Dehydro-ascorbate

A chemical reaction subsequent to a fast (reversible) electrode reaction (Eq. 5.6.1, case b) can consume the product of the electrode reaction, whose concentration in solution thus decreases. This decreases the overpotential of the overall electrode process. This mechanism was proposed by R. Brdicka and D. H. M. Kern for the oxidation of ascorbic acid, converted by a fast electrode reaction at the mercury electrode to form dehydro-ascorbic acid. An equilibrium described by the Nernst equation is established at the electrode between the initial substance and this intermediate product. Dehydroascorbic acid is then deactivated by a fast chemical reaction with water to form diketogulonic acid, which is electroinactive. 

The scavenging of the radical HA (in the case of ascorbate). HA reduces relatively fast (in a non-rate determining step) another Fe(III) the product is dehydro-ascorbate. The rate expression for the production of dissolved iron(II) at a given pH is written as follows ... 

Ascorbic acid oxidase (MW = 1,40,000 8 Cu). It is widely distributed in plants and micro-organisms. It catalyses oxidation of ascorbic acid (vitamin C) to dehydro ascorbic acid. 

Ascorbic acid is readily oxidized to dehydro-ascorbic acid (Box 18-D Fig. 20-2, step e), which may be hydrolyzed to L-bisoxogulonic acid (step/). The latter, after decarboxylation and reduction, is converted to L-xylulose (steps g and h), a compound that can also be formed by a standard oxidation and decarboxylation sequence on L-gulonic acid (step z). Reduction of xylulose to xylitol and oxidation of the latter with NAD+ (steps j and k) produces D-xylulose, which can... 

The red pigment, obtained from the interaction of amino acids and dehydro-ascorbic acid and shown to have structure 31 (p. 54) by Kurata et al.,194 provides another model chromophore for Maillard reaction products. 

PDI disulfide bond formation and isomerization, folding dehydro ascorb ate reduction dehydro ascorb ate, GSH 10,11,22,23, 44,46,54... 

Derivatives of Dehydroascorbic Monomers. Monomeric dehydro-ascorbic acid presumably is formed only in solvents that prevent the formation of dimers, for example, water and alcohols. The unstable evasive nature of the compound impedes its crystallization, and intractable syrups are often the result. However, derivatives of the monomer could be obtained with satisfactory crystalline quality for a regular x-ray study. 

Research on ascorbate and dehydroascorbate can be performed by conventional biochemical methods, but study of the relatively short-lived ascorbate free radical requires methods such as flow techniques with rapid mixing or pulse radiolysis coupled with polarography, ESR, or spectrophotometry. Ascorbate free radicals have been generated preferentially by oxidation of ascorbic acid [enzymatic (14-17), chemical (18-20), radiation chemical (21-26), and photochemical (27)] because ascorbic acid is easily available in a high purity grade but dehydro-ascorbic acid is not. 

The oxidative product of ascorbic acid, dehydroascorbic acid, is the preferred form of the vitamin for uptake by neutrophils, erythrocytes, and lymphocytes (27). Once within the erythrocyte, dehydroascorbic acid is reduced to ascorbic acid by a glutathione-dependent, dehydro-ascorbic-acid-reducing enzyme (20,28). However, the reduced form of ascorbic acid is found in most other tissues, that is, liver, lungs, kidneys, skin, and pituitary and adrenal glands (20,29). From these studies, ascorbic acid is taken up by several tissues by an energy-dependent and Na -sensitive process, but the transport of the oxidized vitamin form follows the principles of diflFusion. 

P. Pollet and S. Gelin, Tetronic acids and derivatives. VII. Structure of dehydro-ascorbic acid osazone and related compounds, Tetrahedron, 36 (1980) 2955-2959. 

Ascorbic acid derivatives inhibit ascorbate-2-sulfate sulfohydro, dehydro-ascorbic acid. 

The reduction of the keto groups to enol groups occurs with many reducing compounds. Of particular interest are the various sulfhydryl compounds (H2S, GSH, homocysteine used to convert dehydroascorhic acid to ascorbic acid for analysis by an oxidation method. Similar compounds can potentially reduce dehydroascorhic acid in biological systems. The redox potential of GSH is now believed to be considerably lower than was once thought (Eo at pH 7, 30° C = —0.32) (B28), considerably below that of ascorbic acid (-(-0.058). This in part accounts for the inefficiency of sulfhydryl reductions of dehydroascorhic acid. Mapson (M6) estimates that Ae half-time of reduction of dehydro-ascorbic acid by GSH is about 15 minutes under physiological conditions of pH, temperature and concentrations. 

The use of ascorbic acid oxidase to convert ascorbic acid to dehydro-ascorbic acid is commonly used as a qualitative test in biochemical ex-... 

There must also be a second mechanism of ascorbic acid transport through certain specialized cells, difierent from the diffusion of dehydro-ascorbic acid followed by intracellular reduction. There are no indications that dehydroascorbic acid is involved in the renal tubular absorption of ascorbic acid, and, contrary to earlier conclusions, it appears that the concentration of ascorbic acid into the aqueous humors of the eye occurs in the form of ascorbic acid itself. 

This system cannot now be fitted into known pathways of electron transport, principally because a system that oxidizes ascorbic acid in animal tissues is unknown. The fact is clear, however, that ascorbic acid is oxidized to dehydroascorbic acid in animal tissues and that dehydro-ascorbic add can be reduced. At the very least, this system emphasizes the potential eflBciency of a form of ascorbic acid as an electron acceptor (K3). 

L17. Linkswiler, H., The effect of the ingestion of ascorbic acid and dehydro-ascorbic acid upon the blood levels of these two components in human subjects. /. Nutrition 64, 43-54 (1958). 

T5. Tewari, C, P., and Kri.shnan, P. S., Enzymatic transformation of dehydro-ascorbic acid to diketogulonic acid. Nature 188, 144 (1960). 

Fox and Levy33 point out that, in some cases, activated carbon can be employed to accomplish a mild oxidation unaccompanied by undesirable by-products thus ascorbic acid is oxidized to dehydro-ascorbic acid. Schwob34 found that uracil is completely oxidized by hydrogen peroxide in the absence of carbon, but in the presence... 

HA = Ascorbate ion DMN = 1, 5 - dimethoxy naphthalene A = Dehydro ascorbic acid... 

Dehydro-ascorbic acid osazones can be rearranged by ring opening of the lactone which upon cyclization gave the respective pyrazolones which can be converted into different functionalized derivatives [49,50,56-60]. 

Monsalve, G.A., Powers, J.A., and Leung, H.K. Browning of dehydro ascorbic acid and chlorogenic acid as a function of water activity, J. Food ScL, 55, 1425, 1990. 

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