A-Tocopherol radical

Scarpa, M., Rigo, A., Maiorino, M., Ursini, F. and Gregolis, C. (1984). Formation of a-tocopherol radical and recycling of alpha tocopherol by ascorbate during peroxidation of phosphatidylcholine liposomes. An electron paramagnetic resonance study. Biochim. Biophys. Acta 801, 215-219. 

The synergistic effect observed in the presence of all three antioxidants implies that there is an interaction between the individual antioxidant components. The direct interaction of the a-tocopherol radical and ascorbic acid is already well established (Bisby and Parker 1995) and a study by Mayne and Parker (1989) on chicks deficient in vitamin E and selenium showed that the... 

One more mechanism responsible for the synergistic action of two phenols has recently been discovered during the study of disproportionation of phenoxyl radicals [42,43], This reaction is possible only for phenoxyls possessing C—H groups in the ortho- or para-position [43], For instance, 2,4,6-tris(l,l-dimethylethyl)phenoxyl is unable to disproportionate, whereas ionol radicals disproportionate. It was found that the cross-disproportionation of ionol and a-tocopherol radicals occurs much more rapidly than homodisproportionation (323 K [42]) ... 

The resultant tocopheroxyl radical is relatively stable and, in normal circumstances, insufficiently reactive to initiate lipid peroxidation itself. It has been demonstrated in vitro that a-tocopherol radical can be converted back to a-tocopherol by reduction with ascorbic acid at the surface of biological membranes... 

Figure 5, Model outlining a proposed mechanism of action for AFM (3). Abbreviations caro-tenoid, activated carotenoid carotenoid, photooxidized carotenoid AFM, activated AFM PVFA-Oft, PUFA peroxide radical Prop Rx, propagation reaction Term Rx, termination reaction and a-T-, a-tocopherol radical (3).

Pazos M, Torres IL, Andersen ML, Skibsted LH, Medina I. Galloylated polyphenols efficiently reduce a-tocopherol radicals in a phospholipid model system composed of sodium dodecyl sulfate (sds) micelles. J Agric Food Chem. 2009 57(ll) 5042-5048. 

Already three decades ago, it was proposed by Tappel (45) that a synergy between Vitamin E and ascorbic acid existed (46), as the reduction potential of the ascorbate radical/ascorbate system at pH 7.0 is 0.28 V, vdiich is above that of the a-tocopherol radical/a-tocopherol (E = 0.5) (47) a-Toc + AscH" —> a-TocH + Asc ... 

Low molecular mass agents shown to have a protective effect against oxidative cell damage (Fig. 1.5) include tocopherols (e.g. a-tocopherol, 77) [32], ascorbic acid (78) [33], flavonoids (e.g. quercetin, 79) [34] and carotenoids (e.g. P-carotene, 80) [35]. The main mechanism of action as antioxidants appears to be the scavenging of ROS. This defence systan often involves complex interplay between various antioxidant species for example ascorbate cooperates with vitamin E, regenerating a-tocopherol (77) from a-tocopherol radicals in membranes (see Sect. 4.4, Scheme 4.5) [36]. 

Walker M, Beckert D, Lash J. Interaction of UV light-induced a-tocopherol radicals with lipids detected by an electron spin resonance prooxidation effect. Photochem Photobiol 1998 68(4) 502-10. 

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