Pro- and Antioxidant Effects of Vitamin C in Cancer in correspondence to Its Dietary and Pharmacological Concentrations

Figure 3

Vitamin C may differently produce reactive oxygen species (ROS) in blood and the extracellular space. After oral or intravenous administration, vitamin C reaches the same concentration in blood and extracellular fluid and loses one electron () to form ascorbate radical Asc^⋅− and reduces a protein-centered metal ion, such as Fe(III). Reduced metal donates an electron to oxygen forming ROS, including superoxide (O₂^⋅−) that can be dismutated to hydrogen peroxide. These reactions in blood are inhibited by plasma and red cell membrane proteins, and hydrogen peroxide in blood is neutralized by antioxidant enzymes hardly present in extracellular fluid. Unless H₂O₂ is decomposed, it may produce hydroxyl radicals in the Fe(II)- or Cu(I)-catalyzed Fenton-like reaction yielding hydroxyl peroxide (HO^⋅).