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DRI DIETARY REFERENCE INTAKES FOR Vitamin C, Vitamin E, Selenium, and Carotenoids
FIGURE 6-5RRR-α-Tocopherol is preferentially resecreted by the liver and distributed to circulating lipoproteins. NOTE: HDL = high-density lipoprotein; LDL = low-density lipoprotein; VLDL = very low-density lipoprotein.
SOURCE: Adapted from Traber (1999).
In three people with ataxia and vitamin E deficiency (AVED) secondary to a defect in the α-TTP gene (Cavalier et al., 1998), the half-lives for both RRR- and SRR-α-tocopherols were approximately 13 hours (Traber et al., 1994b). These studies demonstrate that RRR- and SRR-α-tocopherols in the AVED patients disappear at the same rate as SRR-α-tocopherol in the control subjects. This suggests that α-TTP, which is defective in the AVED patients, is responsible for the longer half-life of RRR-α-tocopherol in the control subjects. It was estimated that resecretion of RRR-α-tocopherol by the liver in the control subjects resulted in the daily replacement of nearly all of the circulating RRR-α-tocopherol. Thus, the liver maintains plasma RRR-α-tocopherol concentrations by a continuous resecretion process. In contrast, other forms of vitamin E (e.g., SRR-α- and γ-tocopherols) are not resecreted into the plasma.
Oxidation Products. α-Tocopherol can be oxidized to the tocopheroxyl radical—one-electron oxidation product—which can be reduced back to the unoxidized form by reducing agents such as vitamin C. Further oxidation of the tocopheroxyl radical forms