Dioxygen toxicity

The enzymatic antioxidants are (a) catalase and the various peroxidases, whose presence lowers the concentration of hydrogen peroxide, thereby preventing it from entering into potentially damaging reactions with various cell components (see Section VI and Reactions 5.82 and 5.83), and (b) the superoxide dismutases, whose presence provides protection against dioxygen toxicity that is believed to be mediated by the superoxide anion, 62 (see Section VII and Reaction 5.95). 

Catalase and peroxidase are heme enzymes that catalyze reactions of hydrogen peroxide.In catalase, the enzymatic reaction is the disproportionation of hydrogen peroxide (Reaction 5.82) and the function of the enzyme appears to be prevention of any buildup of that potentially dangerous oxidant (see the discussion of dioxygen toxicity in Section III). 

Reduced flavoproteins and complex III may undergo side reactions directly with dioxygen, to a limited extent, forming highly reactive and toxic products, hydrogen peroxide, and superoxide. 

Distal fission Although substituted catechols generally undergo extradiol fission, this may produce toxic metabolites, and can be circumvented by regioselective 1,6-dioxygenation. This is rather uncommon but it has been observed in a number of degradations ... 

Trace amounts of copper are essential for life. However, as with iron, excess copper is also toxic, on account of its capacity to catalyse the Fenton reaction. There are analogies and differences between these two elements successively selected by Nature as it was obliged to adapt life to the first general irreversible pollution of the earth, namely the advent of dioxygen. 

The toxicity of cyanide has led to the development of alternative lixiviants for gold and silver see also Sections 9.17.3.2 and 9.17.3.3. Thiosulfate is potentially a cheaper reagent for use in alkaline or near-neutral solutions in the presence of a mild oxidant such as dioxygen,41-48... 

Fee, J. A. Superoxide, superoxide dismutases, and oxygen toxicity. In Metal Ion Activation of Dioxygen (Spiro, T. G, ed.). New York-Chichester-Brisbane-Toronto, Wiley-Interscience, 1980, pp. 209-237... 

Exactly the opposite problem may occur for plants whose roots are growing in anaerobic media. In Hooded soils the roots may be exposed to high levels of irontll). posing potential problems of iron toxicity. Rice plants and water lilies with roots in anaerobic soils transport dioxygen (from the air or photosynthesis, or both) to the periphery of the roots where it oxidizes the iron(II) to irondll). In (his case the insolubility of Irondll) hydroxide is utilized to protect the plant from iron poisoning.113 A similar problem from too much iron occurs in parts of sub-Saharan Africa. 

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