The antioxidant vitamins C and E

Chapter 1
Vitamin C: An Introduction
Jane V. Higdon and Balz Frei
Linus Pauling Institute, Oregon State University, Corvallis, OR 97331
Structure and Chemistry of Vitamin C (L-Ascorbic Acid)
The chemical name for L-ascorbic acid is 2,3-didehydro-L-threo-hexano-1,4-lactone.
Carbon 5 of ascorbic acid (Fig. 1.1) is asymmetric, making two enantiomeric forms
possible; L-ascorbic acid is the naturally occurring and biologically active form.
L-Ascorbic acid is a water-soluble 6-carbon α-ketolactone with two enolic hydrogen
atoms (pK a1 at carbon 3 = 4.17 and pK a2 at carbon 2 = 11.57; Fig. 1.1) (1,2). At physiologic
pH, >99% of L-ascorbic acid is ionized to L-ascorbate, which can donate a
hydrogen atom (H + + e − ) to produce the resonance-stabilized ascorbyl radical (Fig.
1.1). The ascorbyl radical can donate a second electron to form the 2-electron oxidation
product of ascorbate, dehydroascorbic acid (DHA), or dismutate to form ascorbate
and DHA (Fig. 1.1). Alternatively, the ascorbyl radical may be enzymatically
reduced back to ascorbate by NADH-dependent semidehydroascorbate reductase or
the NADPH-dependent selenoenzyme, thioredoxin reductase. DHA can be reduced
back to ascorbate by the glutathione-dependent enzyme, glutaredoxin, or thioredoxin
reductase (3). If not recycled to ascorbate, DHA is irreversibly hydrolyzed to 2,3diketo-L-gulonic
acid (DKG), which does not function as an antioxidant. Further
degradation of DKG results in the formation of oxalic acid and L-threonic acid. Other
catabolites include, among many others, L-xylonic acid, L-lyxonic acid, and L-xylose
(4). The term vitamin C is generally used to describe all compounds that qualitatively
exhibit the biological activity of ascorbate, including ascorbate and DHA (2).
Fig. 1.1. Oxidation of ascorbate (AscH − ) by two successive one-electron oxidation steps
to give the ascorbyl radical (Asc •− ) and dehydroascorbic acid (DHA), respectively.
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