Figure 1. Endogenous sources of ROS signal.

Intracellular ROS is primarily produced by NADPH oxidase enzymes (NOXs), the mitochondria, the endoplasmic reticulum, and the peroxisome. Cytosolic superoxide (O₂⁻) is rapidly converted into hydrogen peroxide (H₂O₂) by superoxide dismutase 1 (SOD1). H₂O₂ can either act as a signaling molecule by oxidizing critical thiols within proteins to regulate numerous biological processes, including metabolic adaptation, differentiation, and proliferation or be detoxified to water (H₂O) by the scavenging enzymes peroxiredoxin (PRX), glutathione peroxidase (GPX), and catalase (CAT). In addition, H₂O₂ can react with metal cations (Fe²⁺ or Cu⁺) to generate the hydroxyl radical (OH•), which causes irreversible oxidative damage to lipids, proteins, and DNA.