**9. Mechanisms of antioxidant defense**

**Figure 3** is a summary of the oxidation mechanisms of a cell, as well as the action that the antioxidant exerts to prevent oxidation. Cell respiration is shown, where molecular oxygen is converted into a superoxide anion, followed by hydrogen peroxide, then a hydroxyl radical and finally water, while the central illustration explains how cell metabolism may form free radicals (superoxide anion and hydroxyl) [48].

According to Londoño [46], when the superoxide anion suffers a mutation catalyzed by the superoxide dismutase enzyme, it becomes less reactive but is still

**Figure 3.** *Pathways of free radical production and action of antioxidants [46].*

toxic for tissues; therefore, it is converted into water by the action of the catalase enzymes, which reduce hydrogen peroxide by oxidizing glutathione; this in turn is generated by the action of glutathione reductase, which uses NADPH as a cofactor. Hydrogen peroxide may also be converted into a hydroxyl radical via a Fenton-type reaction, which is catalyzed by iron. Once produced, the hydroxyl radical attacks proteins, nucleic acids, and mainly polyunsaturated fatty acids, thus generating lipid radicals that quickly react with oxygen to produce peroxyl radicals.

Stabilization of free radicals derived from lipids may be accomplished by phenolic antioxidants such as flavonoids and tocopherols, which stabilize the free radicals (phenoxyl and tocopheryl, respectively). Stabilization may happen inside the molecule by displacement or by reaction with ascorbic acid to generate a reduced compound [49].
