Radical peroxyl ascorbate

The human lens is rich in ascorbate, which is required for normal collagen synthesis and acts as a water-soluble antioxidant, reacting rapidly with superoxide, hydroxyl and peroxyl radicals. However, ascorbic acid can undergo auto-oxidation and, at certain concentrations, can form hydroxyl radicals with hydrogen peroxide in the presence of light and riboflavin as described above (Delaye and Tardieu, 1983 Ueno et al., 1987). 

Nese C, Schuchmann MN, Steenken S, von Sonntag C (1995) Oxidation vs. fragmentation in radiosensitization. Reactions of a-alkoxyalkyl radicals with 4-nitrobenzonitrile and oxygen. A pulse radiolysis and product study. J Chem Soc Perkin Trans 2 1037-1044 Neta P, Huie RE, Mosseri S, Shastri LV, Mittal JP, Maruthamuthu P, Steenken S (1989) Rate constants for reduction of substituted methyl peroxyl radicals by ascorbate ions and N,N,N, N -tetrameth-yl-p-phenylenediamine. J Phys Chem 93 4099-4104 Neta P, Huie RE, Ross AB (1990) Rate constants for reactions of peroxyl radicals in fluid solutions. J Phys Chem Ref Data 19 413-513... 

Uric acid is a water-soluble antioxidant found in relatively high concentrations in plasma. It inhibits lipid peroxidation by tightly binding iron and copper ions into inactive forms, and by scavenging various oxidants such as hydroxyl radicals, peroxyl radicals, singlet oxygen and hypochlorous acid. By complexing with iron, uric acid stabilizes ascorbic acid in human serum. 

Fig. 16.5 Synergistic regeneration of a-tocopherol by quercetin at a lipid-water interphase. a-tocopherol is reacting with a lipid peroxyl radical in a chain-breaking reaction. According to the standard reduction potential, the phenoxyl radical of quercetin can further be regenerated by ascorbate.

The major lipid-soluble antioxidant primarily associated with lipid membranes is a-tocopherol (vitamin E). Circulating a-tocopherol is carried by chylomicrons, LDL and HDL and also has extracellular antioxidant capacities. As a chain-breaking antioxidant, it short circuits the propagation phase of lipid peroxidation because the peroxyl radical will react with a-tocopherol more rapidly than a polyunsaturated ffitty acid (Burton and Traber, 1990). The resulting a-tocopheryl radical reacts with a second peroxyl radical to form an inactive, nonradical complex. In vitro, ascorbate regenerates the tocopheryl radical into its native non-radical form (Burton and Traber, 1990). 

There is evidence from a number of in vitro studies that the vitamin E peroxyl radical formed during fatty-acid degradation may be converted to vitamin E plus nonradical through the actions of vitamin C (Burton et al., 1985). RA patients have reduced serum ascorbate levels (Situnayake et al., 1991) and potentially a reduced capacity for the regeneration of vitamin E. In vitro studies suggest that vitamin E becomes a pro-oxidant when ascorbate levels are low (Bowry and Stocker, 1993). 

Peroxyl radicals are the species that propagate autoxidation of the unsaturated fatty acid residues of phospholipids (50). In addition, peroxyl radicals are intermediates in the metabolism of certain drugs such as phenylbutazone (51). Epoxidation of BP-7,8-dihydrodiol has been detected during lipid peroxidation induced in rat liver microsomes by ascorbate or NADPH and during the peroxidatic oxidation of phenylbutazone (52,53). These findings suggest that peroxyl radical-mediated epoxidation of BP-7,8-dihydrodiol is general and may serve as the prototype for similar epoxidations of other olefins in a variety of biochemical systems. In addition, peroxyl radical-dependent epoxidation of BP-7,8-dihydrodiol exhibits the same stereochemistry as the arachidonic acid-stimulated epoxidation by ram seminal vesicle microsomes. This not only provides additional... 

In the last decade numerous studies were dedicated to the study of biological role of nonenzymatic free radical oxidation of unsaturated fatty acids into isoprostanes. This task is exclusively difficult due to a huge number of these compounds (maybe many hundreds). Therefore, unfortunately, the study of several isoprostanes is not enough to make final conclusions even about their major functions. F2-isoprostanes were formed in plasma and LDL after the treatment with peroxyl radicals [98], It is interesting that their formation was observed only after endogenous ascorbate and ubiquinone-10 were exhausted, despite the presence of other antioxidants such as urate or a-tocopherol. LDL oxidation was followed by... 

As mentioned earlier, ascorbate and ubihydroquinone regenerate a-tocopherol contained in a LDL particle and by this may enhance its antioxidant activity. Stocker and his coworkers [123] suggest that this role of ubihydroquinone is especially important. However, it is questionable because ubihydroquinone content in LDL is very small and only 50% to 60% of LDL particles contain a molecule of ubihydroquinone. Moreover, there is another apparently much more effective co-antioxidant of a-tocopherol in LDL particles, namely, nitric oxide [125], It has been already mentioned that nitric oxide exhibits both antioxidant and prooxidant effects depending on the 02 /NO ratio [42]. It is important that NO concentrates up to 25-fold in lipid membranes and LDL compartments due to the high lipid partition coefficient, charge neutrality, and small molecular radius [126,127]. Because of this, the value of 02 /N0 ratio should be very small, and the antioxidant effect of NO must exceed the prooxidant effect of peroxynitrite. As the rate constants for the recombination reaction of NO with peroxyl radicals are close to diffusion limit (about 109 1 mol 1 s 1 [125]), NO will inhibit both Reactions (7) and (8) and by that spare a-tocopherol in LDL oxidation. 

In 1998, Schlotte et al. [259] showed that uric acid inhibited LDL oxidation. However, subsequent studies showed that in the case of copper-initiated LDL oxidation uric acid behaves itself as prooxidant [260,261]. It has been suggested that in this case uric acid enhances LDL oxidation by the reduction of cupric into cuprous ions and that the prooxidant effect of uric acid may be prevented by ascorbate. On the other hand, urate radicals formed during the interaction of uric acid with peroxyl radicals are able to react with other compounds, for example, flavonoids [262], and by that participate in the propagation of free radical damaging reactions. In addition to the inhibition of oxygen radical-mediated processes, uric acid is an effective scavenger of peroxynitrite [263]. 

Ruhho, H., Radi, R., Ansehni, D., Kirk, M., Bames, S., Butler, J., Fiserich, J. P., and Freeman, B. A., 2000, Nitric oxide reaction with hpid peroxyl radicals spares alpha-tocopherol during hpid peroxidation. Greater oxidant protection from the pair nitric oxide/alpha-tocopherol than alpha-tocopherol/ascorbate, J. Biol. Chem. 275 10812-10818. 

Nishikimi M (1975) Oxidation of ascorbic acid with superoxide anion generated by the xanthine-xanthine oxidase system. Biochem Biophys Res Commun 63 463-468 Niu QJ, Mendenhall GD (1992) Yields of singlet molecular oxygen from peroxyl radical termination. J Am Chem Soc 114 165-172... 

W6. Wayner, D. D., Burton, G. W., Ingold, K. U., Barclay, L. R., and Locke, S. J., The relative contributions of vitamin E, urate, ascorbate and proteins to the total peroxyl radical-trapping antioxidant activity of human blood plasma. Biochim. Biophys. Acta 924, 408—419 (1987). 

Ascorbic acid inhibits light-induced yellowing for a finite time, 1 to 2 hours when irradiated with near-uv light with an intensity of 9.2 mW/cm2 (6). This limitation has been attributed in part to photooxidation of ascorbic acid. In addition to air oxidation, ascorbic acid is oxidized by photochemically produced peroxyl radicals, superoxide radical anion and singlet oxygen (27,28). If ascorbic acid is to be an effective inhibitor of light-induced yellowing it s oxidation must be slowed. 

FIGURE 29.1 Pathways of the chain-breaking action of vitamin E in lipid peroxidation and its subsequent regeneration. LOOH lipid hydroperoxide, LOO lipid peroxyl radical, vitamin C ascorbate radical (semi-dehydroascorbate), vitamin E a-tocopheroxyl radical. The lipid peroxyl radical is reduced to lipid hydroperoxide by tocopherol. The resulting tocopheroxyl radical can be re-reduced by ascorbate. The thus formed ascorbate radical can be reduced to ascorbate by the NADH-dependent semidehydroascorbate reductase. 

The antioxidant activity of ascorbate is variable. From consideration of the chemistry involved, it would be expected that, overall, 2 moles of peroxyl radical would be trapped per mole of ascorbate, because of the reaction of 2 moles of monodehydroascorbate to regenerate ascorbate and yield dehydroascorbate (see Figure 13.3). However, as the concentration of ascorbate increases, so the molar ratio decreases, and it is only at very low concentrations of ascorbate that it tends toward the theoretical 2 1. 

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