Oxidation and Reduction of Ascorbate

Dehydroascorbate is unstable in solution, undergoing hydrolytic ring opening to yield diketogulonic acid. However, in vivo, it is normally reduced to 

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In 1932, Norman Haworth collaborated with Szent-Gyorgyi and demonstrated that ascorbic acid and vitamin C (see chapter 3) are the same substance. Karrer also confirmed the structure of vitamin C (see the structure below). Haworth demonstrated reversible oxidation and reduction of ascorbic acid and also proved that its unique 1,2-enediol structure is also responsible for its acidity. In 1933, both Haworth and Reichstein synthesized ascorbic acid, the first vitamin to be prepared by total artificial synthesis, demonstrating the simple chemical character of a vitamin. 

The complexity of the redox processes involving vitamin C have not prevented numerous studies of the oxidation of L-ascorbic acid and some on the oxidation and reduction of dehydroascorbic acid and other oxidation products. One of the reasons for the great interest in this subject is that the role of vitamin C in living systems is certainly connected to its oxidation-reduction behaviour and this may ultimately be the key to understanding the biological mechanisms of the actions of vitamin C. In this. section we will examine the oxidation and reduction of vitamin C by a wide variety of reagents, with emphasis on the behaviour of L-ascorbic acid since this is the most studied compound in this respect. 

B. Vennesland The title of my talk was Enzymatic Oxidation and Reduction of Glutathione. I find that the literature on the nonenzymatic oxidation is voluminous and, to me, confusing. This is why I made no attempt to cover it in this survey. I didn t mean to imply in any way that the nonenzymatic reactions might not be actually causing a faster rate of oxidation than the enzymatic ones. Certainly, this might be so except for those few tissues where you have a rapid enzymatic reaction with ascorbic acid and there are a few where this is rapid. Except for those I think it s quite possible that the nonenzymatic reactions might be responsible for a large proportion of the oxidation of GSH that occurs. 

Masking by oxidation or reduction of a metal ion to a state which does not react with EDTA is occasionally of value. For example, Fe(III) (log K- y 24.23) in acidic media may be reduced to Fe(II) (log K-yyy = 14.33) by ascorbic acid in this state iron does not interfere in the titration of some trivalent and tetravalent ions in strong acidic medium (pH 0 to 2). Similarly, Hg(II) can be reduced to the metal. In favorable conditions, Cr(III) may be oxidized by alkaline peroxide to chromate which does not complex with EDTA. 

Polypyrrole shows catalytic activity for the oxidation of ascorbic acid,221,222 catechols,221 and the quinone-hydroquinone couple 223 Polyaniline is active for the quinone-hydroquinone and Fe3+/Fe2+ couples,224,225 oxidation of hydrazine226 and formic acid,227 and reduction of nitric acid228 Poly(p-phenylene) is active for the oxidation of reduced nicotinamide adenine dinucleotide (NADH), catechol, ascorbic acid, acetaminophen, and p-aminophenol.229 Poly(3-methylthiophene) catalyzes the electrochemistry of a large number of neurotransmitters.230... 

Drugs can also Interfere with laboratory results by negating certain nonspecific oxidation and reduction reactions essential for the chemical assay. Penicillin, streptomycin and ascorbic acid are known to react with cupric Ion thus, false positive results for glucose may occur If a copper reduction method Is used. If the specific enzymatic glucose-oxidase method Is employed, ascorbic acid can cause a false negative result by preventing the oxidation of a specific chromogen In the reaction. 

Nitro groups can be reduced all the way to amines (Fig. 5.7). The first step requires an enzyme such as cytochrome P450 or anaerobic bacteria, but reduction of nitroso groups is so facile it is usually a simple chemical reduction mediated by biological reducing agents such as ascorbate or NADPH. Although the pathways are shown as two-electron oxidations and reductions, one-electron chemistry can also occur. 

In its biochemical functions, ascorbic acid acts as a regulator in tissue respiration and tends to serve as an antioxidant in vitro by reducing oxidizing chemicals. The effectiveness of ascorbic acid as an antioxidant when added to various processed food products, such as meats, is described in entry on Antioxidants. In plant tissues, the related glutathione system of oxidation and reduction is fairly widely distributed and there is evidence that election transfer reactions involving ascorbic acid are characteristic of animal systems. Peroxidase systems also may involve reactions with ascorbic acid In plants, either of two copper-protein enzymes are commonly involved in the oxidation of ascorbic acid. 

Further mechanistic studies showed that no free peroxynitrite is formed during the reactions of NO with the oxy-forms of these proteins, and that nitrate is formed quantitatively, at both pH 7.0 and pH 9.0 [18]. Analysis of the proteins after ten cycles of oxidation by NO and reduction by ascorbic acid indicated that fewer than 1% of the tyrosine residues are nitrated. These results show that when peroxynitrite is coordinated to the heme of myoglobin or hemoglobin, it rapidly iso-merizes to nitrate, and thus cannot nitrate the tyrosine residues of the globin. 

One example is the known interference by reducing compounds that affect the chemical conversion of substrate to a colored indicator. This is especially true for the tetrazolium assays (Ulukaya, Colakogullari, and Wood 2004 Chakrabarti et al. 2000 Pagliacci et al. 1993 Collier and Pritsos 2003). The growing list of interfering compounds includes ascorbic acid and sulfhydryl reagents such as glutathione, coenzyme A, dithiothreitol, etc. Similar interferences by compounds that affect the oxidation and reduction chemistry of cells are likely to cause artifacts with the resazurin reduction assay. Assays that measure markers of metabolism also can be influenced by the pH of the culture medium and other factors that may stimulate or stress the metabolic rates of cells. 

Luminal thiol oxidation is facilitated by ascorbate (vitamin C) (45) or FAD (12, 13), so the physiologic role of their transport has been proposed. ER membrane is permeable selectively to dehydroascorbate, the oxidized form of ascorbate (10, 11). Luminal reduction of dehydroascorbate to ascorbate is associated with thiol oxidation and leads to ascorbate entrapment (46). 

Figure 13.10. Schematic representation of the oxide dissolution processes [exemplified for Fe(III) (hydr)oxides] by acids (H ions), ligands (example oxalate), and reductants (example ascorbate). In each case a surface complex (proton complex, oxalato and ascorbato surface complex) is formed, which influences the bonds of the central Fe ions to O and OH on the surface of the crystalline lattice, in such a way that a slow detachment of a Fe(III) aquo or a ligand complex [in case of reduction an Fe(ll) complex] becomes possible. In each case the original surface structure is reconstituted, so that the dissolution continues (steady-state condition). In the redox reaction with Fe(III), the ascorbate is oxidized to the ascorbate radical A . The principle of proton-promoted and ligand-promoted dissolution is also valid for the dissolution (weathering) of Al-silicate minerals. The structural formulas given are schematic and simplified they should indicate that Fe(III) in the solid phase can be bridged by O and OH.

A number of enzymes have been characterized that catalyze reactions involving DHA. In addition, other aspects of DHA biochemistry can be deduced from metabolic studies of ascorbic acid. Experiments demonstrating the biological oxidation of AA and reduction of DHA were first made in 1928 (10) and during the next decade several groups studied these reactions. By 1941 Crook (62) was able to separate the ascorbic acid oxidase and DHA reductase activities and to show that glutathione was used in the reductase reaction. 

A preformed support may be impregnated with this HAuCU and then treated with a reducing agent. Alternatively, when the pH of a mixture of HAuCU and various metal ions is raised, gold hydroxide and metal hydroxide coprecipitate. Drying, calcination, and reduction of this mixture provide Au(0) supported upon a metal oxide. Suitable reducing agents include H2, BH4, citrate, and ascorbate. Permutations of... 

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