Ascorbic acid vitamin dehydroascorbate

Because of the time and expense involved, biological assays are used primarily for research purposes. The first chemical method for assaying L-ascorbic acid was the titration with 2,6-dichlorophenolindophenol solution (76). This method is not appHcable in the presence of a variety of interfering substances, eg, reduced metal ions, sulfites, tannins, or colored dyes. This 2,6-dichlorophenolindophenol method and other chemical and physiochemical methods are based on the reducing character of L-ascorbic acid (77). Colorimetric reactions with metal ions as weU as other redox systems, eg, potassium hexacyanoferrate(III), methylene blue, chloramine, etc, have been used for the assay, but they are unspecific because of interferences from a large number of reducing substances contained in foods and natural products (78). These methods have been used extensively in fish research (79). A specific photometric method for the assay of vitamin C in biological samples is based on the oxidation of ascorbic acid to dehydroascorbic acid with 2,4-dinitrophenylhydrazine (80). In the microfluorometric method, ascorbic acid is oxidized to dehydroascorbic acid in the presence of charcoal. The oxidized form is reacted with o-phenylenediamine to produce a fluorescent compound that is detected with an excitation maximum of ca 350 nm and an emission maximum of ca 430 nm (81). 

Vitamin C (Figure 45-19) is a vitamin for human beings and other primates, the guinea pig, bats, passerine birds, and most fishes and invertebrates other animals synthesize it as an intermediate in the uronic acid pathway of glucose metabohsm (Chapter 20). In those species for which it is a vitamin, there is a block in that pathway due to absence of gulonolactone oxidase. Both ascorbic acid and dehydroascorbic acid have vitamin activity. 

Under stress conditions, such as cutting or light exposure, ascorbate oxidase has been described as promoting the transformation of ascorbic acid to dehydroascorbic acid (Wright and Kader 1997b). However, because ascorbic acid can be easily converted into dehydroascorbic acid, it is necessary to measure both ascorbic and dehydroascorbic acids to observe that the content of vitamin C was well preserved in fresh-cut fruit. 

However, its presence is not the only determinant of whether or not oxidative deterioration occurs. Olson and Brown (1942) showed that washed cream (free of ascorbic acid) from susceptible milk did not develop an oxidized flavor when contaminated with copper and stored for three days. Subsequently, the addition of ascorbic acid to washed cream, even in the absence of added copper, was observed to promote the development of an oxidized flavor (Pont 1952). Krukovsky and Guthrie (1945) and Krukovsky (1961) reported that 0.1 ppm added copper did not promote oxidative flavors in milk or butter depleted of their Vitamin C content by quick and complete oxidation of ascorbic acid to dehydroascorbic acid. Krukovsky (1955) and Krukovsky and Guthrie (1945) further showed that the oxidative reaction in ascorbic acid-free milk could be initiated by the addition of ascorbic acid to such milk. Accordingly, these workers and others have concluded that ascorbic acid is an essential link in a chain of reactions resulting in the development of an oxidized flavor in fluid milk. 

Ascorbic acid and dehydroascorbic acid have equivalent biological activities, whereas isoascor-bic acid has only 5% of the biological activity of AA (15,17,19). The absorption and metabolism of vitamin C were recently reviewed (17,20,21). 

Interaction of ascorbic acid (vitamin C) with the silica surface was complicated by the fast oxidation of ascorbic acid in aqueous and ethanol solutions because of dissolved oxygen.12,13 However, both unmodified and modified silica increase the oxidation resistance of vitamin C. In particular, the rate of ascorbic acid oxidation to dehydroascorbic acid was found to be much less in the presence of unmodified or modified silica (Figure 4). Vitamin C is stabilized in the presence of silica, apparently due to interaction of the vitamin with the surface of highly-disperse silica particles, as confirmed by the results... 

Figure 8.34. Forms of Ascorbic Acid (Vitamin C). Ascorbate is the ionized form of vitamin C, and dehydroascorbic acid is the oxidized form of ascorbate.

We have used the growth effects and pathologies associated with L-ascorbic acid deficiency as a basis for the determination of the biological potency of related compounds (Table I). At a dietary concentration of 0.5 mM, L-ascorbic acid and dehydroascorbic acid were fully active, as well as some ester derivatives including the 6-myristate and 2-phosphate compounds. The insect may be metabolically like the guinea pig because both were able to utilize those esters (17), Carboxylesterases and phosphatases probably converted those derivatives to the free vitamin (18). The 6-bromo compound was less active and apparently cannot be metabolized to L-ascorbic acid or only poorly so. 

Research Needs. Over the years L-ascorbic acid has been shown to be an essential nutrient for many insects including species of Lepidoptera, Orthoptera, Coleoptera, and Diptera. Others such as cockroaches, houseflies, and mealworms are reared on simple diets without added ascorbic acid. Perhaps those insects require very low levels of vitamin C in their diets. A sensitive analytical method is needed to measure levels of L-ascorbic acid and dehydroascorbic acid in insect tissue and food. Such a method, which is likely to be developed using HPLC with electrochemical detection, could be used to monitor vitamin C levels in feed ingredients as well as in tissues during an insect s life cycle. This information is needed to determine whether ascorbic acid is used to... 

Vitamin C equals ascorbic acid plus dehydroascorbic acid. 

In recent years more utilities have begun to use ascorbic acid (vitamin C) for dechlorination (23). Vitamin C has long been used in the medical field for dechlorination of tap water prior to use for kidney dialysis treatment. Vitamin C reacts with chlorine to produce chloride and dehydroascorbate. The reactions with chlorine and chloramine are shown below ... 

With adequate intake of vitamin C, plasma concentrations of total vitamin (ascorbic acid plus dehydroascorbic acid) are between 0.4 and 1.5mg/dL (23 to 85 jimol/L). The lower limit value may be seen in some cases with subclinical vitamin C deficiency and in older individuals. A value lower than 0.2mg/dL (llpmol/L) is considered deficient. The guidance reference interval for vitamin C levels in leukocytes is 20 to 53p,g/10 leukocytes (1.14 to 3.01 finol/leukocyte). A value in leukocytes of less than lOpg/10 leukocytes (0.57fmol/leukocyte) is considered deficient. ... 

Oxidation is a common problem associated with some organic acids, for example, ascorbic acid (vitamin C). When dissolved, ascorbic acid is readily oxidized to dehydroascorbic acid, catalyzed by air or light exposure (Wu et al., 1995). Ascorbic acid solutions should be freshly prepared and kept tightly closed and not exposed to light. Organic acids also promote lipid oxidation which increases with increasing acid concentration (Ogden et al., 1995). 

Scheme 8.13. A representation of the oxidation of L-ascorbic acid (vitamin C) to L-dehydroascorbic acid and then to diketogulonic acid.

Ascorbic acid in blood is transported by a reversible complex with serum albumin (Moloy et al., 1980). The main transport form of vitamin C seems to be the reduced molecule. The concentration of dehydroascorbic acid can, however, exceed that of ascorbic acid in vitamin C deficiency and certain diseases (Stone, 1977). The relation of ascorbic acid/dehydroascorbic acid in human plasma decreases also with age (Sasaki et al., 1983). Since the typical screening methods for the determination of vitamin C status in blood are not able to distinguish between the different circulating forms of vitamin C, it has to be clearly stated that the detailed interpretation of vitamin C status and its relation to certain disorders must reflect on both ascorbic acid and dehydroascorbic acid concentration in blood. 

Behrens, W. A., and Madere, R., 1989, Ascorbic acid and dehydroascorbic acid status in rats fed diets varying in vitamin E levels, Int. J. Vitam. Nutr. Res. 59 360-364. 

The structure of vitamin C is shown in Figure 11.27 both ascorbic acid and dehydroascorbic acid have vitamin activity. 

Actually, several chemical compounds have vitamin C activity. So, it is now recommended that the term vitamin C be used as the combined name of all compounds having the biological activ ty of ascoibicadd, and that the temtis ascorbic acid and dehydroascorbic acid be used only when specific reference to them is made. 

The methods of Roe and Kuether (21) and of Roe and Oesterling (22) for the determination of vitamin C do not differentiate between ascorbic acid (AsA), dehydroascorbic acid (DHA), and diketogulonic acid (DKA). The same basic principle is applied to the determination of the three substances (23), the differentiation being made through the use of H2S which reduces DHA to AsA and does not reduce DKA under the conditions used. In analyzing an extract which contains all three forms of vitamin C, one aliquot is reduced with H2S, a second aliquot is left untreated, and a third aliquot is treated with bromine. After coupling with 2,4-dinitrophenyl-hydrazine the derivative in the first aliquot is that of DKA, the one in the second aliquot is that of DHA and DKA, and the one in the third aliquot is that of AsA, DHA, and DKA. Appropriate subtraction of the values obtained by colorimetric measurements will give the value for each component. 

Contains 1-4.5% vitamin C (ascorbic acid) and dehydroascorbic acid, mainly the former, in edible portion of fruit (cf. 0.05% in peeled orange), which makes up about 80% of the fruit. Vitamin C content varies with ripeness of the fruit (highest in green and lowest in fully ripe fruit), season, climate, and locality. 

Absorption, Transport, and Excretion. The vitamin is absorbed through the mouth, the stomach, and predominantly through the distal portion of the small intestine, and hence, penetrates into the bloodstream. Ascorbic acid is widely distributed to the cells of the body and is mainly present in the white blood cells (leukocytes). The ascorbic acid concentration in these cells is about 150 times its concentration in the plasma (150,151). Dehydroascorbic acid is the main form in the red blood cells (erythrocytes). White blood cells are involved in the destmction of bacteria. 

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