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9.3: Vitamin C

Vitamin C is well-known for being a water-soluble antioxidant. Humans are one of the few mammals that don't synthesize vitamin C, making it an essential micronutrient. Other mammals that don't synthesize vitamin C include primates, guinea pigs, and other less prevalent species1. Vitamin C's scientific names are ascorbic acid or ascorbate and the oxidized form is dehydroascorbic acid or dehydroascorbate. The structure of vitamin C is shown below.

Figure 9.31 Structure of ascorbic acid2

When ascorbic acid is oxidized, it forms semidehydroascorbate (1 degree of oxidation) and then dehydroascorbate (2 degrees of oxidation). The structure of dehydroascorbic acid is shown below.

Figure 9.32 Structure of dehydroascorbic acid3

The figure below shows the reaction through which ascorbic acid can stabilize or quench 2 free radicals. The 2 circled hydrogens are lost and replaced by double bonds when ascorbic acid is oxidized to dehydroascorbic acid. Reducing dehydroascorbic acid back to ascorbic acid is the opposite reaction.

Figure 9.33 The oxidation-reduction reaction between ascorbic acid (left) and dehydroascorbic acid (right)2,3

Ascorbic acid is believed to be a part of an antioxidant network (shown below) where it is oxidized to reduce alpha-tocopherol radicals. Dehydroascorbic acid can be reduced by thioredoxin reductase, a selenoenzyme, to regenerate ascorbic acid.

Figure 9.34 The theorized antioxidant network4

 References & Links

  1. Stipanuk MH. (2006) Biochemical, physiological, & molecular aspects of human nutrition. St. Louis, MO: Saunders Elsevier.
  4. Packer L, Weber SU, Rimbach G. (2001) Molecular aspects of alpha-tocotrienol antioxidant action and cell signalling. J Nutr 131(2): 369S-373S.