Ascorbyl radical

In a study directed to the analysis of the role of Fe and the generation of H2O2 in Escherichia coli (McCormick et al. 1998), hydroxyl radicals were specihcally trapped by reaction with ethanol to give the a-hydroxyethyl radical. This formed a stable adduct with a-(4-pyridyl-l-oxide)-iV-t-butyl nitroxide that was not formed either by superoxide or hydroxyl radicals. The role of redox-reactive iron is to use EPR to analyze the EPR-detectable ascorbyl radicals. 

It has been proposed that the a-tocopheroxyl radical can be recycled back to tocopherol by ascorbate producing the ascorbyl radical (Packer etal., 1979 Scarpa et al., 1984). The location of a-tocopherol, with its phytyl tail in the membrane parallel to the fatty acyl chains of the phospholipids and its phenolic hydroxyl group at the memisrane-water interface near the polar headgroups of the phospholipid bilayer, enables ascorbate to donate hydrogen atoms to the tocopheroxyl radical. The suitability for ascorbate and tocopherol as chain-breaking antioxidants is exemplified (Buettner,... 

In 1986, the antioxidant effects of thioredoxin reductase were studied by Schallreuter et al. [81]. It has been shown that thioredoxin reductase was contained in the plasma membrane surface of human keratinocytes where it provided skin protection against free radical mediated damage. Later on, the reductive activity of Trx/thioredoxin reductase system has been shown for the reduction of ascorbyl radical to ascorbate [82], the redox regulation of NFkB factor [83], and in the regulation of nitric oxide-nitric oxide synthase activities [84,85],... 

Vitamin C (ascorbate) (Fig. 9.5) has the ability to act as a reducing agent, i.e. it will tend to reduce more reactive species. This ability to reduce Fe3+ to Fe2+may be important in promoting iron uptake in the gut. Oxidation of ascorbate by reaction with reactive oxygen species or reactive nitrogen species seems to lead to its depletion. In vitro, vitamin C can also exert pro-oxidant properties. Fe3+ can react with ascorbate to form Fe2+ and the semi-dehydroascorbate or ascorbyl radical. The latter can react with hydrogen peroxide to form Fe3+, the hydroxyl radical and a hydroxide anion. A key question with regard to the pro- or anti- oxidant effects of ascorbate may therefore be the availability of transition metal ions. Neurons main-... 

Masumizu T., Noda Y., Mori A., and Packer L. (2005). Electron spin resonance assay of ascorbyl radical generation in mouse hippocampal slices during and after kainate-induced seizures. Brain Res. Brain Res. Protoc. 16 65-69. 

Ascorbate (AH-) can make two e available, and consequently both AH- and its one e oxidation product, the ascorbyl radical (A -), become antioxidants. The latter dismutates to form A - and dehydroascorbicadd (A), as shown in Scheme 3, or is reduced back by GSH or GSH-dependent enzymes (glutaredoxine, thioredoxin). Immediately, A is irreversibly hydrolized to 2,3-diketogluconic acid and then to oxalate, thre-onate, and many other metabolites. This last point is important because products derived from the hydrolysis of A may potentially damage proteins by glycation (47). 

Besides these enzyme substrates, a number of biological molecules are likely to give rise to fairly stable and hence observable free radicals. The more important of these are the quinonoid molecules, particularly vitamin Q quinone (ubiquinone), vitamin E quinone, vitamins K, Ks and vitamin E quinone, the flavins and flavoproteins and the important neurochemicals dopa, dopamine, and closely related phenolic and quinonoid molecules. In many of these cases, the generation of free radicals from these molecules should occur in vivo, but as yet only a few radicals such as the ascorbyl radical and the bacteriochlorophyll radical have been directly identified in intact systems. Free radicals from melanins (polymers from dopaquinone) have been demonstrated both in vivo and in vitro, but these radicals are so stable that it has not yet been possible to identify a biological role for the radicals per se. 

Ramakrishna Rao, D. N., Fischer, V., Mason, R. R Glutathione and ascorbate reduction of the acetaminophen radical formed by peroxidase. Detection of the glutathione disulfide radical anion and the ascorbyl radical. J. Biol. Chem. 1990, 265, 844—847. 

Ascorbic acid (-i-traces of transition metals) Ascorbyl radical -tO2"... 

Ascorbic acid is a strong reducing agent and is able to reduce Fe to Fe-. This might be important for the role of ascorbic acid in iron uptake in the intestine. Several nitroso-compounds are also reduced by ascorbic acid. This contributes to detoxifying reactions against these substances. These reactions produce the ascorbyl radical, an unstable product. The metal-catalyzed oxidation (in vivo most likely iron- or copper-mediated) of ascorbic acid produces reactive oxygen species [11]. 

L-Ascorbate, a water-soluble molecule, protects membranes from oxidative damage by regenerating a-tocopherol from a-tocopheroxyl radical. The ascorbyl radical formed in this process is reconverted to L-ascorbate during a reaction with GSH. 

The second reason that vitamin C is used as an electron donor is that the reaction product is fairly stable and unreactive. When vitamin C gives up an electron, it becomes a free radical called the ascorbyl radical. By free-radical standards, the ascorbyl radical is not very reactive. Its structure is stabilized by electron delocalization — the resonance effect first described by Linus Pauling in the late 1920s. This means that vitamin C can block free-radical chain reactions by donating an electron, while the reaction product, the ascorbyl radical, does not perpetuate the chain reaction itself. 

Despite its slow reactivity, the ascorbyl radical usually gives up a second electron to produce dehydroascorbate. This molecule is unstable and needs to be caught quickly if it is not to break down spontaneously and irrevocably, and be lost from the body. The continual seeping loss of vitamin C in this way accounts for our need to replenish body pools by daily intake. Even so, we can minimize losses by recycling dehydroascorbate. Several different enzymes bind dehydroascorbate to regenerate vitamin C. These enzymes usually take two electrons from a small peptide called... 

When a free radical reacts, it usually snatches an electron from the reactant, turning it into a free radical. This in turn will steal a single electron from another nearby molecule. A chain reaction ensues until two free radicals react together, effectively neutralizing each other, or alternatively, until an unreactive free-radical product is formed. Free radicals are said to be quenched by vitamin C, because the free-radical product — the ascorbyl radical — is so unreactive. As a result, free-radical chain reactions are terminated. Lipid-soluble vitamin E (a-tocopherol) works in the same way, in membranes rather than in solution, often in cooperation with vitamin C at the interface between membranes and the cytosol (the watery ground substance of the cytoplasm that surrounds the intracellular organelles). When vitamin E reacts with a free radical, it too produces a poorly reactive (resonance-stabilized) free-radical product, called the a-tocopheryl radical. Tocopheryl radicals can be reconverted into vitamin E using electrons from vitamin C. 

This new strategy consists of the synthesis of molecules that possess hydroxyl groups in such positions that a radical species can be stabilized by mesomery. This feature is found in natural flavonol such as kaemferol that possesses undeniable antioxidant properties. The first published example is a lipophilic analogue of vitamin C, Fig (13) [40]. In this paper, we have proved that the synthesised substituted 2-hydroxyfuran-2-one is a true ascorbic acid analogue. A radical anion that gives very similar data has been generated under the same conditions as for ascorbic acid with a stability somewhat lower and a redox potential lower than those of ascorbyl radical. Its antioxidant properties are also similar to that of ascorbic acid but it inhibits LDL peroxidation induced by Cu2+ or AAPH more efficiently probably due to a higher lipophilicity. 

Nitric oxide has recently been shown to react very rapidly with organic hydroperoxyl radicals (Padmaja and Huie, 1993). This rapid radical-radical addition could account for the ability of nitric oxide to inhibit lipid peroxidation (Rubbo et al., 1995). Given the relatively high lipid solubility of nitric oxide, it could readily partition into membranes, where it would be sequestered from reactive species such as superoxide and remain for longer periods to act as a chain terminator of radical-mediated lipid peroxidation. Virtually any radical species formed in or near the lipid bilayer could react with nitric oxide (e.g., tocopheryl or ascorbyl radicals) to give nitrosated intermediates or products which could in turn act as nitric oxide reservoirs. 

Oxidation of ascorbate by two successive one-electron oxidation steps results in the formation of the ascorbyl radical and dehydroascorbic acid, respectively. 

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