Figure 1.

Endogenous reactive oxygen species (ROS) production. Intracellular ROS is primarily produced by the mitochondria, in which ROS are largely generated by the electron transport chain. Other sources of ROS include the transmembrane NADPH oxidases; xanthine oxidoreductase in peroxisomes; protein disulfide isomerase, which are involved in the unfolded protein response in the endoplasmic reticulum (ER); and diverse enzymatic reactions that, among others, include prostaglandin synthesis, auto-oxidation of adrenalin, and reduced riboflavin.