Vitamin E Actions and Metabolism

Antioxidant Activity

Vitamin E is the most potent, lipid-soluble antioxi-dant in human plasma and tissues. Thus, vitamin E protects polyunsaturated fatty acids within membrane phospholipids and plasma lipoproteins. When a peroxyl radical forms in a membrane, it is 1000 times more likely to attack a vitamin E molecule than a polyunsaturated fatty acid (Figure 2). The hydroxyl group on the chromanol ring of vitamin E reacts with the peroxyl radical to form the corresponding lipid hydroperoxide and tocopheroxyl radical. Thus, vitamin E acts as a chain-breaking antioxidant, preventing further auto-oxidation of lipids.

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.

Biologic Activity

Biologic activity is a term that has been used historically to indicate a disconnection between vitamin E antioxidant activities and in vivo activities. Observations in rodent experiments carried out in the 1930s formed the basis for determining the 'biologic activity' of vitamin E. Although the various vitamin E

Initiating event

R-OO-H Lipid hydroperoxide

Polyunsaturated fat

Carbon-centered free radical^ o2

Initiating event

Carbon-centered free radical^ o2

R-OO-H Lipid hydroperoxide a-TOH a-TO.

Figure 2 Vitamin E: chain-breaking antioxidant activity. Adapted from Burton GW and Traber MG (1990) Vitamin E: antioxidant activity, biokinetics, and bioavailability. Annu Rev Nutr 10: 357-382.

R-OO-H Lipid hydroperoxide a-TOH a-TO.

Initiation

Propagation

Termination by

«-tocopherol

Figure 2 Vitamin E: chain-breaking antioxidant activity. Adapted from Burton GW and Traber MG (1990) Vitamin E: antioxidant activity, biokinetics, and bioavailability. Annu Rev Nutr 10: 357-382.

forms had somewhat similar structures and antiox-idant activities, they differed in their abilities to prevent or reverse specific vitamin E deficiency symptoms (e.g., fetal resorption, muscular dystrophy, and encephalomalacia). a-Tocopherol with three methyl groups and a free hydroxyl group on the chromanol ring with the phytyl tail meeting the ring in the R-orientation (Figure 1) had the highest biological activity. This specific structural requirement for biological, but not chemical, activity is now known to be dependent upon the hepatic a-tocopherol transfer protein (a-TTP), as discussed below. a-TTP maintains plasma and, indirectly, tissue a-tocopherol concentrations.

Molecular Function

In addition to antioxidant activity, there are specific a-tocopherol-dependent functions that normalize cellular functions in a variety of cells. a-Tocopherol plays a critical role through its ability to inhibit the activity of protein kinase C, a central player in many signal transduction pathways. Specifically, it modulates pathways of platelet aggregation, endothelial cell nitric oxide production, monocyte/macrophage superoxide production, and smooth muscle cell proliferation. Regulation of adhesion molecule expression and inflammatory cell cytokine production by a-tocopherol has also been reported. However, most of the information in this area has been obtained from in vitro studies. More studies in humans are needed to relate a-tocopherol intakes and tissue concentrations to optimal tissue responses.

Vitamin E metabolism a- and 7-tocopherols, as well as a-and 7-tocotrienols, are metabolized to a- and 7-CEHCs (2,5,7,8-tetra-methyl- and 2,7,8-trimethyl-2-(2' carboxyethyl)-6-hydroxychromans), respectively. About 1% of a dose of a-tocopherol or tocotrienol, or 5% of a dose of 7-tocopherol or tocotrienol is excreted in the urine as CEHCs. The importance of vitamin E metabolism in the regulation of vitamin E status is unknown.

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