Ascorbic Acid Chemistry Metabolism and Functions

The term "vitamin C" encompasses L-[+]-ascorbic acid and its derivatives with identical levels of biological effects. Chemically, ascorbic acid is the enolic form of 3-oxo-L-gulofuranolac-tone (A). Plants and many animals can synthesize it from glucuronic acid. Since humans, apes, and guinea pigs lack the last enzyme in the enzyme pathway, L-gulonolactone oxidase, the vitamin is essential for them.

Vitamin C absorption begins at the buccal mucosa, but most of it is absorbed in the proximal small intestine. There are probably several active transport mechanisms. Passive diffusion requires very high concentrations in the intestinal lumen. The absorption rate at physiological doses is ~80%. With megadoses it may drop to 15%. In plasma, about three-fourths is found as free ascorbic acid, the remaining one-fourth is protein bound. Optimal plasma concentrations are estimated to be around 1 mg/l; classic deficiency symptoms occur around 0.2 mg/l. The kidneys are the main excretory organ, whereas after megadose supplementation (>3 g) increasingly large amounts are excreted through feces. Some of the biological effects of vitamin C can be explained by L-ascorbic acid's reductant properties. When L-ascorbic acid is oxidized to dehy-droascorbic acid, the extremely reactive intermediate semidehydro-L-ascorbate is formed. These three forms of vitamin C represent a reversible redox system. The oxidized form can be reduced to ascorbate by a reductase. Other redox systems like glutathione or tocopherol are involved in the latter enzyme's function. This allows ascorbic acid to play a hydrogen donor role in hydroxy-lation reactions, as, for instance, during biosynthesis of the catecholamines, noradrenaline, and adrenaline, where ascorbic acid functions as a cofactor for dopamine-p-mono oxygenase.

Other biological effects are based on different mechanisms, some of which are still unknown. For instance, ascorbic acid is involved in collagen biosynthesis. However, even though intracellular protein modification of pre-collagen occurs through hydroxylation of proline and lysine, ascorbic acid is not involved as a hydrogen donor here. Also, during degradation of tyrosine, activation of one of the enzymes occurs without involvement of the ascorbic acid/ dehydroascorbate redox system. Ascorbic acid is involved in the synthesis of bile acids from cholesterol and the synthesis of carnitine from the amino acids lysine and methionine. Neuroendocrine hormones such as gastrin, bombesin, CRH, and TRH are activated by ascorbic acid-dependent amidation. The synthesis of cytochrome P450 in liver microsomes, required for detoxification reactions, is stimulated by ascorbic acid. Its iron absorption enhancing function has been known for a long time just like its role as inhibitor of nitrosamine formation from nitrite and amines in the stomach. The competitive inhibition that vitamin C exerts on protein glycosylation may be particularly important for long-term prognosis for diabetics. For its antioxidant effects see p. 206.

- A. Chemistry and Function -

CHwOH Ascorbic acid

Cholesterol-7-hydroxyiase

Ascorbic Acid Function

ò Dopamin-ß-monooxygenase

Noradrenaline

Adrenaline

CHwOH Dehydro ascorbic acid

ò Dopamin-ß-monooxygenase

Noradrenaline

Adrenaline

CHwOH Ascorbic acid

CHwOH Dehydro ascorbic acid

Cholesterol-7-hydroxyiase

Catecholamines Synthesis Ascorbic

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Responses

  • yolanda
    What enzymes are involved in ascorbic acid absorption?
    6 years ago

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