Antioxidant activity of a-tocopherol

Other tocopherols and tocotrienols have not been shown to act as prooxidants. For example, antioxidant activity of 8-tocopherol increased parallel to concentration up to 2000 pg/g in a simple oil model (Huang et al., 1995). Adding tocopherols as mixtures is an efficient means to improve oxidative stability of oils (Parkhurst et al., 1968 Huang et al., 1995), because in mixtures, they protect and regenerate each other (Niki et al., 1986 Lampi et al., 1999). Thus, addition of y- or other tocopherols and tocotrienols together with a-tocopherol may enhance the antioxidant activity of a-tocopherol. 

In-vitro rate studies on the antioxidant activity of a-tocopherol has shown that it is one of the most efficient alkylperoxyl radical traps, far better than the commercial hindered phenols, e.g., BHT (2,6-di-t r -butyl-4-methylphenol, AO 1). Its efficiency was attributed to stereo electronic effects the electronic synergy between a fully methylated aromatic ring and the chro-man moiety results in a highly stabilized tocopheroxyl radical, formed during the rate limiting step. Reaction 8, because of the interaction between the p-orbitals on the two oxygen atoms. ... 

Hnang, W., Frankel, E.N., Schwarz, K., and German, B., Effect of pH on antioxidant activity of a-tocopherol and trolox in oil-in-water emnlsions, J. Agric. Food Chem., 42, 2496, 1996. 

Huang, S-W., Hopia, A., Frankel, E.N. and German, J. B. Antioxidant activity of a-tocopherol and Trolox in different lipid substrates bulk oils vs oU-in-water emulsions. J. Agr. Food Chem. 44, 444-452 (1996). 

The synergistic effect of phospholipids is different. Addition of dipalmitoylphos-phatidylethanolamine (0.1-0.2 weight %) to lard enhances the antioxidative activity of a-tocopherol, BHA, BHT and propyl gallate, while phosphatidylcholine shows no activity. 

Frankel et al. (1996a) also observed that camosol and camosic acid were significantly more active antioxidants in oil-in-water emulsions buffered at pH 4 or 5 than at pH 7. Hopia et al. (1996) reported that camosic acid was a better antioxidant than camosol in methyl linoleate, but that the reverse was tme in linoleic acid. In methyl linoleate or linoleic acid, camosic acid and camosol were more active than a-tocopherol. However, a-tocopherol was more active in bulk corn oil triacylglycerols, followed by camosic acid and camosol. The oxidative stability of rapeseed oil treated with rosemary extracts correlated well with the content of camosic acid, but not with content of camosol (Trojakova et al, 2001). In all emulsified systems, a-tocopherol exhibited more antioxidant activity than camosol or camosic acid (Hopia et al., 1996). In bulk com oil, the addition of camosol to a-tocopherol had a negative impact on the antioxidant activity of a-tocopherol, whereas camosic acid enhanced a-tocopherol activity (Hopia et al, 1996). Both camosic acid and camosol disappeared from the test system more quickly than a-tocopherol (Hopia et al, 1996). 

The mechanism of prooxidant effect of a-tocopherol in aqueous lipid dispersions such as LDLs has been studied [22], This so-called tocopherol-mediated peroxidation is considered in detail in Chapter 25, however, in this chapter we should like to return once more to the question of possible prooxidant activity of vitamin E. The antioxidant effect of a-tocopherol on lipid peroxidation including LDL oxidation is well established in both in vitro and in vivo systems (see, for example, Refs. [3,4] and many other references throughout this book). However, Ingold et al. [22] suggested that despite its undoubted high antioxidant efficiency in homogenous solution a-tocopherol can become a chain transfer agent in aqueous LDL... 

Vitamin F, substances are necessary for the normal growth of animals. Without vitamin E, the animals develop infertility, abnormalities of the central nervous system, and myopathies involving both skeletal and cardiac muscle. The antioxidant activity of the tocopherols is in reverse order to that of their vitamin activity. Muscular tissue taken from a deficient... 

Morreale, M. and Livrea, M.A., Synergistic effect of glycolic acid on the antioxidant activity of alpha-tocopherol and melatonin in lipid bilayers and in human skin homogenates, Biochem. Mol. Biol. Int., 42, 1093-1102, 1997. 

Kontush, A., Finckh, B., Karten, B., Kohlschutter, A., and Beisiegel, U. (1996). Antioxidant and prooxidant activity of a-tocopherol in human plasma and low density lipoprotein. /. Lipid Res. 37,1436-1448. 

M. oleifera seed oil is high in the natural antioxidants - tocopherols (128) with homologues (a- P, y-, and 8-tocopherol) (42). Extraction methods influence the quantities of tocopherols extracted (2). a-Tocopherol is the primary vitamer with biological activity, while the other vitamers have been shown to have decreased activity, y-tocopherol has 10% of the activity of a-tocopherol, and 5-tocopherol has 1% of the activity of a-tocopherol (38,115). a-tocopherol content of M. oleifera leaves averages 90.0 mg/kg. However, a-, y- and 5-tocopherols are detected up to levels of 105.0, 39.5 and 77.6 mg/kg of oil, respectively as shown in Table VII (2,82). 

The naturally occurring vitamin E consists of four components, namely a, and S tocopherols (TOH). These four molecules, which differ from each other by the number and position of methyl groups attached to the phenol ring, reveal a rather different antioxidant activity. The following results show that the ordering of antioxidant activity of the tocopherols in vitro is a > y > 5, which is almost the same order as their in vivo... 

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