Ascorbic acid vitamin reducing agent

FIGURE 18.30 The physiological effects of ascorbic acid (vitamin C) are the result of its action as a reducing agent. A two-electron oxidation of ascorbic acid yields dehy-droascorbic acid. 

To enhance iron excretion, intensive chelation therapy is used. The most successful drug is desferrioxamine B, a powerful Fe3+-chelator produced by the microbe Streptomyces pilosus,6 The formation constant for the Fe(III) complex, called ferrioxamine B, is 103afi. Used in conjunction with ascorbic acid—vitamin C, a reducing agent that reduces Fe3+ to the more soluble Fe2+— desferrioxamine clears several grams of iron per year from an overloaded patient. The ferrioxamine complex is excreted in the urine. 

Because the pH inside a plant or animal cell is about 7, reduction potentials that apply at pH 0 are not particularly appropriate. For example, at pH 0, ascorbic acid (vitamin C) is a more powerful reducing agent than succinic acid. However, at pH 7, this order is reversed. It is the reducing strength at pH 7, not at pH 0, that is relevant to a living cell. 

Ascorbic acid (vitamin C fig. 10.16) is the reducing agent required to maintain the activity of a number of enzymes, most notably proline hydroxylase, which forms 4-hydroxyproline residues in collagen. Hydroxyproline (see fig. 10.16c) is not synthesized biologically as a free amino acid but rather is created by modification of proline residues already incorporated into collagen. The hydroxylation reaction occurs as the protein is synthesized in the endoplasmic reticulum. At least a third of the numerous proline residues in collagen are modified in this way, substantially increasing the resistance of the protein to thermal denaturation. 

Browning can be controlled by adding reducing agents or acidic solutions that inhibit the activity of the PPO enzyme. Solutions of sodium bisulfite, ascorbic acid (vitamin C), and lemon juice are commonly added to freshly cut fruit to retard browning. 

Table 8.9 lists the water-soluble vitamins —ascorbic acid (vitamin C) and a series known as the vitamin B complex (Figure 8.32). Ascorbate, the ionized form of ascorbic acid, serves as a reducing agent (an antioxidant), as will be discussed shortly. The vitamin B series comprises components of coenzymes. Note that, in all cases except vitamin C, the vitamin must be modified before it can serve its function. 

It has been known previously that substances which oxidize readily, such as ascorbic acid, are reduced in the aged 141) and that the use of antioxidants such as Vitamin E and selenium compoimds can inactivate free radicals 142). It also has been observed that agents which bind ultratrace metals, particularly protein-rich sulfahydryl groups such as cysteines and glutathione have the capability of neutralizing the eflFect of free radicals 140). 

Not surprisingly, the reducing agent ascorbic acid (vitamin C) has certain chemical features common with the reducing sugar glucose. Briefly, observations made on the effect of ascorbic acid supplementation in diabetic individuals, the prooxidant ability of ascorbic acid in vitro, and the ability of ascorbic acid to modify proteins are of interest in the context of the development of diabetic complications. 

Nonaqueous Systems In nonaqueous (nonpolar) solvent systems, nitrosatlon also proceeds. In these solvents, alpha-tocopherol acts as a lipid soluble blocking agent in much the same fashion as ascorbic acid functions in the aqueous phase. Alpha-tocopherol reacts with a nitrosating agent and reduces it to nitric oxide. At the same time, alpha-tocopherol is oxidized to tocoquinone, which is the first oxidation product of vitamin E and also a normal metabolite in vivo. 

Antioxidants Reducing agents, such as vitamins C (ascorbic acid) and E (a-tocopherol), which scavenge toxic free radicals generated by oxidative reactions in the cell. 

The active form of vitamin C is ascorbate acid (Figure 28.8). The main function of ascorbate is as a reducing agent in several different reac lions. Vitamin C has a well-documented role as a coenzyme in hydroxy lation reactions, for example, hydroxylation of prolyl- and lysyl-residues of collagen (see p. 47). Vitamin C is, therefore, required for the mainte nance of normal connective tissue, as well as for wound healing. Vitamin C also facilitates the absorption of dietary jron from the intestine. 

The fundamental role of ascorbic acid in metabolic processes is not well understood. There is some evidence that it may be involved in metabolic hydroxylation reactions of tyrosine, proline, and some steroid hormones, and in the cleavage-oxidation of homogentisic acid. Its function in these metabolic processes appears to be related to the ability of vitamin C to act as a reducing agent. 

Compounds that can scavenge radicals are also referred to as antioxidants. The best known anti-oxidants are vitamin C and vitamin E. Vitamin C is L-ascorbate (7.8), a good reducing agent that prevents oxidation of other molecules. The oxidized form of L-ascorbate is L-dehydroascorbic acid (7.9). Vitamin E is a mixture of a-, [3-, y-, and 8-tocopherol (7.10a-d). Of these four compounds, a-tocopherol is the most effective. Vitamin E is lipid-soluble and has the ability to disrupt the chain reaction during lipid peroxidation (see Chapter 2, Section 1.9). 

---