Antioxidant and Signal-Modulating Effects of Brown Seaweed-Derived Compounds against Oxidative Stress-Associated Pathology

Figure 6

The enzymatic and nonenzymatic antioxidant protection mechanism. (1) Superoxide radical (O₂^⋅−) is formed by a single-electron reduction of oxygen. In a reaction catalyzed by superoxide dismutase (Cu/ZnSOD or MnSOD), superoxide radical binds an electron, which leads to the formation of hydrogen peroxide (H₂O₂). (2) In the further reduction of hydrogen peroxide (H₂O₂) to water and oxygen by catalase (CAT) and glutathione peroxidase (GPx) enzymes. (3) In the Fenton reaction, the H₂O₂ is then transformed into hydroxyl radical (HO^⋅) by catalyzing the transition metal, which is further participating in the free radical chain reactions. (4) H₂O₂ is reduced by glutathione (GSH) and produced glutathione disulfide (GSSH). (5) The glutathione disulfide is then reduced by glutathione reductase (GR) using the hydrogen of NADPH which is oxidized to NADP⁺. (6) Besides, the nonenzymatic antioxidants such as carotenoids, vitamin C, and tocopherol support the regeneration of GSSG and back to GSH. Vitamin C, carotenoids, and tocopherol donate the hydrogen to free radicals that directly scavenges the free radical and terminates the lipid peroxidation chain reaction [66, 91, 92].