Ascorbic acid vitamin metabolic functions

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. 

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. 

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. 

Since ascorbate reduces photooxidation of lipid emulsions and multivitamin preparations (see Figure 4) [19], Lavoie et al. [34] studied the formation of oxidative by-products of vitamin C in multivitamins exposed to light. They found that the loss of ascorbic acid in photoexposed multivitamin preparations was associated with the generation of products other than dehydroascorbate and 2,3-diketogulonic acid, which are the usual products of vitamin C oxidation. The authors showed that hydrogen peroxide at concentrations found in TPN solutions induced the transformation of dehydroascorbate into new, biologically active compounds that had the potential to affect lipid metabolism. They believe that these species have peroxide and aldehyde functions [35]. 

Vitamins are chemically unrelated organic compounds that cannot be synthesized by humans and, therefore, must must be supplied by the diet. Nine vitamins (folic acid, cobalamin, ascorbic acid, pyridoxine, thiamine, niacin, riboflavin, biotin, and pantothenic acid) are classified as water-soluble, whereas four vitamins (vitamins A, D, K, and E) are termed fat-soluble (Figure 28.1). Vitamins are required to perform specific cellular functions, for example, many of the water-soluble vitamins are precursors of coenzymes for the enzymes of intermediary metabolism. In contrast to the water-soluble vitamins, only one fat soluble vitamin (vitamin K) has a coenzyme function. These vitamins are released, absorbed, and transported with the fat of the diet. They are not readily excreted in the urine, and significant quantities are stored in Die liver and adipose tissue. In fact, consumption of vitamins A and D in exoess of the recommended dietary allowances can lead to accumulation of toxic quantities of these compounds. 

Physiological Function. The mechanism by which L-ascorbic acid benefits an insect is unknown. The vitamin is found in many tissues where it probably plays a variety of roles related to its redox potential. Besides the possible general function of detoxifying superoxide and hydrogen peroxide, L-ascorbic acid may be involved in metabolic processes such as tyrosine metabolism, collagen formation, steroid synthesis, detoxification reactions, phagostimulation, or neuromodulation. At this time one can only speculate about the function of vitamin C in some specific tissues. 

Ascorbic acid performs important metabolic functions, as evidenced by the severe manifestations of its deficiency in humans. This vitamin is involved in metabolic hydroxyla-lions in numerous important mclaholic processes (e.g.. the synthesis of. steroids and of neurotransmiticrs and in collagen and drug metabolism). Ascorbic acid has also been implicated as important in other critical oxidation-reduction pro-ccsse,s in human metabolism. ... 

Most vitamins function either as a hormone/ chemical messenger (cholecalciferol), structural component in some metabolic process (pantothenic acid), or a coenzyme (phytonadi-one, thiamine, riboflavin, niacin, pyridoxine, biotin, folic acid, cyanocobalamin). At least one vitamin has more than one biochemical role. Vitamin A as an aldehyde (retinal) is a structural component of the visual pigment rhodopsin and, in its acid form (retinoic acid), is a regulator of cell differentiation. The precise biochemical functions of ascorbic acid and a-tocopherol still are not well defined. 

Ascorbic Acid A six carbon compound related to glucose. It is found naturally in dtrus fruits and many vegetables. Ascorbic acid is an essential nutrient in human diets, and necessary to maintain connective tissue and bone. Its biologically active form, vitamin C, functions as a redudng agent and coenzyme in several metabolic pathways. Vitamin C is... 

Ascorbic acid occurs in all the tissues of living organisms where it is responsible for the normal functioning of important metabolic processes. Vegetable foodstuffs (oranges, blackcurrants, parsley, green peppers, etc.) are the richest sources of vitamin C, but animal products contain relatively little ascorbic acid, l-Ascorbic acid is a six carbon weak acid with a pA", of 4.2, which is reversibly oxidized due to its enediol structure with the loss of an electron to form the free radical semihy-droascorbic acid. ... 

Ascorbic acid has, so far, resisted the advances of the teleologists, none of whom has yet been able to convince a sufficient body of opinion that it has any particular purpose or function in metabolism. It has therefore stimulated more enquiry than perhaps any other vitamin into what it does in the body, in default of any easy answer to the question, what is it /or In this way it has proved a useful stimulus to the scientific, inductive study of many bodily processes. It has been observed in an extraordinary number of experimental situations partly because it is usually supposed that any technician can measure it simply and accurately. Let us hope that no one will too soon ve it a function — unless it is the one here suggested, of stimulating further thought and experiment. 

One has only to think of the extraordinarily varied metabolic functions of thiamine, riboflavin, pantothenic acid, pyridoxine, and biotin to realize that it is most unlikely that ascorbic acid could possibly replace every one of these. Moreover, one would have to postulate a quite different mechanism for the large number of other substances, such as sorbitol, sorbose, arabitol, and starch, which spare B vitamins even more readily than ascorbic acid, but which do not have its redox properties. 

Vitamin E is often recommended and used in the therapy of AD [207,208] however, results of clinical studies are controversial [209]. In dietetics, there is the idea that a combination of extra- and intracellular antioxidant (ascorbic acid -I- vitamin E) provides adequate antioxidant status of the cells however, it seems that it does not meet at least in part. The interest in the field of toco-pherols moves to tocotrienols increasingly [210]. In addition, certain epoxides of tocotrienols proved as an effective elicitors of internal biosynthesis of coenzyme Q [211], which can enhance protection of mitochondrial metabolism. Reduced mitochondrial function is in terms of current views the first factor in the development of neurodegenerative diseases [212]. 

Metabolic sparing of ascorbic acid by flavonoids. The fourth possible mechanism for the flavonoid-induced increases in tissue ascorbic acid is that flavonoids, by taking over some of the biological functions of the acid, thereby reduce the metabolic demand for the vitamin and permit its increased accumulation in the tissues. The inability of flavonoids to prolong the life of scorbutic guinea-pigs [43,46] implies that if substitution occurs then it must be in an area other than that normally associated with the development of scurvy — namely, the defective hydroxylation of the collagen molecule precursor. A possible corollary of this is that ascorbic acid is a multifunctional vitamin, with functional substitution by flavonoids possible only in certain metabolic areas. Such partial substitution is not unknown in nutrition the partial replacement in animals of the vitamin E function of tocopherols by antioxidants is a case in point. 

Water-Soluble Vitamins. Vitamin G (ascorbic acid) functions in the formation of collagen, wound healing, metabolic functions, and other roles. Foods high in vitamin G include citrus fruits, strawberries, cantaloupe, and cruciferous vegetables. B vitamins are important in energy metabolism. Thiamin (Bj) is called the antineuritic vitamin. Riboflavin (B ), rarely deficient in the diet, is found most abundantly in milk and dairy products. Niacin (Bj) is prevalent in meats, poultry, fish, peanut butter, and other foods. Other major B vitamins include folic acid (B ), B, and Bj2-... 

Although ascorbic acid has been shown to have many biologic functions, little is known of its chemical role in metabolic processes. Recently, Sealock and Goodland and Knox and LeMay-Knox have demonstrated that ascorbic acid is a coenzyme in the oxidation of tyrosine. This is the only section of the vitamin s activity in which the molecular processes that depend on it are clearly understood (Chapter 11). This explains the abnormal excretion of tyrosine metabolites in human scurvy and the return to... 

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