Ascorbate and Free Radicals

FIGURE 1. The univalent pathway of O2 reduction gives rise to three active oxygen species. SOD superoxide dismutase, CAT = catalase, GPx = GSH peroxidase, GR = GSH reductase, ASC ascorbate. 

FIGURE 2. The antioxidant-prooxidant balance. An excess of active oxygen species and transition metals in relation to antioxidants leads to oxidative stress. The converse situation is observed in the presence of relatively high levels of antioxidants. URI = uric acid, VITE = vitamin E. 

FIGURE 3. Relationships between ascorbate and other antioxidants and lipid peroxidation. L = lipid, L = alkyl lipid radical, LOO = peroxyl lipid radical, LOOK = lipid hydroperoxide, LO = alkoxyl lipid radical, LOH = lipid hydroxide, AFR = ascorbate free radical, VEOH = vitamin E, VEO = tocopheroxyl radical. 

Madere, 1989) and in guinea pig liver (Cadenas et al., 1995) and heart (Barja et al., unpublished) was observed in unstressed normal animals. Perhaps the lack of interaction observed in vivo between vitamins E and C is due to the presence of very low levels of free radicals in intact animals under basal conditions. Further work is needed to clarify if an in vivo synergic effect is present only when the animals are subjected to a physiologically relevant additional oxidative stress. An alternative way in which ascorbate could directly inhibit lipid peroxidation without mediation of vitamin E would be by intercepting antioxidants in the aqueous phase, before they can reach and attack membrane lipids (Frei, 1991). 

The main metabolic pathways of ascorbate synthesis, redox chemistry, and catabolism are summarized in Fig. 4. Ascorbate is synthesized from D-glucuronic acid in three steps D-glucuronic acid is reduced to L-gulonic acid, which is epi- 

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Buettner GR, Jurkiewicz BA. Catalytic metals, ascorbate and free radicals Combinations to avoid. 

Roginski, V. A., and 8tegmann, H. B., 1993, Kinetics of the reaction between ascorbate and free radical from vitamin E as studied by E8R steady-state method, Chem. Phys. Lipids 65 103-112. 

The most significant chemical characteristic of L-ascorbic acid (1) is its oxidation to dehydro-L-ascorbic acid (L-// fi (9-2,3-hexodiulosonic acid y-lactone) (3) (Fig. 1). Vitamin C is a redox system containing at least three substances L-ascorbic acid, monodehydro-L-ascorbic acid, and dehydro-L-ascorbic acid. Dehydro-L-ascorbic acid and the intermediate product of the oxidation, the monodehydro-L-ascorbic acid free radical (2), have antiscorbutic activity equal to L-ascorbic acid. 

Braun, L., Puskas, F., Csala, M., et al., 1997. Ascorbate as a substrate for glycolysis or glnconeogenesis Evidence for an interorgan ascorbate cycle. Free Radical Biology and Medicine 23 804—808. 

The chemistry of ascorbic acid free radicals is reviewed. Particular emphasis is placed on identification and charac-terization of ascorbate radicals by spectrophotometric and electron paramagnetic resonance techniques, the kinetics of formation and disappearance of ascorbate free radicals in enzymatic and nonenzymatic reactions, the effect of pH upon the spectral and kinetic properties of ascorbate anion radical, and chemical reactivity of ascorbate free radicals. 

Radiolysis of oxygenated water or photolysis of hydrogen peroxide solutions yields two oxidative species, the -OH radical and the perhy-droxyl radical (HO2 02" + H ). As previously discussed, the final reaction product of hydroxyl radicals interaction with ascorbate above pH 6 is predominantly the ascorbate anion radical (A ). To account for the stoichiometry of ascorbic acid consumption in a Co gamma ray study of oxygenated ascorbic acid solutions, the ascorbic acid free radical was thought (3) to react with molecular oxygen to yield a transient adduct ... 

During the past two decades ascorbic acid free radicals have become recognized and their kinetics studied (281-287) in the oxidation of ascorbic acid. Interactions between certain of the vitamins or ingredients... 

Formation of the free radical of ascorbic acid (measured at 360 nm) accompanied reduction of tocopherol free radicals (38). Bielski et al. (18,46) showed that the ascorbic acid free radical in the 3-position absorbs at 360 nm and that in the 2-position absorbs at 290 nm also, ascorbic acid reacts with superoxide. 

In certain systems ascorbic acid free radical on the 2- and 3-positions may be an intermediate in the antioxidant function, but it is a short-lived intermediate (18). An electron spin resonance flow system was used to study scavenging of a nitrosating agent by ascorbic acid, and a total spin free radical, which is the same as that produced by radiolytic oxidation (48), was determined (47). 

Oxidative stress and free-radicals The production of free-radicals and other reactive species has been much implicated in cytotoxicity. This topic is discussed in more detail elsewhere see ANTIOXIDANTS PREE-RADICAL SCAVENGERS nitric oxide synthase inhibitors. The production of these species in oxidative stress is well established. Advances in therapy may well be in the direction of boosting and augmenting natural defence processes, for example, by superoxide dismutase (SOD), catalase, a-tocopherol, glutathione, ascorbic acid and perhaps melatonin. Surprisingly, some forms of the enzyme SOD can be administered in vivo and are protective. 

Muckenschnabel, 1., B.A. Goodman, B. Williamson, G.D. Lyon, and N. Deighton Infection of leaves of Arabidopsis thaliana by Botrytis cinerea Changes in ascorbic acid, free radicals and lipid peroxidation products J. Exp. Bot. 53 (2002) 207-214. 

The reader will also find in the Index certain broad classifications of components, like oxidases and free radicals. These and similar examples in the Index are not there to confuse the reader, as many of the individual components in the broad classifications have specific CAS numbers. Generally, the references associated with these classes of components (found within the chapters noted in the Index) will provide the reader with information of a common nature. In nearly all cases, individual components such as ascorbate oxidase, choline oxidase, cytochrome oxidase, and glycolate oxidase follow after the broadly classified component, oxidase. Likewise, specific free radicals such as methyl-acyl radical, ethyl-acyl radical, and propyl-acyl radical 2 isomers may be found in the Index. For some components in the Index, several partially identified isomers exist, their number noted, and included in the total number of components identified in tobacco and/or smoke. 

Smoking is considered a risk factor for several diseases associated with oxidative stress [180-183]. Cigarette smoke contains NO and NO2, as well as several lipid per-oxy radicals and free radicals [184]. All these components may serve as substrates for oxidation and generate ONOO", likely furthering the cascade of oxidative damage [184,185]. Smokers also have lower amounts of plasma antioxidants, such as ascorbate and vitamin E [186,187] and the consumption of antioxidants attenuate the production of inflammatory mediators [188]. Evidence also suggests that smoking is... 

Various carbonyl compounds and decomposition products of methyl linolenate hydroperoxides were tested for their interaction with DNA by measuring fluorescence in the presence of ferric chloride and ascorbic acid. 2,4-Alkadienals and 2,4,7-decatrienals were among the most active decomposition products of linolenate hydroperoxides (Table 5.8). To determine the type of reactive species involved in fluorescence formation with DNA, the effect of free radical antioxidants and a singlet oxygen quencher were examined. )3-Carotene, a-tocopherol and phenolic antioxidants strongly inhibited DNA fluorescence formed by decomposition of linolenate hydroperoxides in the presence of ferric chloride and ascorbic acid (Table 5.9). These results indicate that singlet oxygen and free radical species are important intermediates in the interaction of linolenate hydroperoxides with DNA in the presence of iron and ascorbic acid. 

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