Ascorbic acid vitamin functions

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. 

A corollary of the oxidation hypothesis of atherogenesis is that antioxidants may reduce the progression of the disease (114). Antioxidants present in LDL, including alpha-tocopherol, and antioxidants present in the extracellular fluid of the arterial wall, including ascorbic acid (vitamin C), inhibit LDL oxidation (132), and this action is extended to multiple oxLDL-mediated signaling pathways (133). Vitamin C may potentiate NO activity and normalize vascular function in patients with CHD and classical risk factors (132). Thus, NO may restore endothelial dysfunction and ameliorate vascular remodeling in several clinical correlates to experimental... 

Ascorbic acid (vitamin C) also functions as an antioxidant and is added to medicines and foodstuffs for this reason. Food manufacturers enthusiastically label their products as having added vitamin C What they are not so keen to tell you is that the vitamin is not there for the consumers benefit but rather as an antioxidant to stop their product decomposing oxidatively (see Figure 8.14). 

The next step in the catecholamine biosynthesis is side-chain hydroxylation of DA to NE. The enzyme dopamine (3-hydroxylase (DBH) catalyzes this reaction. This enzyme, like TH, is a mixed-function oxidase utilizing molecular 02, in this case to add the OH onto the (3-carbon of the phenelthylamine side chain. DBH is a Cu2+-containing enzyme that, with ascorbic acid (Vitamin C) as a cofactor, carries out the necessary electron transfers. 

Neurons that secrete norepinephrine synthesize it from dopamine in a hydroxylation reaction catalyzed by dopamine (3-hydroxylase (DBH). This enzyme is present only within the storage vesicles of these cells. Like tyrosine hydroxylase, it is a mixed-function oxidase that requires an electron donor. Ascorbic acid (vitamin C) serves as the electron donor and is oxidized in the reaction. Copper (Cu ) is a bound cofactor required for the electron transfer. 

Recent studies in biological systems have revealed the presence of a variety of compounds of many structural types that appear to function as antioxidants. The plant kingdom is especially rich in these materials (Larson, 1988). They include flavonoids and other phenolic compounds, alkaloids and other amines, reduced sulfur compounds, uric acid, ascorbic acid. Vitamin E, carotenoids, and many other substances. Often, their principal mechanism of action appears to be the quenching of peroxy radicals, removing them from the autooxidation chain. Vitamin E, for example, reacts with these radicals in a manner entirely analogous to that of BHT ... 

Vitamins are any organic dietary substance necessary for life, health, and growth that do not function by supplying energy. They usually function as coenzymes. Vitamins for one species may not be vitamins for another. Only humans, monkeys, and guinea pigs lack the ability to synthesize ascorbic acid (vitamin C), for instance. Indeed, bacteria in the gut synthesize some essential vitamins, which are absorbed in amounts sufficient to meet daily requirements. The administration of antibiotics for a long period of time could thus result in a vitamin deficiency of the bacterial host. 

The proper functioning of the mind is known to require the presence in the brain of molecules of many different substances. For example, mental disease, usually associated also with physical disezise, results from a low concentration in the brain of thiamine (Vitamin B,), nicotinic acid or nicotinamide (B,), pyridoxine (BJ, cyanoco-balamin (6 ), biotin (H), and ascorbic acid (Vitamin C). 

Most media contain water-soluble B vitamins. Common to many formulations are vitamins Bi (thiamine), B2 (riboflavin), B3 (niacinamide), Bj (pantothenic acid). Be (pyridoxine), and Bg (folic acid). Biotin (vitamin H), cyanocobalamin (vitamin B]2 ), and ascorbic acid (vitamin C) are also common vitamin components. Although choline and inositol are classically grouped with vitamin components, in cell culture they function as metabolic substrates rather than as catalysts. 

L-ascorbic acid (vitamin C) Antioxidant function and promote coUagen synthesis... 

The bis-enol, L-ascorbic acid (vitamin C) (Scheme 8.13), is easily oxidized. Therefore, in addition to its function as a vitamin (vide infra), L-ascorbic acid and related materials (derivatives of L-ascorbic acid) are often used as antioxidants, ... 

Tomato (Solarium lycopersicwn L.) is extensively cultivated worldwide, and its fruits have assumed the status of functional foods as a result of epidemiological evidence of reduced risks of certain types of cancers and cardiovascular diseases [180,181]. They are a reservoir of diverse antioxidant molecules, such as lycopene, ascorbic acid, vitamin E, carotenoids, flavraioids, and phenolics, and may provide a significant part of the total intake of beneficial phytochemicals, as a result of then-high consumption rates. Among carotenoids, lycopene has a strong antioxidant activity and is able to induce cell-to-ceU communications and modulate hormones, immune systems, and other metabolic pathways [182]. 

In vitro studies indicate that folic acid may be converted to citrovorum factor by liver slices and that this conversion is aided by ascorbic acid. Vitamin B12 may also have a role in the conversion of folic acid to citrovorum factor, and both vitamin B12 and ascorbic acid may stimulate synthesis of folic acid in the body. Relationships between vitamin B12 and folic acid are complex both function in hematopoiesis, probably in nucleoprotein synthesis, and in the metabolism of labile methyl groups and choline. 

Vitamin C occurs as L-ascorbic acid and dihydroascorbic acid in fruits, vegetables and potatoes, as well as in processed foods to which it has been added as an antioxidant. The only wholly undisputed function of vitamin C is the prevention of scurvy. Although this is the physiological rationale for the currently recommended intake levels, there is growing evidence that vitamin C may provide additional protective effects against other diseases including cancer, and the recommended dietary allowance (RDA) may be increased in the near future. Scurvy develops in adults whose habitual intake of vitamin C falls below 1 mg/d, and under experimental conditions 10 mg/d is sufficient to prevent or alleviate symptoms (Bartley et al., 1953). The RDA is 60 mg per day in the USA, but plasma levels of ascorbate do not achieve saturation until daily intakes reach around 100 mg (Bates et al., 1979). Most of the ascorbate in human diets is derived from natural sources, and consumers who eat five portions, or about 400-500 g, of fruits and vegetables per day could obtain as much as 200 mg of ascorbate. 

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. 

As has been explained in previous chapters, the antioxidant capacity of fruits and vegetables is a function of the amounts and types of phytochemicals that are present in the fresh tissues. However, the individual contribution to the total antioxidant capacity varies widely. Various studies have demonstrated that phenols and flavonoids contribute to a higher extent than ascorbic acid, carotenoids, and others to the antioxidant capacity of fmits and vegetables (Robles-Sanchez and others 2007). It has been observed that a given content of vitamin E in fruits contributes significantly more to the antioxidant capacity than the same content of ascorbic acid. 

Physiological functions, of vitamins, 25 784 Physiology, of ascorbic acid, 25 766-773 Physisorbed water, 23 71-72 Physostigmine, 2 817-818 Phytane, 18 592... 

Inhibitors of nitrosation generally function by competing with the amine for the nitrosating agent. An inhibitor would thus react with nitrite at a faster rate than with amines. The inhibition reaction has recently been reviewed ( 35). The ability of ascorbate to act as a potent inhibitor of nitrosamine formation has resulted in the use of the vitamin in nitrite-preserved foods and pharmaceuticals. Furthermore, the effectiveness of ascorbate in inhibiting nitrosamine formation is dependent on (1) the concentration of ascorbate (an excess is required) (2) pH (ascorbate is nitrosated 240 times more rapidly than ascorbic acid) (3) the reactivity of the amine toward nitrosation and (4) the extent of oxygenation of the system. 

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