Schematic of SCD-associated oxidative stress pathways and antioxidant systems. In the intravascular space, sickled RBCs undergo hemolysis, releasing hemoglobin (Hb) and heme. Hemoglobin reacts with (1) NO^·, forming nitrate (NO₃⁻), and (2) hydrogen peroxide (H₂O₂), producing hydroxyl radical (⁻OH). Endothelial-bound xanthine oxidase (XO) generates superoxide (O₂^·−) and H₂O₂. Free heme binds to Toll-like receptor 4 (TLR4) producing reactive species through the activation of the NFκB pathway. SOD3 is found in the extracellular compartment and converts O₂^·− to H₂O₂. In the endothelial cell (EC) cytoplasm, endothelial NO synthase (eNOS) uncoupling and NADPH oxidase 2 (Nox2), Nox4, and Nox5 produce O₂^·−, which is dismutated by SOD1. eNOS normally generates NO^·, which is capable of reacting with O₂^·− to form peroxynitrite (ONOO⁻). In the mitochondrial matrix, electrons leaked from complexes I and III of the respiratory chain react with oxygen (O₂), forming O₂^·−. SOD2 dismutates O₂^·− to H₂O_2, which is further broken down to water by catalase (CAT). SOD2 expression and activity is maintained at a precise balance. Overexpression of SOD2 decreases O₂^·− levels and increases H₂O₂ levels, which are then free to oxidize protein thiols. SOD2 underexpression or dysfunction increases O₂^·− levels, increasing nitration and oxidation of iron sulfur clusters and decreasing metabolism and adenosine triphosphate (ATP) production.