Tocopheryl quinones

The recommended daily allowance for vitamin E ranges from 10 international units (1 lU = 1 mg all-rac-prevent vitamin E deficiency in humans. High levels enhance immune responses in both animals and humans. Requirements for animals vary from 3 USP units /kg diet for hamsters to 70 lU /kg diet for cats (13). The complete metaboHsm of vitamin E in animals or humans is not known. The primary excreted breakdown products of a-tocopherol in the body are gluconurides of tocopheronic acid (27) (Eig. 6). These are derived from the primary metaboUte a-tocopheryl quinone (9) (see Eig. 2) (44,45). 

Very high intakes may antagonize vitamin K and hence potentiate anticoagulant therapy. This is probably the result of inhibition of the vitamin K quinone reductase, but a-tocopheryl quinone may compete with vitamin K hydroquinone and hence inhibit carboxylation of glutamate in target proteins (Section 5.3.1). 

One complication that can arise when using Fremy s salt is that the substrate may be insoluble in the aqueous reaction medium, despite the use of organic cosolvents. Olson et al introduced an organic-soluble version of Fremy s salt in their synthesis of a-tocopherol. Hexahydrofamesolacetone (13) was transformed to the phenol (14), which was oxidized in nearly quantitative yield to the tocopheryl quinone (15) using a mixture of Fremy s salt and tricaprylylmethylammonium chloride in a two-phase water/ben-zene solvent system. The oxidizing agent was presumed to be the bis(tricaprylylmethyl)ammonium ni-trodisulfonate. The quinone (15) is a known precursor of a-tocopherol (16 Scheme 4). 

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). 

The natural RRR-a-Toc isomer meets the above criteria, with the possible exception of mobility , and more than this it has been shown that a-tocopheryl quinone, the common oxidation product of a-Toc, is converted back into vitamin E in man. ... 

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. 

Most of the proteins of PS II are embedded within the photosynthetic membrane although portions are exposed to the aqueous media on the inside and on the outside of the membrane. A fully developed PS II is composed of hundreds of Chi a and Chi b molecules, carotenoids, plastoquinones (Figure 1), a-tocopheryl quinone or a-tocopherol, cytochrome b-559, the Mn-protein responsible for O2 evolution, and other electron transport agents. The diameter of a fully-developed PS II has been estimated at 160 A from electron micrographs of freeze-fractured photosynthetic membranes (46). The bulk of the chlorophyll molecules in PS II have only an antenna function, i.e., they absorb photons (reaction 1) and transfer the resultant electronic... 

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). ... 

The tocopherols may be metabolized to bioactive compounds, such as Simon metabolites, CEHCs, or tocopheryl quinones (see sections on tocopherol metabolites and tocopheryl quinones), which can bind to transcription factors and enzymes and modulate their activity. 

Cornwell, D.G., Williams, M.V., Wani, A.A., Wani, G., Shen, E., and Jones, K.H., Mutagenicity of tocopheryl quinones evolutionary advantage of selective accumulation of dietary alpha-tocopherol, Nutr. Cancer 43 (1), 111-118, 2002. 

Calviello, G., Di Nicuolo, E, Hccioni, E, Marcocci, M.E., Serini, S., Maggiano, N., Jones, K.H., Cornwell, D.G., and Palozza, R, gamma-Tocopheryl quinone induces apoptosis in cancer cells via caspase-9 activation and cytochrome c release. Carcinogenesis 24 (3), 427 33, 2003. 

On the addition of acids, this compound is transformed into tocopheryl-quinone. 

Egger and Kleinig [41] have succeeded in isolatiug a-tocopheryl-quinone from mixtures with related quinones, using polyamide layers. This technique may be suitable for other E-compounds also. 

Leneo-methylene bine for ubi-, plasto- and tocopheryl quinones. 

Methyl-7-cholestene-3,5,6-triol, M-80061 ) -Tocopherol, T-80200 y-Tocopheryl quinone, T-80201... 

Tocopherols and tocotrienols are monoethers of the respective hydroquinones and are therefore readily oxidised, for example by ferric ions, Hpid hydroperoxides and other oxidants. This creates the corresponding quinones (tocopheryl quinones or tocoquinones). Tocopheryl quinones can be reduced to tocopheryl hydroquinones (tocohydroquinones). The most important reactions are those with oxidised lipids (Figures 5.7 and 5.8). 

Tocopheryl quinones were efficiently prepared from the corresponding tocopherols by polymer-supported MTO/H2O2 catalytic system. Homogeneous MTO/H2O2 proved to be more reactive, but less selective than the heterogeneous catalyst, this may be explained by the presence of a kinetic barrier for the substrate to approach the supported rhenium complexes. Heterogeneous systems were stable to perform at least four recycling experiments with similar conversion and selectivity (eq 56). ... 

Table 3 Prenylquinones in the photosynthetic membranes of bacteria and higher plants. UQ-10 = ubiquinone-10 MKn = menaquinone homologues (vitamin K2I PQ-9 = plastoquinone-9 1 = phylloquinone (vitamin Kj) oC-tocopheryl-quinone (= o(.-tocoquinone)sot-1 . 

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. 

Kobayashi, M. (1994b) Marine terpenes and terpenoids Part 18. First natural 4-tocopherol and A—tocopheryl quinone derivatives with oxygenated side chain, isolated from soft coral Sinularia mayi. J. Chem, Res, (S), 494- 95. 

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