Ascorbic acid biological functions

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]. 

A number of copper -containing protein compounds are enzymes with an oxidase function (ascorbic acid oxidase, urease, etc 1 and these play an important role in Ihe biological oxidation-reduction system. There is a definite relationship of copper with iron in connection with utilization of iron in hemoglobin function. 

Although many in vitro studies on the antioxidative property of food constituents have been reported, little is known about the biological functions of dietary antioxidants in vivo, except for several well-known antioxidants such as tocopherols, (i-carotene, and ascorbic acid. Because the bioavailability of food constituents is limited by their digestibility and metabolic fate, an oral administration trial of a dietary antioxidant is favored to evaluate its biological function. 

SmirnoffN (2000) Ascorbic acid metabolism and functions ofamulti-facettedmolecule. Current Opinion Plant Biology 3,229-35. 

Ascorbic acid (1) has many biological functions besides its well-known antioxidant activity. It is a functional component of many pharmaceutical and cosmetic preparations. Antioxidants such as glutathione (45) or sodium metabisulfite (Na2S205) are added... 

Some itamirLS are water soluble, while others are fat soluble. This classification is valuable as it indicates whether the vitamin is likely to be absorbed similarly to lipids or like other water-soluble nutrients. The fat-soluble vitamins are A, D, E, and K. The water-soluble vitamins arc ascorbic acid, biotin, folate, niacin, pantothenic acid, riboflavin, thiamin, vitamin B i, and vitamin B 2. The classification is also valuable, as it helps chemists decide on the best way to extract and analyze a particular vitamin in foods and biological tissues. Aside from having some bearing on the path ways of absorption and distribution throughout the body, the question of whether a particular vitamin is fat soluble or water soluble has little or no relevance to its function in the body. 

Oxidation of the reductone functionality of ascorbic acid is certainly its single most important reaction and results in the formation of its most biologically important derivative, dehydroascorbic acid, 28. As chemistry and biochemistry of dehydroascorbic acid will be covered in a separate section of this volume, only a few of its reactions will be covered here. 

Metabolic Roles. Ascorbic acid is an electron donor required for a variety of oxidative processes. It is readily regenerated by glutathione, NAD, and NADP and thus has a long biological half-life. Currently, there are eight known human enzymes that require ascorbic acid, and they are listed in Table 8.5. The precise metabolic roles have not been completely elucidated, but it appears that in the met-alloenzymes, ascorbate reduces the active metal site. In addition to these specific enzymes, ascorbic acid seems to function as a free-radical scavenger in the aqueous phase of plasma and cells. 

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