Plant ascorbic acid

Conklin PL, Norris SR, Wheeler GL et al (1999) Genetic evidence for the role of GDP-mannose in plant ascorbic acid (vitamin C) biosynthesis. Proc Natl Acad Sci USA 96 4198-4203... 

Ascorbic acid is oxidized by plants with ascorbate oxidase to yield dehydroascorbic acid (3,6-anhydro-L-Ay/o-hex-ulono-1,4-lactone hydrate) (76) (Loewus, 1980, 1988). In plants, ascorbic acid is rarely accumulated, but is usually converted into tartaric (77) and oxalic (78) acids (Fig. 15.14). 

Ascorbate and derived compounds are normal components of higher plants. Ascorbic acid is generated from glucose in the cytosol of actively growing plants by oxidation, epimerization, and new oxidation. Putative intermediates in the ascorbate biosynthesis pathway are D-glucosone and L-sorbosone (Loewus, 1988 Loewus et al., 1990 Saito et al., 1990) (see also Chapter 2). 

A much better case can be made for the possible function of GSH in the hydrogen transport systems of higher plants. Ascorbic-acid oxidase appears to be characteristically a plant enzyme. Phenol oxidases and peroxidase systems are also found in many higher plants. Such enzymes can cause an oxidation of ascorbate indirectly by virtue of the nonenzymatic oxidation of ascorbic acid by compounds of quinoid structure formed from the phenolic substrate (39). Such systems are not universally distributed in plants, but where they are present in sufficient amount it is not unreasonable to suppose that some substrates may be oxidized by way of a respiratory chain consisting of substrate, TPN, GSH, ascorbic acid, and a terminal oxidase. 

Conklin, P.L. etal. (2006) Arabidopsis thaliana VTC4 encodes L-galactose-l-P phosphatase, a plant ascorbic acid biosynthetic enzyme. J. Biol. Chem. 281,15662-15670... 

M.p. 190-192 C. The enolic form of 3-oxo-L-gulofuranolactone. It can be prepared by synthesis from glucose, or extracted from plant sources such as rose hips, blackcurrants or citrus fruits. Easily oxidized. It is essential for the formation of collagen and intercellular material, bone and teeth, and for the healing of wounds. It is used in the treatment of scurvy. Man is one of the few mammals unable to manufacture ascorbic acid in his liver. Used as a photographic developing agent in alkaline solution. 

Trichloroethanoic acid, CCI3COOH. A crystalline solid which rapidly absorbs water vapour m.p. 58°C, b.p. 196-5" C. Manufactured by the action of chlorine on ethanoic acid at 160°C in the presence of red phosphorus, sulphur or iodine. It is decomposed into chloroform and carbon dioxide by boiling water. It is a much stronger acid than either the mono- or the dichloro-acids and has been used to extract alkaloids and ascorbic acid from plant and animal tissues. It is a precipitant for proteins and may be used to test for the presence of albumin in urine. The sodium salt is used as a selective weedkiller. 

L-Ascorbic acid biosynthesis in plants and animals as well as the chemical synthesis starts from D-glucose. The vitamin and its main derivatives, sodium ascorbate, calcium ascorbate, and ascorbyl palmitate, are officially recognized by regulatory agencies and included in compendia such as the United S fates Pharmacopeia/National Formula (USP/NF) and the Food Chemicals Codex (FCC). 

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. 

In all plants and most animals, L-ascorbic acid is produced from D-glucose (4) and D-galactose (26). Ascorbic acid biosynthesis in animals starts with D-glucose (4). In plants, where the biosynthesis is more compHcated, there are two postulated biosynthetic pathways for the conversion of D-glucose or D-galactose to ascorbic acid. 

Fig. 10. Suggested pathway for the biosynthesis of L-ascorbic acid (with retention of configuration) in higher plants based on D-glucose-l- C...

L-Ascorbic acid, better known as vitamin C, has the simplest chemical structure of all the vitamins (Figure 18.30). It is widely distributed in the animal and plant kingdoms, and only a few vertebrates—humans and other primates, guinea pigs, fruit-eating bats, certain birds, and some fish (rainbow trout, carp, and Coho salmon, for example)—are unable to synthesize it. In all these organisms, the inability to synthesize ascorbic acid stems from a lack of a liver enzyme, L-gulono-y-lactone oxidase. 

Ascorbic acid is a reasonably strong reducing agent. The biochemical and physiological functions of ascorbic acid most likely derive from its reducing properties—it functions as an electron carrier. Loss of one electron due to interactions with oxygen or metal ions leads to semidehydro-L-ascorbate, a reactive free radical (Figure 18.30) that can be reduced back to L-ascorbic acid by various enzymes in animals and plants. A characteristic reaction of ascorbic acid is its oxidation to dehydro-L-aseorbie add. Ascorbic acid and dehydroascor-bic acid form an effective redox system. 

For wheat plants grown to maturity under irrigation (43) in a soil of neutral pH (44), one can calculate that a mature wheat plant (yield 13.2 g d m [grams of dry matter)] with a Mn concentration of 42 mg/kg took up 556 pg of Mn [i.e., (556/55) X 2 microequivalents of Mn], If ascorbic acid (M, a = 176), or another reducing agent of similar molecular weight and C content is assumed to be the... 

Several vitamins have been recognized in green coffee beans, as might be expected in a plant seed. A relatively high ascorbic acid content is associated with fine-grade coffee beans to be used as seeds.162 Vitamin E has already been mentioned as a component of coffee oil.114... 

Vasudeva, N., Gopal, N. H., Studies on ascorbic acid in coffee plants. II. Distribution in ripe fruits and its relation with coffee quality, J. Coffee Res., 4, 25, 1974. (CA85 59606k)... 

Alpha hydroxy acids (AHAs) are water-soluble substances and thereby penetrate the outermost epidermal skin layers. In contrast, beta hydroxy acids (BHAs) are lipid (fat) soluble and are capable of penetrating to the underlying layers of skin (the dermis) located 1-5 mm below the surface of the skinJ2 Most AHAs are derived from plant materials and marine sources. Commonly used AHAs include malic acid (found in apples), ascorbic acid (a common ingredient in numerous fruits), glycolic acid (a constituent of sugar cane), lactic acid (a component of milk), citric acid (naturally abundant in citrus fruits), and tartatic acid (found in red wine). A common BHA is salicylic acid (an ingredient in aspirin). 

Senesi N, Loffredo E (1999) The chemistry of soil organic matter. In Sparks DL (ed) Soil physical chemistry. CRC Press, Boca Raton, FL, pp 239-370 Sujatha TV, Hegde MJ (1998) C-mitotic effects of Trichloroethylene (TCE) on bone marrow cells of mice. Mutat Res 413 151-158 Varanini Z, Pinton R (2001) Direct versus indirect effects of soil humic substances on plant growth and nutrition. In Pinton R, Varanini Z, Nannipieri P (eds) The rizosphere. Marcel Dekker, Basel, pp 141-158 Vijayalaxmi KK, Venu R (1999) In vivo anticlastogenic effects of L-ascorbic acid in mice. Mutat Res 438 47-51... 

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. 

Phytochemicals present in fruits and vegetables are very diverse, such as ascorbic acid, carotenoids, and phenolic compounds (Liu 2004 Percival and others 2006 Syngletary and others 2005 Yahia and others 2001a, 2001b). Plant polyphenols are ubiquitous in the diet, with rich sources being tea, wine, fruits, and vegetables they demonstrate considerable antioxidative activity in vitro, which can have important implications for health (Duthie and others 2000). 

The situation with wholemeal flour is refreshingly simple. Flour treatments are banned and there are no statuary additions. The addition of ascorbic acid to wholemeal flour is forbidden but the use of ascorbic acid in wholemeal bread is allowed. Presumably, it was thought beneficial to allow the change so that the Chorleywood plants could make wholemeal bread. The ascorbic acid presumably goes in as an improver with other ingredients. 

---