Hemoglobin and Erythrocytes as a Systemic Redox Buffer: Evidence from Comparative Physiology.
Abstract
Blood is classically described as a powerful acid–base buffer, largely due to hemoglobin and erythrocytes. In contrast, its role as a systemic redox buffer remains underappreciated. Here, we review experimental and comparative evidence indicating that erythrocytes—and hemoglobin in particular—constitute a quantitatively significant and physiologically relevant redox buffering system. Emphasis is placed on thiol chemistry, hemoglobin–glutathione coupling, oxygen-dependent redox dynamics, and data from hypoxia-tolerant vertebrates. We propose that blood functions as a circulating redox buffer, analogous to its role in pH homeostasis, with hemoglobin acting as a central mediator of reversible redox exchange between tissues [1–4].
1. Introduction
Redox homeostasis is fundamental to cellular and organismal physiology. Traditionally, antioxidant defense has been framed as a tissue-localized, enzyme-centered process involving superoxide dismutase, catalase, glutathione peroxidase, and intracellular glutathione pools [4,5]. However, this perspective underestimates the potential role of circulating components, particularly erythrocytes and hemoglobin, in systemic redox regulation.
Erythrocytes circulate continuously through tissues with widely varying oxygen tensions and redox environments. Hemoglobin, present at extraordinarily high concentrations, contains multiple reactive cysteine residues capable of reversible thiol chemistry [2,6]. These features position blood as a plausible redox buffering compartment, conceptually analogous to the well-established bicarbonate/hemoglobin buffering of pH.
2. Thiol Chemistry and Redox Capacity of Hemoglobin
Hemoglobin contains solvent-accessible cysteine residues whose redox potentials fall within an intermediate range (e.g., β93-Cys in mammals) [2]. Such potentials are not optimized for terminal antioxidant reactions, but rather for reversible redox exchange, a defining property of buffering systems.
Reversible processes including S-glutathionylation (Hb–SSG), mixed disulfide formation, and intramolecular thiol oxidation and reduction allow hemoglobin to absorb, store, and later release reducing equivalents without irreversible loss of function [2,7]. This chemistry is consistent with a buffering role, rather than a sacrificial antioxidant function.
3. Quantitative Evidence from Hypoxia-Tolerant Vertebrates
A quantitative analysis by Reischl (1986) in the freshwater turtle Phrynops hilarii demonstrated that erythrocytes contain ~2 mM glutathione, ~5 mM non-protein sulfhydryl groups, and a total reducing capacity of ~26 mM when hemoglobin is included [1]. Thus, hemoglobin accounted for the dominant fraction of erythrocyte reducing capacity.
Moreover, incubation with oxidized glutathione induced reversible electrophoretic changes in hemoglobin, consistent with mixed disulfide formation rather than irreversible damage [1]. These results strongly suggest that, in hypoxia-tolerant species, hemoglobin represents the primary redox buffer within blood, far exceeding low-molecular-weight antioxidants in quantitative capacity.
4. Oxygen-Dependent Redox Coupling Between Hemoglobin and Glutathione
Recent work in human erythrocytes demonstrates that intracellular glutathione levels are dynamically modulated by hemoglobin oxygenation state [8]. Partial deoxygenation (~50% O₂ saturation) increases intracellular GSH without de novo synthesis, indicating direct redox coupling between hemoglobin and the glutathione pool.
These findings establish a mechanistic link between oxygen transport, erythrocyte redox buffering, and systemic physiological state, supporting the view that hemoglobin integrates gas transport and redox regulation into a unified functional system [2,8].
5. Transmembrane Redox Exchange and Systemic Integration
For blood to function as a systemic redox buffer, redox equivalents must be exchangeable between erythrocytes and tissues. Multiple mechanisms support this requirement, including export of oxidized glutathione (GSSG), plasma membrane oxidoreductases, redox-sensitive membrane hubs such as Band 3, and nitric oxide/S-nitrosothiol metabolism [6,7,9].
Band 3 functions as a redox stress sensor and metabolic integrator [9], enabling erythrocytes to participate in redox communication with the extracellular environment.
6. Evolutionary Considerations
The occurrence of hemoglobins with high thiol content in turtles, crocodilians, birds, and other physiologically extreme lineages suggests evolutionary selection for enhanced erythrocyte redox buffering under conditions of chronic or cyclic hypoxia, ischemia–reperfusion, or high metabolic flux [1,10].
Rather than being a universal vertebrate trait, hemoglobin-dominated redox buffering appears accentuated in lineages with exceptional physiological demands, consistent with adaptive specialization [1,10,11].
7. Conceptual Model: Blood as a Redox Buffer
We propose a model in which blood operates as a circulating redox buffer characterized by high capacity (dominated by hemoglobin thiols), intermediate redox potential favoring reversibility, oxygen-dependent modulation, and integration with tissue redox metabolism [2,4,8].
This model parallels acid–base buffering, where hemoglobin buffers protons and facilitates exchange between tissues and lungs.
8. Limitations and Open Questions
Despite strong conceptual and quantitative support, several gaps remain, including limited taxonomic sampling, insufficient whole-organism redox flux measurements, and the absence of integrative models combining erythrocyte and tissue redox regulation [4,5].
Conclusion
Accumulating evidence supports a reinterpretation of hemoglobin and erythrocytes as active participants in systemic redox homeostasis. In hypoxia-tolerant organisms, hemoglobin can constitute the dominant erythrocyte redox buffer [1], while in mammals it remains a dynamic and oxygen-responsive redox mediator [2,8]. Recognizing blood as a redox buffer expands our understanding of circulatory physiology and opens new avenues for comparative, evolutionary, and clinical research.
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