Ascorbic acid in plants

Methods for the preparation of L-ascorbic acids having isotopic C, H, O in various positions have been described and reviewed (104,105). Labeled L-ascorbic acid has played an important role in the elucidation of the metaboHc pathway of L-ascorbic acid in plants and animals. 

Biosyntheses of hexuronic acids and L-ascorbic acid in plants and animals are closely related. Hexuronic acids, L-ascorbic acid, and L-tartaric acid (a possible precursor of dihydroxyfumaric acid) commonly occur together in plants. If a rat is given chloretone (an antispasmodic), both L-ascorbic acid and D-glucuronic acid are excreted in increased quantity.244 Unlike humans, rats can synthesize their own vitamin C, and are therefore independent of outside sources. Here, D-glucose and D-galactose can be utilized, but not D-mannose. 

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. 

As a number of excellent articles have been published that review various aspects of the biological roles of L-ascorbic acid,8 the biosynthesis of L-ascorbic acid in plants and animals,7-11 and radicals derived from L-ascorbic acid,12 these topics will not be treated. Likewise, the methods by which L-ascorbic acid is assayed13 and the uses of L-ascorbic acid and related molecules in a wide variety of assays and oxidation-reduction systems will not be discussed. 

Loewus FA (1999) Biosynthesis and metabolism of ascorbic acid in plants and of analogs of ascorbic acid in fungi. Phytochemistry 52, 193-210. 

Compounds known to behave in this way in vivo are listed in recent reviews in this Series.1 2 The structures of some of the /3-D-glucopyranosiduronic acids isolated from urine have been proved by chemical synthesis.3 A few similar derivatives of flavones and triterpenes have been isolated from plants. D-Glucuronic acid also occurs in mammalian tissues as a constituent of acid mucopolysaccharides (aminodeoxypolysaccharides, containing uronic acid), such as hyaluronic acid, chondroitinsulfate, and heparin,4 and it is a direct precursor of L-ascorbic acid in plants and mammals.6 It is present in many of the plant polysaccharides classified as hemicelluloses6 and gums,7 and it has also been found in certain bacterial polysaccharides.4... 

SMIRNOFF, N., WHEELER, G.L., Ascorbic acid in plants biosynthesis and function, Crit. Rev. Biochem. Mol. Biol., 2000,35,291-314. 

Conklin, PL., 2001, Recent advances in the role and biosynthesis of ascorbic acid in plants. Plant Cell Environ. 24 383-394. 

Smirnoff, N., Conklin, P.L., and Loewus, F.A., 2001, Biosynthesis of ascorbic acid in plants A renaissance. Anna. Rev. Plant Physiol. Plant Mol. Biol. 52 437 167. 

Linster CL, Gomez TA, Christensen KC, Young BD, Brenner C. Arabidopsis VTC2 encodes a GDP-L-galactose phosphorylase, the last unknown enzyme in the Smirnoff-Wheeler pathway to ascorbic acid in plants. J. Biol. Chem. 2007 282 18879-18885. Davidson VL. Electron transfer in quinoproteins. Arch. Biochem. Biophys. 2004 428 32-40. 

Side-chain oxidized derivatives of ascorbic acid are also implicated in the catabolism of ascorbic acid in plants. Loewus et al. (62) have established the intermediacy of ascorbic acid in the biosynthesis of tartaric acid in the grape. Labeling studies have established a metabolic pathway that must involve C5 and C6 oxidation of ascorbic acid. 

Figure 2. Cleavage of i.-ascorbic acid in plants of the Geraniaceae. Oxalate accumulating plants also cleave i.-ascorbic acid between C2 and C3 but the fragment does not accumulate as tartrate.

Sunlight is not necessary for the synthesis of ascorbic acid in plants but is needed to produce optimal vitamin C concentrations. Photosynthesis produces the precursor hexoses needed for ascorbic acid synthesis (38,51),... 

The most important recent addition to our knowledge of ascorbic acid is the series of stepwise enzyme reactions by which it is made in animal tissues and through which it is related to the reactions of other carbohydrates in the body. The biosynthetic pathways of ascorbic acid in plants have been partially elucidated, but less successfully, since it appears that two different pathways may operate (L21). These important advances have been the subject of a number of reviews (M7, B38, B39, K5). 

M7. Mapson, L. W., The biosynthesis of L-ascorbic acid in plants and animals. Proc. Intern, Congr. Biochem. 4th Congr. Vienna 11, 1-16 (1960). 

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. 

Labeling studies indicate that in the conversion of D-glu-cose to L-ascorbic acid in plants, the carbon chain is conserved. Although the steps of biosynthesis are not clear, they involve oxidation at C-1, an internal oxidation of C-2 or C-3, and epimerization at C-5 of glucose. One possible sequence is as follows D-glucose is converted to D-glucosone, which compound to L-sorbosone, which compound to l-ascorbic acid (Loewus, 1988). 

Scheme 11.19. A representation of what is currently (2007) understood abont the pathway from D-glucose to L-ascorbic acid in plants. L-ascorbic acid is not biosynthesized by some mammals (including humans). EC nnmbers and some graphic materials provided in this scheme have been taken with permission from appropriate links in a URL starting with http // www.chem.qmul.ac.uk/iubmb/enzyme/.

The stability of ascorbic acid in plant products is very important in the food industry. Oxidation in milk is accelerated by copper and sunlight. Low-temperature storage of foods (below 42 F.) is helpful in preventing loss. 

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