Figure 1.

Intracellular sources and main properties of ROS. Within the cell ROS can be generated from different sources as follows: 1) as O₂^•- released at the level of complex I and III of mitochondrial electron chain reaction, with O₂^•- being immediately dismutated to H₂O₂ by mtSOD; 2) as O₂^•- following the reaction of 5-lypoxygenase on arachidonic acid, then dismutated to H₂O₂ by cSOD; 3) as O₂^•-, then dismutated into H₂O₂, following activation (i.e., ligand-receptor interaction) of NADPH oxidase in either phagocytic or non-phagocytic cells; 4) as different ROS generated following the redox reactions catalyzed by a number of cytochromes and enzymes. Critical properties of major ROS are also briefly summarized.