Tocopheryl quinones radicals

The ot-tocopheryl dimer continues to possess antioxidant activity. Also, two toco-pheryl semiquinone radicals can form one tocopheryl quinone molecule and regenerate one tocopherol molecule. The decomposition products of tocopherols (during thermal oxidation) can slowly oxidize and release tocopherol that can act as an antioxidant (100). 

Fig 24.18. Vitamin E (a-tocopherol) terminates free radical hpid peroxidation by donating single electrons to hpid peroxyl radicals (LOO ) to form the more stable lipid pieroxide, LOOK. In so doing, the a-tocopherol is converted to the fully oxidized tocopheryl quinone. 

The tocopherols are converted by oxidation to tocopheryl quinones, which upon reduction become tocopheryl hydroquinones. The reduction of a-tocopheryl quinone to a-tocopheryl hydroquinone occurs either via NADPH-cytochrome P45Q reductase, NAD(P)H quinone oxidoreductase 1, or ascorbate for the other tocopherols these pathways have not been tested. The tocophrayl hydroquinones can regenerate the toeopheroxyl radical and thus preserve a-tocophraol with different effieieneies (a > p > y-toeopheryl hydroquinone). ... 

Tocopherols behave as chain breaking antioxidants by competing with the substrate (RH) for the chain peroxyl radicals (ROO ) that are normally present in the highest concentration in the system. Tocopherols donate hydrogen to a peroxyl radical resulting in an a-tocopherol semiquinone radical (reaction 1). This may further donate hydrogen to produce methyl tocopherol quinone or react with another a-tocopheryl semiquinone radical (reactions 2 and 3) to produce an a-tocopherol dimer which also possesses antioxidant activity. ... 

The tocopheroxyl radical has a number of possible fates. It can react with another radical to form non-reactive products. Alternatively, it can be further oxidized to the tocopheryl quinone, a two-electron oxidation product. Another possibility is vitamin E recycling, where the tocopheroxyl radical is restored to its unoxidized form by other antioxidants such as vitamin C, ubiquinol, or thiols, such as glutathione. This process will deplete these other antioxidants. For this reason, it is important to maintain a good intake of other dietary antioxidants. 

FIGURE 6.9 Confirmed heterolytic formation pathway for 5a-a-tocopheryl benzoate (11) without involvement of 5a-C-centered radicals and its proof by trapping of ortho-quinone methide 3 with ethyl vinyl ether to pyranochroman 13. Shown are the major products of the reaction of a-tocopherol (1) with dihenzoyl peroxide. 

Lipid radicals and other radicals may be removed by a number of endogenous compounds as well as GSH. One is vitamin E, (which includes a-tocopherol) a lipophilic substance. It can react with and neutralize lipid radicals and hydroperoxides, in the process becoming a free radical itself. This a-tocopheryl radical is relatively stable and can then be converted back into the a-tocopherol by vitamin C (ascorbate) or react with more radicals and become a-tocopherol quinone ... 

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