Nebraska Redox Biology Center Educational Portal

Ascorbic Acid or Vitamin C

Ascorbic acid or Vitamin C is essential nutrient that involved in wide range of biological processes as antioxidant. Ascorbic acid is synthesized by most mammals hovewer, humans lack the terminal enzyme, gulonolactone oxidase, in biosynthesis pathway and dependant on dietary intake for ascorbate [ 1, 2, 3 ].

Ascorbic Acid or Vitamin C.

Serum concentrations of ascorbic acid is regulated by kidney by filteration and reabsorbtion. Ascorbic acid is proposed to be transported across cellular membranes most likely by two different mechanisms. (I) Ascorbic acid may be easily oxidized to the dehydroascorbic acid which is rapidly transported to the cells and reduced to ascorbic acid by glutathione system F [ 4, 5 ]. Two specific glucose transporters, GLUT1 and GLUT3, mediate dehydroascorbic acid transport (II) Ascorbic acid may be transported into the cell by sodium-dependent ascorbic acid transporters SVCT1 and SVCT2 [ 6, 7 ]. It is unclear which transport system provides a major contribution in ascorbic acid transport in vivo. Recommended Dietary Allowances for is 40 mg/day for 0-6 month old, 25 mg/day for 4-8 year old, 45 mg/day for 9-13 year old and 75-90 mg/day for 19+ year old. Acute vitamin C deficiency leads to scurvy. Ascorbic acid has low toxicity and is cause serious adverse effects at high intake. Tolerable Upper Intake Levels 400 mg/day for 1-3 year old, 1200 mg/day for 9-13 year old and 2000 mg/day for 19+ year old [ 8, 9, 10 ]. Fruits and vegetables are the best sources of vitamin C Citrus fruits, tomatoes and potatoes are major contributors of ascorbic in tipical diet. Other good food sources include red and green peppers, kiwifruit, broccoli, strawberries, Brussels sprouts, and cantaloupe. Ascorbic acid is not naturally present in grains [ 8, 11 ].

Ascorbic acid exist in wide range of form depending on pH and its oxidation state. 99.95% of vitamin C is present as ascorbate monoanion (AscH-). Ascorbate releases one electron in reactions with reactive radicals or lipid hydroperoxides rusulting in formation of much less reactive ascorbyl radicals. Such ascorbyl radicals are not stable and are converting to ascorbate and dehydroascorbate in disproportional reaction. Dehydroascorbate is unstable and degradates resulting in oxalic and L-threonic acid [ 2, 11, 12 ].

The equilibrium and redox species in the ascorbic acid-dehydroascorbic acid system. AscH2 (ascorbic acid), AscH- (ascorbate monoanion), Asc2- (ascorbate dianion), AscH. (ascorbyl radical), Asc.-(ascorbate radical), DHA (dehydroascorbate), DHAA(1) and DHAA(2) (dehydroascorbate hydrolyzed forms) [ 3 ].

In addition to antioxidant role of ascorbic acid, this compound may work as prooxidand in case of oversupplementation. Ascorbic acid may contribute to DNA damage, lipids and proteins oxidation by promoting of reactive oxygen species formation in the presence of traces of transition metal ions [ 14, 15, 16 ].

ASCH- + O2 → ASCH.- + O2.- ;
ASCH- + O2.- → ASCH.- + H2O2 ;
ASCH- + Fe3+ → ASCH.- + Fe2+ ;
Fe2+ + H2O2 → Fe3+ + OH.- + OH- ;

Ascorbic acid biosynthesis in animals starts from UDP-glucuronate wich is converted to d-glucuronate, then reduced to l-gulonate, which leads to inversion of the numbering of the carbon chain since the aldehyde function of d-glucuronate becomes a hydroxymethyl group in the resulting l-gulonate. The latter is converted to its lactone, which is oxidized to l-ascorbate by l-gulonolactone oxidase. In plants, the pathway starts with GDP-d-mannose, which is converted to l-galactonolactone, the substrate for l-galactonolactone dehydrogenase [ 17, 18, 19, 20 ].

Ascorbic acid biosynthesis pathway in mouse liver [ 20 ].

Ascorbic acid supplementation showed positive results in numerous health conditions, acting primarily as anantioxidant. Ascorbic acid rich diets helps in prevention against cardiovascular disease and cancer. Ascorbic acid supplements are helpfull in wound healing. It has effect on preventing plasma lipid peroxidation, and decreasing serum fibrinogen and cholesterol values as risk factors for cardiovascular disease. However, some positive effects could be achieved only by intraarterial or intravenous administration of ascorbic acid. In addition, adverse health effects of ascorbic acid are not induced in healthy persons by ingesting high doses supplementation [ 3, 21, 22, 23, 24, 25, 26 ].

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