Radicals, derived from ascorbic acid

As a number of excellent articles have been published that review various aspects of the biological roles of L-ascorbic acid,8 the biosynthesis of L-ascorbic acid in plants and animals,7-11 and radicals derived from L-ascorbic acid,12 these topics will not be treated. Likewise, the methods by which L-ascorbic acid is assayed13 and the uses of L-ascorbic acid and related molecules in a wide variety of assays and oxidation-reduction systems will not be discussed. 

Ascorbic acid is a reasonably strong reducing agent. The biochemical and physiological functions of ascorbic acid most likely derive from its reducing properties—it functions as an electron carrier. Loss of one electron due to interactions with oxygen or metal ions leads to semidehydro-L-ascorbate, a reactive free radical (Figure 18.30) that can be reduced back to L-ascorbic acid by various enzymes in animals and plants. A characteristic reaction of ascorbic acid is its oxidation to dehydro-L-aseorbie add. Ascorbic acid and dehydroascor-bic acid form an effective redox system. 

The radical form 9.4 has an unpaired electron and may undergo fast reactions with redox partners that also undergo one-electron processes. Such a redox partner is the triplet radical, dioxygen. The copper complex of ascorbic acid undergoes rapid aerial oxidation to give the dione, dehydroascorbic acid, which may be viewed as being derived by electron loss from the radical (Fig. 9-4). 

Ascorbate (AH-) can make two e available, and consequently both AH- and its one e oxidation product, the ascorbyl radical (A -), become antioxidants. The latter dismutates to form A - and dehydroascorbicadd (A), as shown in Scheme 3, or is reduced back by GSH or GSH-dependent enzymes (glutaredoxine, thioredoxin). Immediately, A is irreversibly hydrolized to 2,3-diketogluconic acid and then to oxalate, thre-onate, and many other metabolites. This last point is important because products derived from the hydrolysis of A may potentially damage proteins by glycation (47). 

Either Fenton reagent or a mixture of ascorbic acid, Fe, and ethyl-enediaminetetraacetic acid catalyzed the production of acetyldehyde from ethanol, ethylene from methional derivative, and methane from dimethyl sulfoxide (34). The authors claimed that both hydroxy radicals and singlet oxygen were found as intermediates, and, indeed, ascorbic acid scavenged both hydroxyl radical and singlet oxygen. 

The brown pigments which were formed through the oxidative pathway are almost twice those formed through the anaerobic pathway. This effect could be derived from the involvement of H2O2 and oxy--radicals in the aerobic oxidation pathway of ascorbic acid (20,21). The oxy-radicals may generate carbonyls and amino acid free radicals which, by polymerization, could accelerate the formation of more brown pigments. 

The traditional approach has been to accept that flavonoids retard the copper-catalysed oxidation of ascorbic acid by chelating with copper and possibly other trace elements. Harper, Morton and Rolfe have shown that the protective mechanism is possibly more complex than this they found that flavonoids exerted a strong protective action under conditions where EDTA (a potent inhibitor of copper-catalysed ascorbic aci oxidation) was ineffective [67]. As an alternative and possibly complementary mechanism, they suggested that the protective capacity is derived from the ability of flavonoids to act as free radical acceptors free radical formation is believed to be an important phase of ascorbic acid oxidation [67]. It should be noted, however, that the model system used for these studies was designed primarily to elucidate the mechanism of ascorbic acid protection by flavonoid in fruit juices at a low pH it would be improper, without qualification, to extrapolate them to physiological conditions of pH, temperature and concentration. 

The synergism exhibited by the ternary mixture of a-tocopherol, ascorbic acid and phospholipids has been shown to be due to the stabilization of a-tocopherol, on the basis of ESR studies with methyl linolenate oxidized at 90°C to detect the free radicals of a-tocopherol and ascorbic acid. Evidence was obtained by this technique for the formation of nitroxide radicals (R-N-0 ) in the presence of phosphatidylserine or phosphatidylethanolamine or soybean lecithin and oxidized methyl linolenate. However, as pointed out earlier (Section C), the synergistic activity of this ternary mixture may be derived from antioxidant products formed from the phospholipids at elevated temperatures by the Maillard browning reaction (Chapter 11). 

Ellipticine (E) 1 is an indolic alkaloid with antitumor activity. Some of its phenolic derivatives as N-methyl-9-hydroxyellipticine (Celiptinium), obtained from 9-hydroxy ellipticine (9-OH E), exhibit high cytotoxicity. 38 Syntheses of 9-OH E are not satisfactory considering yields and cost, justifying the attempts at the direct conversion of E to 9-OH E via 9-oxoellipticine (9-0X0 E). Potassium nitrosodisulfonate (Fremy s salt), a valuable oxidant for the synthesis of quinone-imine from heterocyclic amines,139 was used. Under these conditions the conversion of E to 9-0X0 E was observed for the first time. Its reduction to 9-OH E is then easily achieved with ascorbic acid. The radical nature of Fremy s salt [ 0-N(S03K)2] led us to carry out the oxidation reaction under sonication in order to increase the yields via an easier electron transfer.1 1... 

Under physiological conditions, ascorbic acid is mainly used as a reductant in a variety of functions. It is essential in chloroplasts for the removal of photoproduced active oxygen species (Miyake and Asada, 1992). Ascorbate can also regenerate some membrane-bound radical quenchers such as a-tocopherol and zeaxanthin (Foyer et al, 1991). Ascorbate function is in addition extended to the removal of free radical oxygen species produced by air pollutants, certain herbicides, and other cytotoxic compounds derived from lipid peroxidation (Polle et al., 1990 Penel and Castillo, 1991 Luwe et al., 1993). 

Our laboratories have been carrying out a large series of studies on the effect of various antioxidant micronutrients on both animal and human cancer cell lines in culture (Schwartz and Shklar, 1994). These studies were carried out to augment our extensive animal studies. Various molecular parameters can be evaluated more easily in cell culture studies, and one can study many human cancer cell lines and compare them to cell lines derived from animal studies. In vivo studies have shown that ascorbic acid s primary biochemical activity is as an aqueous antioxidant that interacts with other antioxidants. Fundamental to the activity of ascorbic acid is the formation of the ascorbate radical, which forms under various oxidizing conditions. The development of the ascorbate radical during carcinogenesis could then replace... 

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