13'-carboxy-alpha-tocopherol is a dehydrogenation carboxylate product of 13'-hydroxy-a-tocopherol by an unidentified microsomal enzyme(s) probably via an aldehyde intermediate. The tocopherols ( a-tocopherol , b-tocopherol ,r-tocopherol and d-tocopherol ) and their corresponding tocotrienols are synthesized by plants and have vitamin E antixoidant activity (see pathway vitamin E biosynthesis ). They differ in the number and location of methyl groups on the chromanol ring. The naturally occurring form of a-tocopherol is (2R,4'R,8'R)-a-tocopherol (synonym (R,R,R)-a-tocopherol). Synthetic a-tocopherols are a racemic mixture of eight different R and S stereoisomers. Only the 2R forms are recognized as meeting human requirements. The in vivo function of vitamin E is to scavenge peroxyl radicals via its phenolic (chromanol) hydroxyl group, thus protecting lipids against free radical-catalyzed peroxidation. The tocopheryl radical formed can then be reduced by reductants such as L-ascorbate. Other major products of a-tocopherol oxidation include &#945;-tocopherylquinone and epoxy-a-tocopherols. The metabolites a-tocopheronic acid and its lactone, known as the Simon metabolites, are generally believed to be artefacts. In addition to these oxidation products, the other major class of tocopherol metabolites is the carboxyethyl-hydroxychromans.These metabolites are produced in significant amounts in response to excess vitamin E ingestion. Vitamin E is fat-soluble and its utilization requires intestinal fat absorption mechanisms. It is secreted from the intestine into the lymphatic system in chylomicrons which subsequently enter the plasma. Lipolysis of these chylomicrons can result in delivery of vitamin E to tissues, transfer to high-density lipoproteins (and subsequently to other lipoproteins via the phospholipid exchange protein), or retention in chylomicron remnants. These remnants are taken up by the liver. Natural (R,R,R)-&#945;-tocopherol and synthetic 2R-&#945;-tocopherols are then preferentially secreted from the liver into plasma as a result of the specificity of the &#945;-tocopherol transfer protein. This protein, along with the metabolism of excess vitamin E in the liver and excretion into urine and bile, mediate the supply of a-tocopherol in plasma and tissues.