We present here current data on the distribution and metabolism of vitamin E analogs in vivo. There are eight different naturally occurring forms of vitamin E : four tocopherols (α-,β, γ- and δ-toc) and four tocotrienols (α-,β-,γ and δ-toc). With regard to the bioavailability of vitamin E, it has been established that the affinity of various vitamin E analogs for α–tocopherol transfer protein (α-TTP), which may determine their plasma levels, is a major determinant if their biological activity however, a novel function of toc-3 has been noted as a result of its unique distribution in the skin and the adipose tissue. In addition, following the discovery that the final metabolites of Toc and Toc-3 are in the form of carboxyethyl hydroxychroman, it is now possible to examine the intermediary metabolites of vitamin E analogs. The metabolites of vitamin E is known to be involved in the actions of drug metabolic enzymes (CYP3A, CYP4F2). However, the relationship between α –TTP and the metabolic enzymes that are responsible for the regulation of vitamin E metabolism has yet to be clarified. Future research will focus on the elucidation of the vitamin E metabolic regulation system.

Tocochromanols (tocopherols and tocotrienols), collectively known as vitamin E, are essential antioxidant components of both human and animal diets. Because of their potential health benefits, there is a considerable interest in plants with increased or customized vitamin E content. Here, we have explored a new strategy to reach this goal. In plants, phenylalanine is the precursor of a myriad of secondary compounds termed phenylpropanoids. In contrast, much less carbon is incorporated into tyrosine that provides p-hydroxyphenylpyruvate and homogentisate, the aromatic precursors of vitamin E. Therefore, we intended to increase the flux of these two compounds by deriving their synthesis directly at the level of prephenate. This was achieved by the expression of the yeast (Saccharomyces cerevisiae) prephenate dehydrogenase gene in tobacco (Nicotiana tabacum) plants that already overexpress the Arabidopsis p-hydroxyphenylpyruvate dioxygenase coding sequence. A massive accumulation of tocotrienols was observed in leaves. These molecules, which were undetectable in wild-type leaves, became the major forms of vitamin E in the leaves of the transgenic lines. An increased resistance of the transgenic plants toward the herbicidal p-hydroxyphenylpyruvate dioxygenase inhibitor diketonitril was also observed. This work demonstrates that the synthesis of p-hydroxyphenylpyruvate is a limiting step for the accumulation of vitamin E in plants.