Glucose, the energy source for RBC metabolism, is used in two different pathways: glycolysis and the pentose shunt (Fig. 1). Glycolysis is the primary means of obtaining energy and provides up to two molecules of ATP. Glycolysis also reduces NAD to NADH, which is needed for protection of heme ferro-iron to oxidized ferri-iron (methemoglobin). The energy shunt (Rapoport-Luebering pathway) in glycolysis also generates 2,3-BPG. 2,3-BPG is an important glycolytic intermediate that facilitates hemoglobin oxygen delivery. If 2,3-BPG is formed, only one ATP molecule is generated.

Fig1. PRINCIPAL COMPONENTS OF THE ERYTHROCYTE METABOLISM WITH CLINICAL RELEVANCE. Shown are glycolysis, glutathione production (purple shaded area), pentose shunt (orange shaded area), and Rapoport-Luebering shunt (blue shaded area). 1,3-BPG, 1,3-bisphosphoglycerate; 2,3-BPG, 2,3-bisphosphoglycerate; 6PG, 6-phosphogluconate; ADP, adenosine diphosphate; ATP, adenosine triphosphate; DHAP, dihydroxy acetone phosphate; GSH, reduced glutathione; GSSG, oxidized glutathione; NAD, oxidized form of nicotinamide adenine dinucleotide; NADH, reduced form of nicotinamide adenine dinucleotide; NADP, oxidized form of nicotinamide adenine dinucleotide phosphate; NADPH, reduced form of nicotinamide adenine dinucleotide phosphate.

Some glucose is also metabolized in the pentose shunt, which generates five and seven carbon carbohydrates and reduces NADP to NADPH, the sole source of NADPH in red cells. Under physiologic conditions, approximately 90% of glucose is consumed in the glycolytic pathway and about 10% is used in the pentose shunt. In conditions of increased oxidant stress, however, the contribution of the pentose shunt may be significantly increased.

RBCs also have an active glutathione metabolism comprised of its synthesis and reduction (Fig. 2). Maintaining a high ratio of GSH to its oxidized form GSSG, utilizing NADPH conversion to NADP, is the principal mechanism for protecting RBCs from ROS generated from the release of oxygen. Other lines of defense against endogenous ROS in RBCs are catalase, superoxide dismutase (SOD1), peroxiredoxin, and the active efflux of GSSG. These defense mechanisms protect the RBCs not only from ROS generated as a byproduct of released oxygen from hemoglobin but also from ROS generated by NADPH oxidase and, in reticulocytes, from mitochondria-derived ROS during oxidative phosphorylation. Non-quenched ROS is detrimental to RBCs by affecting membrane deformability and hemoglobin solubility.

Fig2. GLUTATHIONE PATHWAY. ADP, Adenosine diphosphate; ATP, adenosine triphosphate; F6P, fructose-6-phosphate; G6P, glucose-6-phosphate; G6PD, glucose-6-phosphate dehydrogenase; GSH, reduced glutathione; GSSG, oxidized glutathione; NADP, oxidized form of nicotinamide adenine dinucleotide phosphate; NADPH, reduced form of nicotinamide adenine dinucleotide phosphate.

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Chemistry, Biosynthesis, and Secretion of Endothelins

Chemistry, Biosynthesis, and Secretion of ANP

Hemoglobins with Increased Oxygen Affinity

Specific Enzymes are Associated with Compartments Separated by the Mitochondrial Membranes

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Two-Dimensional Polyacrylamide Gel Electrophoresis

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