Fig 1. The general scheme of action of reactive oxygen species (ROS) and the antioxidant defense system.

Main sources of ROS generation include the mitochondrial electron transport chain, endoplasmic reticulum system, and NAD(P)H oxidase (NOX) complex. The oxygen utilized for respiratory purposes can be converted to ROS such as O₂⁻∙, H₂O₂, and ∙OH. Three key enzymes forming the defensive complex against ROS are SOD, CAT and GPx. Symbols used: GPx–glutathione peroxidase; GRx–glutathione reductase; GSSG–oxidized glutathione; GSH–reduced glutathione; H₂O₂ –hydrogen peroxide; NADPH–reduced nicotinamide adenine dinucleotide phosphate; SOD–superoxide dismutase.