Vitamin ascorbyl radical

Vitamin C (ascorbate) (Fig. 9.5) has the ability to act as a reducing agent, i.e. it will tend to reduce more reactive species. This ability to reduce Fe3+ to Fe2+may be important in promoting iron uptake in the gut. Oxidation of ascorbate by reaction with reactive oxygen species or reactive nitrogen species seems to lead to its depletion. In vitro, vitamin C can also exert pro-oxidant properties. Fe3+ can react with ascorbate to form Fe2+ and the semi-dehydroascorbate or ascorbyl radical. The latter can react with hydrogen peroxide to form Fe3+, the hydroxyl radical and a hydroxide anion. A key question with regard to the pro- or anti- oxidant effects of ascorbate may therefore be the availability of transition metal ions. Neurons main-... 

Besides these enzyme substrates, a number of biological molecules are likely to give rise to fairly stable and hence observable free radicals. The more important of these are the quinonoid molecules, particularly vitamin Q quinone (ubiquinone), vitamin E quinone, vitamins K, Ks and vitamin E quinone, the flavins and flavoproteins and the important neurochemicals dopa, dopamine, and closely related phenolic and quinonoid molecules. In many of these cases, the generation of free radicals from these molecules should occur in vivo, but as yet only a few radicals such as the ascorbyl radical and the bacteriochlorophyll radical have been directly identified in intact systems. Free radicals from melanins (polymers from dopaquinone) have been demonstrated both in vivo and in vitro, but these radicals are so stable that it has not yet been possible to identify a biological role for the radicals per se. 

The second reason that vitamin C is used as an electron donor is that the reaction product is fairly stable and unreactive. When vitamin C gives up an electron, it becomes a free radical called the ascorbyl radical. By free-radical standards, the ascorbyl radical is not very reactive. Its structure is stabilized by electron delocalization — the resonance effect first described by Linus Pauling in the late 1920s. This means that vitamin C can block free-radical chain reactions by donating an electron, while the reaction product, the ascorbyl radical, does not perpetuate the chain reaction itself. 

Despite its slow reactivity, the ascorbyl radical usually gives up a second electron to produce dehydroascorbate. This molecule is unstable and needs to be caught quickly if it is not to break down spontaneously and irrevocably, and be lost from the body. The continual seeping loss of vitamin C in this way accounts for our need to replenish body pools by daily intake. Even so, we can minimize losses by recycling dehydroascorbate. Several different enzymes bind dehydroascorbate to regenerate vitamin C. These enzymes usually take two electrons from a small peptide called... 

When a free radical reacts, it usually snatches an electron from the reactant, turning it into a free radical. This in turn will steal a single electron from another nearby molecule. A chain reaction ensues until two free radicals react together, effectively neutralizing each other, or alternatively, until an unreactive free-radical product is formed. Free radicals are said to be quenched by vitamin C, because the free-radical product — the ascorbyl radical — is so unreactive. As a result, free-radical chain reactions are terminated. Lipid-soluble vitamin E (a-tocopherol) works in the same way, in membranes rather than in solution, often in cooperation with vitamin C at the interface between membranes and the cytosol (the watery ground substance of the cytoplasm that surrounds the intracellular organelles). When vitamin E reacts with a free radical, it too produces a poorly reactive (resonance-stabilized) free-radical product, called the a-tocopheryl radical. Tocopheryl radicals can be reconverted into vitamin E using electrons from vitamin C. 

This new strategy consists of the synthesis of molecules that possess hydroxyl groups in such positions that a radical species can be stabilized by mesomery. This feature is found in natural flavonol such as kaemferol that possesses undeniable antioxidant properties. The first published example is a lipophilic analogue of vitamin C, Fig (13) [40]. In this paper, we have proved that the synthesised substituted 2-hydroxyfuran-2-one is a true ascorbic acid analogue. A radical anion that gives very similar data has been generated under the same conditions as for ascorbic acid with a stability somewhat lower and a redox potential lower than those of ascorbyl radical. Its antioxidant properties are also similar to that of ascorbic acid but it inhibits LDL peroxidation induced by Cu2+ or AAPH more efficiently probably due to a higher lipophilicity. 

In an in vitro model, tumour metastases (B16BL6 cells) and invasion human fibrosarcoma HT-1080) were inhibited by repeated addition of 300 iiM 2-0-phosphorylated ascorbate (Nagao et al. 2000). Intracellular vitamin C increased and both hydroxyl and ascorbyl radicals decreased as quantified by electron spin resonance spectroscopy. 

The name vitamin C refers not only to L-ascorbic acid, but also to the whole reversible redox system that includes the one-electron oxidation product of L-ascorbic acid, known as L-ascorbyl radical (or L-monodehydroascorbic acid or semidehydroascorbic acid), and the two-electron oxidation product of L-ascorbic acid known as L-dehydroascorbic acid (Figure 5.26). Ascorbic acid and ascorbyl radical mainly occur as anions in solutions at physiological pH. 

Ascorbic acid plays an important role in the regeneration of vitamin E from vitamin E radical. This recycling effect has been specifically shown in skin. Therefore, sun-care products should ideally combine vitamin E acetate and sodium ascorbyl phosphate to protect the skin from harmful UV irradiation. To test this, a new method was developed by Hanson (2003) to visualize UV-induced free radical formation in human skin. Results show that SAP in combination with vitamin E acetate reduces the formation of free radicals in the skin by almost 50% compared to a placebo sun-care formulation having an SPF of 8. 

By this synergistic mechanism, tocopherols and ascorbic acid can mutually reinforce one another by regenerating the oxidized form of the other. Radical exchange reactions among lipid radicals, tocopherols, and ascorbic acid are the basis of numerous approaches for stabilizing oil and foods with their mixtures. It is however important to note that vitamin C is not soluble in the lipid phase that is most susceptible to oxidation. This was the reason why L-ascorbic esters were developed, e.g., ascorbyl palmitate that has a lipid solubility superior to that of ascorbic acid. Mixtures of ascorbyl palmitate with tocopherols are well known for their synergistic activity. 

Vitamin E and other free radical scavengers have been shown to inhibit the glycation of proteins, in vivo and in vitro (Rosen et aL, 1991 Ceriello et al., 1991 Aoki et al., 1992 LeGuen et al., 1992). This might also apply to ascorbylation of proteins, given that the effect of such free radical scavengers is likely to be inhibition of enediol-mediated oxidant formation which involves an enediol-radical intermediate (Hunt, 1994), shown in Figs. 6 and 8. 

The antioxidant activity of vitamin E in emulsions depends on the structure of the emulsions and the presence of other antioxidants, such as 3,5-di-tert-butyl-4-hydroxytoluene (BHT) and ascorbyl pahnitate. Temperature plays an important part, as does, particularly, the presence of oxygen and the stability of the radicals of tocopherols produced as intermediates in reactions with oxidised lipids. At 80 °C in the presence of air, 5-tocopherol, for example, is the only vitamin form which partially withstands heating for 6 h, when used as an antioxidant to protect linoleic acid against autoxidation. In an atmosphere containing only 10% oxygen (hah of the amount of oxygen in air), P- and y-tocopherols are also present, but a-tocopherol and aU tocotrienols are absent. At 60 °C in the absence of oxygen, all tocopherols and tocotrienols are present. 

Vitamins (see Section 8.6), often added to cosmetic formulations, act as antioxidant preservatives due to their general antioxidant properties towards free radicals. Examples are retinol (vitamin A) and its precursor j5-carotene, tocopherol (vitamin E) and ascorbic acid (vitamin C). Moreover, vitamin derivatives, such as retinyl acetate, retinyl palmitate, ascorbyl palmitate, magnesium ascorbyl phosphate and tocopheryl acetate among others, are also employed as antioxidant agents. 

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