Diketogulonic acid formation

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]. 

The concentration of ascorbic acid in milk (11.2-17.2mgl-1) is sufficient to influence its redox potential. In freshly drawn milk, all ascorbic acid is in the reduced form but can be oxidized reversibly to dehydroascorbate, which is present as a hydrated hemiketal in aqueous systems. Hydrolysis of the lactone ring of dehydroascorbate, which results in the formation of 2,3-diketogulonic acid, is irreversible (Figure 11.2). 

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. 

The formation of phenylhydrazones and osazones from dehydroas-corbic acid is the basis of the most commonly used method of ascorbic acid analysis. The reactions are described in the section on determinations. There is no great specificity to this reaction, which is given by most compounds containing keto groups. However, most phenylhydrazones become intensely colored only in alkaline solution, while the dehy-droascorbic acid type of compound is intensely colored in acid solution. Both dehydroascorhic acid and 2,3-diketogulonic acid are converted to the same osazone under the conditions used. 

Another enzyme which will cause loss of ascorbic acid in plant material is phenolase. This is the substance which helps to produce the browning offruit such as apples when polyphenolic species are oxidised by oxygen from the air. The enzyme functions with oxygen and ascorbic acid and reduces or/Ao-quinones back to orfAo-diphenols. This results in the formation of dehydroascorbic acid, which is rapidly converted to 2,3-diketogulonic acid. The process is catalysed by copper(ii) and other transition metal ions, which therefore accelerate the loss of ascorbic acid from vegetables and fruit, when for example they are cooked in copper or iron containers. Of course the major factor in the removal of vitamin C from cooked vegetables is simple dissolution in the cooking water. 

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