Also known as
Vitamin C or
ascorbic acid, ascorbate serves as an
antioxidant and
redox factor for certain
hydroxylating enzymes. Most
mammals can synthesize ascorbate from
glucose in the
liver, but this capacity has been lost by higher
primates (including humans) and
guinea pigs. Thus, ascorbate is required in the diet of these animals. Its antioxidant properties derive from the high reducing potential of the
carbon-carbon double bond in its five-membered
lactone ring.
It undergoes one-electron oxidation to become ascorbyl radical; a further one-electron transfer yields dehydroascorbate (DHA). These oxidation steps are reversible. DHA in erythrocytes can be reduced to ascorbate by two-electron transfer from NADPH or reduced glutathione (GSH). Enzymes that require ascorbate are prolyl hydroxylase, lysyl hydroxylase and dopamine b-hydroxylase. The first two enzymes are critical in the production of collagens. Without ascorbate, collagen is insufficently hydroxylated, and this abnormal collagen accounts for the the symptoms of scurvy, such as skin lesions and blood-vessel fragility.