Figure 1.
Oxygen-free radicals are produced through the leakage of electrons from electron transport chains in the mitochondria and endoplasmic reticulum on to molecular oxygen. The superoxide anion generated is not freely diffusable through cell membranes and must be dismutated in situ by either Cu/Zn or Mn SOD. Although H2O2 is not a free radical it can react with O2·− to form the extremely reactive hydroxyl radical. Catalase and GPX must therefore operate in concert with the SODs to keep concentrations at physiological levels.