Oxidants and Antioxidants in the Redox Biochemistry of Human Red Blood Cells

Abstract

Red blood cells (RBCs) are exposed to both external and internal sources of oxidants that challenge their integrity and compromise their physiological function and supply of oxygen to tissues. Autoxidation of oxyhemoglobin is the main source of endogenous RBC oxidant production, yielding superoxide radical and then hydrogen peroxide. In addition, potent oxidants from other blood cells and the surrounding endothelium can reach the RBCs. Abundant and efficient enzymatic systems and low molecular weight antioxidants prevent most of the damage to the RBCs and also position the RBCs as a sink of vascular oxidants that allow the body to maintain a healthy circulatory system. Among the antioxidant enzymes, the thiol-dependent peroxidase peroxiredoxin 2, highly abundant in RBCs, is essential to keep the redox balance. A great part of the RBC antioxidant activity is supported by an active glucose metabolism that provides reducing power in the form of NADPH via the pentose phosphate pathway. There are several RBC defects and situations that generate oxidative stress conditions where the defense mechanisms are overwhelmed, and these include glucose-6-phosphate dehydrogenase deficiencies (favism), hemoglobinopathies like sickle cell disease and thalassemia, as well as packed RBCs for transfusion that suffer from storage lesions. These oxidative stress-associated pathologies of the RBCs underline the relevance of redox balance in these anucleated cells that lack a mechanism of DNA-inducible antioxidant response and rely on a complex and robust network of antioxidant systems.

Figure 1

Reactive species produced in the vascular system relevant to RBCs. Oxygen and NO^• are the two main ingredients required for the generation of reactive species that will lead to biomolecular damage. Details about each pathway are given in the text.

Figure 2

Peroxiredoxin activity. The reduced Cys52 in Prx2 (C_PSH) is oxidized by H₂O₂ and other oxidants to sulfenic acid (C_P-SOH). This C_PSOH reacts with the C_RSH, forming an intermolecular disulfide bridge. The disulfide-oxidized Prx2 is predominantly a dimer and is reduced by Trx, TR, and NADPH. The oxidized C_PSOH can alternatively react with a second oxidant molecule to yield the hyperoxidized sulfinic acid (C_PSO₂). The latter can either be repaired to the active enzyme by sulfiredoxin (Srx) or form stacked decamer high molecular weight structures. The structure of decameric Prx2 (5IJT) shows reactive cysteine residues in yellow, and each dimer is shown in green and blue, as sides of the pentagon.

Figure 3

Main low molecular weight antioxidants in RBCs are the water-soluble urate, glutathione, and ascorbate and the lipid-soluble α-tocopherol. These antioxidants can form stable radicals after one-electron oxidation that can be repaired by other antioxidants. The ultimate source of reducing power is given by NADPH, which can be used to yield GSH, which can be used to reduce DHA to ascorbate, which can reduce the urate radical and also the α-tocopheroxyl radical back to their reduced forms.

Figure 4

Antioxidant systems in RBCs are robust and redundant, allowing the detoxification of several oxidants (shown in red boxes) such as O₂^•–, H₂O₂, and ONOO^– that could lead to more potent and less selective oxidants such as NO₂^• and HO^•, which could result in hemoglobin damage, affecting RBC functionality. Oxidants can be generated endogenously or can be from other cells in the blood vessels. The RBC contains low molecular weight antioxidants, such as GSH, ascorbate, and α-tocopherol, and several enzymatic systems (in ovals of different colors). Reduction reactions are shown by green arrows. The reducing power for the antioxidant systems in RBCs is ultimately provided by NADPH from glucose-6-P and the pentose phosphate pathway. Details of the different pathways are given in the text.