Ascorbic acid function

The mechanism, then, by which ascorbic acid functions to block these reactions Is one of competitive kinetics with the susceptible amine for the nltrosatlng agent. Hence, the reactivity of any given amine will be an Important parameter. 

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. 

Vitamin C (ascorbic acid) functions as an antioxidant and as a cofactor for hydroxyla tion reactions in procollagen. A deficiency of vitamin C results in scurvy, a disease char acterized by sore, spongy gums, loose teeth, and poor wound healing. There is no known toxicity for this vitamin. 

Much of the work on model systems was stimulated by the observation of Udenfriend and co-workers in 19546S4a,b that a mixture of Fe(II), EDTA, ascorbic acid, and molecular oxygen could hydroxylate arenes to phenols under mild conditions. Udenfriend s reagent also hydroxylates alkanes to alcohols and epoxidizes olefins.670 6 74 The EDTA in Udenfriend s reagent probably reduces the redox potential of the Fe(II)/Fe(III) couple. The ascorbic acid functions as an electron donor, analogous to the cofactor in monooxygenases, and can be replaced by other enediols.672... 

The striking increases in the formation of tritiated water and tritiated hydroxyproline on in vitro addition of ascorbate are consistent with a function of this vitamin in hydroxylation—probably at step 3. The present results do not support a systemic ascorbic acid-mediated effect, the belief that ascorbic acid functions in the maintenance of collagen, or acts by stimulating maturation of the fibroblasts in the system under study here. The present data do not support the possibility that intermediates containing hydroxyproline accumulate in scurvy. The proposal that ascorbic acid is involved in the hydroxylation reaction itself is consistent with studies on the nonenzymatic hydroxylation of proline (4) and on enzymatic hydroxylation of other compounds (5, 20, 21, 44, 51). 

The co-substrate for the mono-oxygenase is tetrahydrobiopterin (THB) and again it is likely that ascorbic acid functions to restore this substrate from the oxidised dihydrobiopterin (DHB) as in Figure 5.17. 

Moreover, an NADH-dependent semidehy-droascorbate reductase is thought to be involved in the regeneration or restoration of ascorbate (Diliberto et al, 1982 Chow, 1988). Under certain circumstances, ascorbic acid functions as a prooxidant rather than an antioxidant. Similarly to superoxide, ascorbate is able to reduce Fe to Fe +, and in the presence of H2O2, it can promote (HO) production. In vitro concentrations of ascorbate up to 0.2 mM can induce LPO in rat liver mi-crosomes (Samuni et al, 1983 Shinar et al, 1983). 

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-... 

The mechanism of inactivation of PPO in the presence of ascorbic acid is shown in Fig. 6 The mechanism of the effect of ascorbic acid on PPO is controversial Some workers reported that ascorbic acid inactivated the enzyme (Baruah and Swain, 1953 Ponting, 1954 Mlhalyi and Vamos-Vigyazo, 1976) while other workers reported activation (Krueger, 1950). Ingraham (1956) and Scharf and Dawson (1958) reported no effect of ascorbic acid on PPO activity More recently, Markakls and Embs (1966) and Padron et al (1975) reported no direct effect of ascorbic acid on the enzymatic activity We believe ascorbic acid functions primarily... 

In addition to its antioxidant role, ascorbic acid functions to keep various metallic ions in catalytic centers in their reduced forms. For example, some oxygenases require iron or copper in their Fe + or Cu+ states of oxidation. If these protein-bound ions are accidentally left in a more oxidized state they may need to be reduced by ascorbate ions. While this is a protectant role, there are some enzymes for which ascorbate has become a cosubstrate. An example is dopamine -hydroxylase, which converts dopamine to the neurotransmitter noradrenaline. The enzyme contains copper which cycles between Cu+ and Cu +, as it incorporates one atom of oxygen from O2 into its substrate. Ascorbate supplies the electrons for reduction of the second atom of the O2 to H2O. A recent report describes another distinct function for ascorbate ion. It apparently acts as a basic catalytic group for proton abstraction from a water molecule during the action of a glycosyltransferase enzyme, becoming part of the active site of that enzyme. 

Ascorbic acid functions as a relatively nonspecific, radical-trapping antioxidant and also reduces the tocopheroxyl radical formed by oxidation of vitamin E. It has a specific metabolic function as the redox coenzyme for dopamine /3-hydroxylase and peptidyl glycine hydroxylase, and it is required to maintain the iron of 2-oxoglutarate-dependent hydroxylases in the reduced state. 

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