Figure 1

Molecular oxygen use by enzyme systems leading to reactive oxygen species production and downstream consequences. Oxygen (O₂) not only leads to superoxide anion (·O₂^-) generation by mitochondria and monooxygenases, but is also required for the enzymatic production of the important signaling molecules nitric oxide (NO) and carbon monoxide (CO). Some oxygen-derived reactive oxygen intermediates such as hydrogen peroxide (H₂O₂) have pluripotent effects in the cell that are not only detrimental, such as protein and DNA oxidation and lipid peroxidation, but are beneficial and adaptive, for instance by enhancement of the antioxidant defenses. Ask1, apoptosis-signaling kinase 1; Fe, iron; HIF-1, hypoxia inducible factor 1; iNOS/eNOS, inducible nitric oxide synthase/endogenous nitric oxide synthase; ONOO^-, peroxynitrite anion; PI3K, phosphoinositide 3-kinase; SOD, superoxide dismutase.