Vitamin E refers to a group of fat-soluble vitamins, the tocopherols, e.g. a-, p-, y-, and S-tocopherols (Figure 4.53), which are widely distributed in plants, with high levels in cereal seeds such as wheat, barley, and rye. Wheat germ oil is a particularly good source. The proportions of the individual tocopherols vary widely in different seed oils, e.g. principally p- in wheat oil, y- in corn oil, a- in safflower oil, and y- and S- in soybean oil. Vitamin E deficiency is virtually unknown, with most of the dietary intake coming from food oils and margarine, though much can be lost during processing and cooking. Rats deprived of the vitamin display reproductive abnormalities. a-Tocopherol has the highest activity (100%), with the relative activities of p-, y-, and S-tocopherols being 50%, 10%, and 3% respectively. a-Tocopheryl acetate is the main commercial form used for food supplementation and
a-tocopherol initiation of free radical reaction by peroxy radical
resonance-stabilized o ¡V
ô-tocopherol quenching of second a-tocopherol resonance-stabilized quenching of second
loss of peroxide leaving group loss of peroxide leaving group a-tocopherolquinone
for medicinal purposes. The vitamin is known to provide valuable antioxidant properties, probably preventing the destruction by free radical reactions of vitamin A and unsaturated fatty acids in biological membranes. It is used commercially to retard rancidity in fatty materials in food manufacturing, and there are also claims that it can reduce the effects of ageing and help to prevent heart disease. Its antioxidant effect is likely to arise by reacting with peroxyl radicals, generating by one-electron phenolic oxidation a resonance-stabilized free radical that does not propagate the free radical reaction, but instead mops up further peroxyl radicals (Figure 4.54). In due course, the tocopheryl peroxide is hydrolysed to the tocopherolquinone.
more favoured aromatic tautomer from the hydrolysis of the coenzyme A ester. This compound is now the substrate for alkylation and methylation as seen with ubiquinones and plastoquinones. However, the terpenoid fragment is found to replace the carboxyl group, and the decarboxylated analogue is not involved. The transformation of
1,4-dihydroxynaphthoic acid to the isoprenylated naphthoquinone appears to be catalysed by a single enzyme, and can be rationalized by the mechanism in Figure 4.56. This involves alkylation (shown in Figure 4.56 using the diketo tautomer), decarboxylation of the resultant P-keto acid, and finally an oxidation to the p-quinone.
Michael-type HO2C. ^ addition
isochorismic acid co2h
TPP succinic semi-aldehyde -TPP anion isochorismic acid
TPP-dependent decarboxylation of a-keto acid to aldehyde; nucleophilic addition of TPP anion on to aldehyde then allows removal of aldehydic proton which has become acidic
TPP succinic semi-aldehyde -TPP anion
2-oxoglutaric acid ho2C(^
Claisen-like condensation (Dieckmann reaction)
1,4-elimination of pyruvic acid
hydrolysis of thioester; enolization to more stable tautomer
CO2 C-alkylation with concomitant decarboxylation
O dehydration to O
o-succinyl form aromatic ring benzoic acid (OSB)
C-methylation menaquinone-n (vitamin K2)
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