Figure 2.

Mechanism of ROS production and neutralization in eukaryotic cells. Redox homeostasis of eukaryotic cell is presented. ROS are generated in the cell by enzymatic systems which include endothelial NOS, cyclooxygenase (COX), xanthine oxidase (XOD), P450 enzymes and mitochondrial ETC. eNOS is involved in the generation of O₂^•- and its transformation into ONOO^- with the participation of NO^•. COX, XOD and P450 enzymes also generate peroxide anion. In the VI complex of OXPHOS, a sequential univalent reduction of O₂ occurs, which during the exchange of 4 electrons is reduced to H₂O. However, about 5% of O₂ is transformed to H₂O₂ in a process of electron leakage when only 2 electron reduction occurs. Moreover, extracellular H₂O₂ may enter the cell through membrane transporters e.g. aquaporins (AQP). H₂O₂ in specific conditions is transformed to ^•OH through Fenton reactions. ^•OH is highly reactive and potentially mutagenic to the cell. Antioxidant defense systems include SOD, TRX, GPX, PRX and chemical antioxidants (e.g. vitamins (C, A, E), β-carotene, GSH). Chemical antioxidants inhibit reaction cascades of ROS formation. Enzymatic antioxidants transform ROS into inactive molecules, e.g. H₂O. GSH is treated as a non-enzymatic antioxidant as it is a substrate for the GPX. It reduces H₂O₂ and oxidizes GSH to form glutathione disulfide (GSSG). GSSG is then reduced to GSH by GR. CAT reduces H₂O₂ directly to H₂O. PRX is another enzyme reducing reactive H₂O₂ molecules. PRX becomes its oxidized form (ox-PRX), reducing H₂O₂ to H₂O. PRX is regenerated by TRX which becomes oxidized in this process (ox-TRX).