Oxygen: how do we stand it?

Abstract

The electronic structure of ground state oxygen, which is essential for the life of all aerobic organisms, makes it potentially dangerous for those organisms. Atmospheric oxygen contains two unpaired electrons with parallel spin states, which predisposes it to reduction by a univalent pathway. As a consequence, normal aerobic metabolism generates dangerous reactive intermediates of the reduction of O2. These include superoxide radical (O2*-), hydrogen peroxide (H2O2), and hydroxyl radical (HO*). These reactive oxygen species and others that they can engender can damage all cellular macromolecules and unless opposed by cellular defenses, would make aerobic life impossible. Such defenses include superoxide dismutases, catalases, and peroxidases, enzymes that decrease the concentration of the reactive oxygen species that are their substrates, and others that repair or recycle oxidatively damaged macromolecules. Any factor that stimulates reactive oxygen species production or suppresses the antioxidant systems would inevitably cause cell damage. The role of such oxidative damage in various diseases is well documented. In vivo detection of O2- and other reactive oxygen species is however hampered by the lack of easy, specific, and sensitive analytical methods. Potential artifacts and limitations of the most common detection methods currently in use are briefly discussed.

Fig. 1

Electronic configurations energy levels of ground state and singlet molecular oxygen.

Fig. 2

Sequential one-electron reduction of molecular oxygen to water.