O2 leakages

Although O2 leakage compromises the root s internal aeration, some leakage is desirable for a number of purposes. These include oxidation of toxic products of anaerobic metabolism in submerged soil such as ferrous iron (van Raalte, 1944 Bouldin, 1966 van Mensvoort et al., 1985) nitrification of ammonium to nitrate, there being benefits in mixed nitrate-ammonium nutrition (Kronzucker et al., 1999, 2000) and mobilization of sparingly soluble nutrients such as P (Saleque and Kirk, 1995) and Zn (Kirk and Bajita, 1995) as a result of acidification due to iron oxidation and cation-anion intake imbalance. 

Superoxide dismutases (SODs) are a family of cytosolic metalloenzymes that specifically remove (reviewed by Omar etal., 1992). SOD distribution within the body is ubiquitous, being found in erythrocytes as well as most organs and cell types. Three distinct mammalian SOD forms exist CuZnSOD, MnSOD and extracellular SOD (EC-SOD). Their amino-acid sequences differ as well as the transition metals at their active sites. Rheumatoid synovial fluid contains low levels of SOD activity and hence little protection from ROM generated by infiltrating PMNs (Blake etcU., 1981). Furthermore, leucocytes from patients with RA are deficient in MnSOD, which might promote the extracellular leakage of O2 (Pasquier et al., 1984). 

The floodwater and uppermost part of the soil are oxygenated by photosynthetic organisms, and the rhizosphere is oxygenated by leakage of O2 from plant roots. 

A GCE coated with a film of Prussian Blue (Fe4[Fe(CN)6]3) mimics HRP as catalyst for selective electrochemical reduction of H2O2 in the presence of O2. A careful deposition method has to be followed to prevent leakage of the oxidized form of the pigment from the coating into the solution. Amperometric determinations in phosphate buffer at pH 6.0, with an apphed potential of —50 mV vs. SCSE, shows Unearity in the 0.1 to 100 p,M range. ... 

Leaf discs have commonly been used for bioassays to determine if herbicides inhibit photosynthesis (Table 16.2). The simplest leaf-disc bioassay uses small discs cut from fully expanded cucumber or pumpkin cotyledons, floated in the light on a phosphate buffered medium containing suspected photosynthesis inhibitors.115 Qualitatively, if photosynthesis is inhibited, the leaf disc sinks. There are several variations of this method that can provide quantitative data. Evolution of O2 in the test solution can be measured with an oxygen electrode, CO2 induced pH changes colorimetrically determined with bromothymol-blue, or electrolyte leakage monitored with a conductivity meter. Leaf strips, algae, isolated chloroplasts, and duckweed (Lemna minor) have been used as test subjects. Although the bioassays presented in Table 16.2 are fairly easy to perform, few allelochemicals have been tested as possible inhibitors of photosynthesis. Many... 

Several heterodinuclear Fe-Cu or Co-Cu complexes that closely resemble the native enzyme active sites have recently been prepared to elucidate the catalytic mechanism of cytochrome c oxidases [231-245], The use of a covalently attached axial ligand seems essential to achieve efficient electroreduction of O2 to H2O [235, 238, 241-243], The closest structural analogs of the heme as/Cus active site of cytochrome c oxidases reported so far are Fe-Cu complexes (1 and 2) in which the Cu coordination site is provided by three imidazole ligands [242], These biomimetic model complexes afford clean electroreduction of O2 to H2O over a wide range of pH with no leakage of H2O2 [243],... 

Aerobic organisms produce minor fluxes of superoxide ion during respiration and oxidative metabolism. Thus, up to 15% of the O2 reduced by cytochrome-c oxidase and by xanthine oxidase passes through the HOO /O2 - state.70 The reductase of the latter system is a flavoprotein i that probably reduces O2 to HOOH via a redox cycle similar to that outlined by Eqs. (7-19) - (7-22). Thus, the observed flux of O2 -, which is the carrier of the auto-oxidation cycle, is due to leakage during turnover of xanthine/xanthine oxidase (see Scheme 7-14 for a reasonable mechanistic pathway). 

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