Dehydro ascorbic acid

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. 

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. 

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. 

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. 

Two reactions used in steroid chemistry were modified by Bennett for histochemical use. Frozen sections of either unfixed or formalin-fixed tissue were used, with no differences reported in their reactivity (see, however, Section V.2) the sections were 80 to 100 microns in thickness. In the first method, sections were treated with phenylhydrazine hydrochloride (1 %) in acetate buffer, pH 6 to 6.5, overnight. The formation of yellow phenylhydrazones indicated the presence of carbonyl groups. The pH of the solution was kept low enough to prevent extensive accumulation of the decomposition products of phenylhydrazine, which are yellow and soluble in lipid. In order to avoid reaction with ascorbic acid the sections were first oxidized briefly with iodine or indophenol. Since dehydro-ascorbic acid, which is formed by the oxidation of ascorbate, also forms phenylhydrazones, it is doubtful that this procedure had any value. However, since ascorbic acid and its oxidation product are soluble in most aqueous mixtures, they probably would not remain in sections as ordinarily treated. 

First, their total ascorbic acid (49) with both reduced ascorbic acid and dehydro ascorbic acid, and reduced ascorbic acid increased after both then-supplementations of one gold kiwifruit/day or tw o gold kiwifieits/day when compared to those [total ascorbic acid (49) and reduced ascorbic acid] of before. 

Phenylhydrazine (Rgt. No. 205) is a specific reagent for dehydro-ascorbic acid it yields orange red spots with diketogulonic acid also. The reaction with o-phenylenediamine (Rgt. No. 203) appears to be even more characteristic in yielding an intense blue fluorescence. Ascorbigen can be identified on the layer as free ascorbic acid after acid hydrolysis by spraying with 30% hydrochloric acid and drying at ca. 95° C. 

L-Ascorbic acid is very easily oxidized to L-dehydro-ascorbic acid, a diketone. Both L-ascorbic acid and... 

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