Vitamin E Trafficking


Address for correspondence: Maret G. Traber, Ph.D., Linus Pauling Institute, Oregon State University, Corvallis, OR 97331-6512. Voice: 541-737-7977; fax: 541-737-5077.


Abstract: The α-tocopherol transfer protein (α-TTP) is required to prevent vitamin E deficiency in humans and in α-TTP null mice. Whereas α-TTP is not required to facilitate intestinal absorption of vitamin E, it is required to maintain normal α-tocopherol concentrations in plasma and extrahepatic tissues. α-Tocopherol secretion from the liver in very low density lipoproteins (VLDLs) is impaired in humans with a defect in the α-TTP gene. In perfusions of isolated cynomolgus monkey livers, VLDLs were preferentially enriched in RRR-α-tocopherol. The mechanism by which α-TTP incorporates α-tocopherol into nascent VLDLs is the topic of this report. VLDL assembly is a multistep secretory process that occurs within the membrane compartments of the endoplasmic reticulum and Golgi apparatus. Thus, we postulated that α-TTP might transfer α-tocopherol onto nascent VLDLs either in the endoplasmic reticulum or in the Golgi apparatus. To test these possibilities, we isolated nascent VLDLs from highly purified RER and Golgi apparatus membrane fractions from livers of rats fed equimolar ratios of RRR- and SRR-α-tocopherols labeled with different amounts of deuterium. Although the plasma was enriched in RRR-α-tocopherol 14 hours after the dose, no enrichment of nascent VLDL precursors from either of the secretory compartments was detected, indicating that VLDL enrichment with α-tocopherol may occur as a post-VLDL secretory process. Therefore, we hypothesize that α-TTP may facilitate movement of α-tocopherol to the hepatocyte plasma membrane (by unknown mechanisms) where newly secreted, nascent VLDLs could acquire both α-tocopherol and unesterified cholesterol while within the space of Disse. Clearly, critical information is lacking in our understanding of the mechanism by which α-TTP facilitates the preferential enrichment of VLDLs with α-tocopherol.