Absorption Metabolism and Excretion

Because of its hydrophobicity, vitamin E requires special transport mechanisms in the aqueous environment of plasma, body fluids, and cells. In humans, vitamin E is taken up in the proximal part of the intestine depending on the amount of food lipids, bile, and pancreatic esterases that are present. It is emulsified together with the fat-soluble components of food. Lipolysis and emulsification of the formed lipid droplets then lead to the spontaneous formation of mixed micelles, which are absorbed at the brush border membrane of the mucosa by passive diffusion. Both a- and 7-tocopherol and dietary fat are taken up without preference by the intestine and secreted in chylo-micron particles together with triacylglycerol and cholesterol (Figure 2). The nearly identical incorporation of a- and 7-tocopherol in chylomicrons after supplementation with equal amounts of the two tocopherols indicates that their absorption is not selective (Figure 2). The chylomicrons are stored as secretory granula and eventually excreted by exocytosis to the lymphatic compartment, from which they reach the bloodstream via the ductus thoraczcus. The exchange between the apolipoproteins of the chylomicrons (types AI, AII, and B48) and high-density lipoprotein (HDL) (types C and E) triggers the intravascular degradation of the chylomicrons to remnants by the endothelial lipoprotein lipase (LPL) and is a prerequisite for the hepatic uptake of tocopherols (Figure 2). During LPL-mediated catabolism of chylomicron particles, some of the chylomicron-bound vitamin E appears to be transported and transferred to peripheral tissues, such as muscle, adipose, and brain (Figure 2). The formation of remnants favors the rapid uptake of the tocopherols via the hepatic receptors for apo-E and apo-B.

The chylomicron remnants are subsequently taken up by the liver, where a-tocopherol is preferentially

Chylomicron

Chylomicron

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