Oxygen univalent reduction

Asada, K., Kiso, K. Toshikawa, K. (1974). Univalent reduction of molecular oxygen by spinach chloroplasts on illumination. Journal of Biological Chemistry, 249, 2175-81. 

H.J. Forman and I. Fridovich, Electrolytic univalent reduction of oxygen in aqueous solution demonstrated with superoxide dismutase. Science. 175, 339 (1972). 

Fridovich recently summarized important aspects concerning the accurate detection and measurement of superoxide. He indicates that univalent reduction of O2 to superoxide is a facile process, but the instability of superoxide in aqueous solutions hinders its detection and measurement. To measure intracellular superoxide, he favors use of the rapid inactivation of [4Fe-4S]-containing dehydratases (such as aconitase) by oxidation of their iron-sulfur clusters. See Oxygen, Oxides Oxygen Radicals... 

Successive, univalent reduction of oxygen may yield reactive oxygen species. This reduction can be effected photochemically, chemically or enzymatically (Korycka- Dahl and Richardson, 1980). A number of enzymes, including Xanthine oxidoreductase in milk, are capable of producing large amounts of superoxide via the univalent reduction of oxygen (Fridovich, 1976). 

Fig. 2 Formation of various reactive oxygen species in biological systems derived from univalent reduction of molecular oxygen. GSH-POD glutathione peroxidase...

Table 3 Biological components involved in the univalent reduction of oxygen...

Asada K, Kiso K and Yoshikawa K (1974) Univalent reduction ofmolecular oxygen hy spinach chloroplasts on illumination. J Biol Chem 247 2175-2181... 

The mitochondrial electron transport chain reduces oxygen to water, where oxygen is reduced by accepting four electrons, leading to the formation of two water molecules. However, upon mitochondrial damage, a variable fraction of electrons can leak from the mitochondrial chain, leading to the univalent reduction of molecular oxygen, which... 

Excitation or reduction of molecular oxygen results in a number of chemically reactive oxygen species including, O2 (univalent reduction), H2O2 (bivalent reduction) and OH (three-electron reduced state) (Hill 1979 Halliwell and Gutteridge 1985 Sies 1985). These three reactive species are produced in vivo in... 

A potentially harmful chemical property of iron and its compounds is the ability to catalytically generate potent hydroxyl radicals that are oxidatively toxic to cells [3,4]. This catalysis (Haber-Weiss and Fenton reactions) occurs with activated oxygen species (superoxide and hydrogen peroxide, O3 and H2O2) as shown below. Superoxide is formed by the univalent reduction of oxygen (xanthine plus xanthine oxidase). 

It is well known that in presence of xanthine oxydase (XO) the oxidation of oxypurines by oxygen occurs with concomitant formation of superoxide nion (Op, product of the univalent reduction of molecular oxygen ( 0 ). 

The latter system employs electrons from NADPH for the univalent reduction of oxygen to superoxide radical (SOR), and is under the influence of the enzyme NADPH oxidase. Subsequent reactions lead to the production of hydrogen peroxide, hydroxyl radicals, and singlet oxygen, all of which possess antimicrobial activity... 

Atmospheric oxygen ( O2) is relatively unreactive because it is a diradical with parallel spin state. Thus its divalent reduction is kinetically limited by the relatively slow spin inversion process. The spin conservation rule states that spin must be conserved during the time for a chemical reaction to occur. The spin restriction means that when O2 is involved in metaboUc oxidation it has to be activated, allowing for spin inversion of one electron at a time, in order to have productive collision. This univalent pathway requires the generation of intermediates, among superoxide (02 ) is the first reduced product. 

FIGURE 1. The univalent pathway of O2 reduction gives rise to three active oxygen species. SOD superoxide dismutase, CAT = catalase, GPx = GSH peroxidase, GR = GSH reductase, ASC ascorbate. 

This hypothesis has a quantitative limitation opened to experimental verification. Four electrons are required for each molecule of oxygen utilized by the oxidation-reduction chain. Thus, in turn, four or fewer univalents ions must be transported in the reaction for every molecule of oxygen utilized. 

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