α-Tocopherol binding proteins

Vitamin E is the generic term for two groups of naturally occurring fat-soluble compounds, the tocopherols and the tocotrienols (Kasperek, 1980). In the tocopherols, the side chain is saturated; in the tocotrienols, it is unsaturated. The members of each group are designated α, β, γ, and δ depending on the number and position of methyl groups attached to the chroman nucleus. The most biologically active of these compounds is α-tocopherol, which inhibits the peroxidation of membrane lipids. This protection is dependent on the incorporation of α-tocopherol into membranes, and the extent of protection is related to the quantity of tocopherol present in membranes. Vitamin E is found primarily in intracellular membranes at concentrations of one to two orders of magnitude higher than that in the soluble fraction of various tissues (Taylor et al., 1976). In tissue homogenates, α-tocopherol is associated with amphiphilic and protein molecules and does not occur in the free form.

The differing biological activities of vitamin E vitamers could be the result of mechanisms operating at various levels: intestinal absorption, lymph and plasma transport, level of uptake and retention by tissues, and finally, their rates of catabolism. Orally administered radioactive tocopherol appears rapidly in the plasma and tissues of the rat and chicken (Krishnamurthy & Bieri, 1963) The vitamin is transported by lymph and plasma proteins (Peake et al., 1972). Despite various studies on the absorption of α-tocopherol in animals and in man, the mechanism of absorption is still not well understood.