Ascorbic biosynthesis

Fig. 12. Lactonase reactions involved in ascorbate biosynthesis and degradation of L-rhamnose and D-xylose...

Lorence, A., Chevone, B.I., Mendes, P., and Nessler, C.L., 2004, wyo-Inositol oxygenase offers a possible entry point into plant ascorbate biosynthesis. Plant Physiol. 134 1200-1205. 

Smirnoff N (2000) Ascorbate biosynthesis and function in photoprotection. Philosophical Transactions of the Royal Society of London B Biological Sciences 355,1455-64. 

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). 

Ishikawa, T. and Shigeoka, S. (2008) Recent advances in ascorbate biosynthesis and the physiological significance of ascorbate peroxidase in photosynthesizing organisms. Biosci. Biotechnol. Biochem. 72, 1143-1154... 

Laing, W.A. et al. (2007) The missing step of the L-galactose pathway of ascorbate biosynthesis in plants, an L-galactose guanyltrans-... 

Linster, C.L., Clarke, S.G., 2008. L-ascorbate biosynthesis in higher plants the role of VTC2. Trends in Plant Science... 

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). 

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. 8. Pathway for the biosynthesis of L-ascorbic acid in rats using C-l-labeled D-glucose indicates position of C.

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

Biochemical Functions. Ascorbic acid has various biochemical functions, involving, for example, coUagen synthesis, immune function, dmg metabohsm, folate metaboHsm, cholesterol cataboHsm, iron metaboHsm, and carnitine biosynthesis. Clear-cut evidence for its biochemical role is available only with respect to coUagen biosynthesis (hydroxylation of prolin and lysine). In addition, ascorbic acid can act as a reducing agent and as an effective antioxidant. Ascorbic acid also interferes with nitrosamine formation by reacting direcdy with nitrites, and consequently may potentially reduce cancer risk. 

Ascorbic acid is involved in carnitine biosynthesis. Carnitine (y-amino-P-hydroxybutyric acid, trimethylbetaine) (30) is a component of heart muscle, skeletal tissue, Uver and other tissues. It is involved in the transport of fatty acids into mitochondria, where they are oxidized to provide energy for the ceU and animal. It is synthesized in animals from lysine and methionine by two hydroxylases, both containing ferrous iron and L-ascorbic acid. Ascorbic acid donates electrons to the enzymes involved in the metabohsm of L-tyrosine, cholesterol, and histamine (128). 

L-Tyrosine metabohsm and catecholamine biosynthesis occur largely in the brain, central nervous tissue, and endocrine system, which have large pools of L-ascorbic acid (128). Catecholamine, a neurotransmitter, is the precursor in the formation of dopamine, which is converted to noradrenaline and adrenaline. The precise role of ascorbic acid has not been completely understood. Ascorbic acid has important biochemical functions with various hydroxylase enzymes in steroid, dmg, andhpid metabohsm. The cytochrome P-450 oxidase catalyzes the conversion of cholesterol to bUe acids and the detoxification process of aromatic dmgs and other xenobiotics, eg, carcinogens, poUutants, and pesticides, in the body (129). The effects of L-ascorbic acid on histamine metabohsm related to scurvy and anaphylactic shock have been investigated (130). Another ceUular reaction involving ascorbic acid is the conversion of folate to tetrahydrofolate. Ascorbic acid has many biochemical functions which affect the immune system of the body (131). 

By analogy with the biosynthesis of L-ascorbic acid and with other oxidation-reduction enzyme systems, it seems likely that D-galactose is oxidized to D-galacturonic acid (XXIII) on reduction at Cl, this yields L-galactonic acid (XXIV), the 1,5-lactone (XXV) of which, on reduction at Cl, would give L-galactose (XXII). 

Fig. 8(a) and (b).—Hypothetical Routes Suggested for the Biosynthesis of Hex-uronic Acid and l-Ascorbic Acid. 

Glucosyl-fructose, 24 237 D-Glucuronic acid, 4 711 Glucuronic pathway, of ascorbic acid biosynthesis, 25 762-763, 763-764 Glue, kaolin application, 6 688t Glueckauf approximation, adsorption, 1 608, 609... 

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