BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 8. VITAMINS

8.3. Water-soluble vitamins

8.3.8. Vitamin C

Structure and physicochemical properties. L-Ascorbic acid is a γ-lactone of 2,3-dehydrogulonic acid. It undergoes rapid and reversible oxidation to dehydroascorbic acid, forming a redox couple. Ascorbic acid is readily soluble in Water and sparingly soluble in ethanol. Its aqueous solutions have an acidic taste; a 0.1 N solution has a pH of 2.2. Dehydroascorbic acid is a neutral substance. The vitamin is easily oxidized by atmospheric oxygen, particularly in the presence of Cu²⁺, and is sensitive to high temperatures. Under such conditions, it irreversibly decomposes into two inactive compounds (Fig. 8.21).

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Fig. 8.21. Oxidation and degradation of ascorbic acid:

A - L-ascorbic acid; B - L-dehydroascorbic acid;

C - L-diketogulonic acid; D - threonic acid; E - oxalic acid

Biochemical Functions. As a strong reducing agent, ascorbic acid participates in numerous hydroxylation reactions. Hydroxylase Enzymes require an additional reduced cosubstrate for their catalytic activity, with Vitamin C being most commonly utilized for this purpose. This applies, in particular, to all α-ketoglutarate-dependent hydroxylases. Two such enzymes are involved in the hydroxylation of Lysine and Proline residues within the procollagen molecule. Free proline and lysine do not serve as substrates for the respective enzymes; instead, hydroxylation occurs on sufficiently long, yet uncoiled, protein chains. Unhydroxylated Collagen fails to form structurally normal fibers. Ascorbic acid is essential for humans, primates, guinea pigs, and certain other species because these organisms lack gulonolactone oxidase (EC 1.1.3.8), the enzyme that catalyzes the final step of glucose conversion into ascorbate. Ascorbic acid is capable of reversible oxidation to dehydroascorbic acid, establishing a redox system. It also participates in the synthesis of noradrenaline from dopamine via dopamine β-hydroxylase. This reaction takes place in Brain Neurons and the Adrenal Glands.

Ascorbic acid is also classified among antioxidant Vitamins.

Nutritional requirement. The daily requirement for ascorbic acid is 60 mg, which is unusually high compared to most other vitamins. Dietary sources for humans include fresh fruits, greens, and various vegetables. High concentrations of the vitamin are found in rose hips, black currants, various types of cabbage, citrus fruits, and potatoes. It is absent in dry cereal grains and legumes. Canned fruits and vegetables contain only negligible amounts of the vitamin. Vitamin deficiency (avitaminosis) manifests after several months as scurvy. Specialized Cells lose their ability to synthesize bone collagen and deposit Extracellular matrix, and glycoprotein-proteoglycan formation is impaired. The resulting condition leads to Connective Tissue atrophy, hematopoietic disorders, and tooth loss. Affected individuals experience weight loss, fatigue, and cardiac pain.