Oxidative stress is a condition characterized by elevated levels of intracellular reactive oxygen species (ROS). ROS either are, or break down to form, free radicals. ROS include superoxide anion (O2-), hydrogen peroxide (H2O2), and hydroxyl radicals (OH-) that are capable of reacting with, and damaging DNA, proteins, and lipids. R&D Systems offers a range of enzymes related to the study of oxidative stress including glutathione peroxidases (GPX), nitric oxide synthase (eNOS, iNOS, and nNOS), peroxiredoxins, super oxide dismutases (SOD), thioredoxins (Trx), and more. Some of these enzymes are involved in generating deleterious free radical species while others have antioxidant properties. For example, nitric oxide synthase (NOS) generates nitric oxide (NO) which is a pleiotropic signaling molecule implicated in diverse biological processes including inhibition of platelet aggregation, regulation of neurotransmission, vasodilatation, immune responses, and inflammation. In contrast, antioxidant enzymes, such as the glutathione peroxidase family, protect cell surfaces, extracellular fluid components, and other enzymes from oxidative stress by catalyzing the reduction of hydrogen peroxide, lipid peroxides, and organic hydroperoxide using reduced glutathione. Physiological homeostasis demands enzymes that exert antioxidant properties.