Dehydroascorbic biological activity

Because potatoes are good source of vitamin C, it is important to point out that irradiation does not adversely affect the vitamin C levels [23,30]. Although some ascorbic acid is converted into dehydroascorbic acid on irradiation, the latter is also biologically active. 

Since ascorbate reduces photooxidation of lipid emulsions and multivitamin preparations (see Figure 4) [19], Lavoie et al. [34] studied the formation of oxidative by-products of vitamin C in multivitamins exposed to light. They found that the loss of ascorbic acid in photoexposed multivitamin preparations was associated with the generation of products other than dehydroascorbate and 2,3-diketogulonic acid, which are the usual products of vitamin C oxidation. The authors showed that hydrogen peroxide at concentrations found in TPN solutions induced the transformation of dehydroascorbate into new, biologically active compounds that had the potential to affect lipid metabolism. They believe that these species have peroxide and aldehyde functions [35]. 

Both forms are biologically active. Ascorbic acid (the reduced form) is relatively stable to heat however, dehydroascorbic acid (the oxidized form) is unstable. The lactone ring is easily hydrolyzed to diketogulonic acid, which has no antiscurvy activity. When fruits, vegetables, and other foods are heated, there is some loss of active vitamin C by conversion to diketogulonic acid. 

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). 

The ease of oxidation of reduced ascorbic acid is the basis for a simple method of analysis by dye titration (58j. Ascorbic acid as it occurs in citrus juice is in the reduced form. When subjected to oxidation, ascorbic acid changes to the dehydro form. Dehydroascorbic acid has nearly the same physiological activity as the reduced form and is easily converted to the latter. Further oxidation of the dehydroascorbic acid converts it to 2,3-diketo-gulonic acid. This reaction is irreversible, and the oxidized product is devoid of biological activity. These reactions are shown in Figure 2. Nearly 90 percent or more of the vitamin C found in citrus juice and citrus products is in the reduced form (Table X) (59). 

Oxidation of L-ascorbic acid produces a variety of chemical species depending on the nature of the solvent, the time, and the strength of the oxidation agent. By careful, mild oxidation two hydrogen atoms are given off in successive steps. In the first step, an unstable radical semi-dehydroascorbic acid is formed. In the second, the biologically active dehydroascorbic acid results. 

Fluorometric Methods. One of the most specific methods for the determination of ascorbic acid and its biologically active oxidation product, dehydroascorbic acid, is the fluorometric method introduced by Deutsch and Weeks (43), which is an official AOAC method (44). It is... 

DHA can be reduced to RAA by chemical agents, such as hydrogen sulfide or enzymatically, by dehydroascorbic acid reductase. The conversion of DHA to diketogulonic acid (DKG) is irreversible and occurs both aerobically and anaerobically, particularly during heating. This reaction results in loss of biological activity. The total oxidation of RAA may result in the formation of furfural by decarboxylation and dehydration. With subsequent polymerization, the formation of dark-colored pigments results. These compounds affect the color and flavor of certain foods, such as citrus juices, and decrease nutritive value. 

Ascorbic acid (I) has an acidic hydroxyl group (pKi = 4.04, pK2 = 11.4 at 25 °C). Its UV absorption depends on the pH value (Table 6.9). Ascorbic acid is readily and reversibly oxidized to dehydroascorbic acid (II), which is present in aqueous media as a hydrated hemiketal (IV). The biological activity of II is possibly weaker than that of I because the plasma and tissue concentrations of II are considerably lower after the administration of equal amounts of I and II. The activity is completely lost when the dehydroascorbic acid lactone ring is irreversibly opened, converting II to 2,3-diketogulonic acid (III), cf. Formula 6.18 ... 

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. 

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. 

The vitamin C activity of L-ascorbic acid or reduced ascorbic acid (RAA) and its oxidized form, dehydroascorbic acid (DHA) is essentially the same, while D-ascorbic acid (isoascorbic acid or erythroascorbic acid) has little of the vitamin s biological potency (1). The readiness with which RAA is reversibly oxidized to DHA is the basis of its physiological activity, and of its use as an antioxidant in food systems. 

Dehydroascorbic acid hydrolysis yields biologically inactive 2,3-dioxogulonic (L-threo-hexo-2,3-diulosonic) acid. In aqueous solutions, this acid is present as a dihydrate (Figure 5.31). The reaction is generally acid-base catalysed. The activity of important ions and undissociated molecules decreases in the order hydroxyl ions (HO ) > hydronium ions (H3O+) > anions of carboxylic acids R-COO ) > undissociated carboxylic acids (R-COOH) > water. The catalytic effect of hydroxyl ions is about 15x10 times higher than that of hydronium ions. This means that the reaction rate in the solution of pH 4 is 15x 10 times lower than in solution of pH 10. Dehydroascorbic acid is most stable in solutions of pH 2.S-5.5, where the reaction is only catalysed by undissociated water molecules and is rapidly hydrolysed in neutral and alkaline solutions. 

L-Ascorbic acid (AA) and L-dehydroascorbic acid (DHAA) are the two biologically vitamin C active compounds AA is a strong reducing agent and DHAA is the... 

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