phenotype of the mouse model used in this work illustrates the critical physiological importance this mechanism has in regulating tissue oxygenation.…”

Redox and Peroxidase Activities of the Hemoglobin Superfamily: Relevance to Health and Disease

“…Roles for this NO binding chemistry have been suggested to be a mechanism for the regulation of hypoxic vasodilation. This cysteine is highly conserved in the mammal and avian species (and is also present in fetal Hb as γCys93), so much so that the only other amino acids that are as highly conserved as β/γ Cys93 are a histidine and phenylalanine involved in heme stabilization (132). The relevance of hemoglobin forming and carrying NO relates to conditions of hypoxia, with the hypothesis that hypoxic conditions trigger the release of NO to induce a vasodilation response.…”

“…The relevance of hemoglobin forming and carrying NO relates to conditions of hypoxia, with the hypothesis that hypoxic conditions trigger the release of NO to induce a vasodilation response. Indeed mice with a βCys93Ala mutation appear to have deficient hypoxic vasodilation and enhanced mortality with transient hypoxia (132). Thus βCys93 appears to have an antioxidant role, protecting mice against lipopolysaccharide-induced reactive oxygen species and hypertension (121).…”