Vitamin E is a group of eight compounds, which fit into one of two classes, the tocotrienols and the tocopherols. Each class is composed of four isomers (α, β, γ and δ) which differ based on the location of the methyl groups on the chromanol ring. The tocopherols have a saturated side chain, whereas the tocotrienols have an unsaturated side chain. Vitamin E is an important lipophilic antioxidant in vivo, and plays an important role in protecting phospholipids membranes from peroxide formation. Vitamin E is thought to fit in cell membranes with the chromanol head outermost and closest to the phospholipids, and the hydrophobic phytil tail within the hydrocarbon region. Plants synthesise vitamin E and it is associated with foods high in polyunsaturated fats such as seeds. The high oxidation potential of these fats suggests that vitamin E serves the same purpose in plants as it does in animals.
Vitamin E is found throughout the body in the membranes of cells, including those of the mitochondria and endoplasmic reticulum. Evidence suggests that adipose tissue may represent a significant storage capacity for vitamin E. In adipose tissue, vitamin E is stored in an unesterified form and concentrations generally rise linearly with increasing intakes. This is in contrast to the liver, heart, lungs, muscle and adrenal glands which tend to maintain a constant concentration. In times of depletion vitamin E tends to withdraw slowly from adipose tissue, but depletion of vitamin E from other tissues including the plasma is more rapid. Obese subjects on a restricted calorie diet show rapid depletion of triglycerides from adipocytes, but the vitamin E content of the cells does not change significantly. This suggests that different mechanisms are in place for the removal of triglycerides and vitamin E.
The main storage location in adipocytes is the bulk lipids, where over 99% of the vitamin E is stored. Only a small fraction of the vitamin E is stored in the plasma membrane of adipocytes. Vitamin E is brought to the adipocytes via the plasma in chylomicrons and very low density lipoproteins. Lipoprotein lipase, an enzyme located on the endothelial lining of the capillary walls, hydrolyses the triglycerides present in the lipoproteins to form free fatty acids. The free fatty acid along with vitamin E diffuse through the capillary wall into the adipocyte. Estimates suggest that total adipocyte content of vitamin E is around 4 mmol, which is considerably higher than the content of plasma (0.4 µmol) and the liver (0.07 nmol). Over 90% of the vitamin E in the body may be stored in the adipocytes, but this may be unavailable as a labile pool to plasma because of its slow release.
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