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Membrane compartments

Figure 2 Absorption, transport, and metabolism of a-tocopherol (a-T) and 7-tocopherol (7-T) in peripheral tissues. 1: Both a-T and 7-T are absorbed without preference by the intestine along with lipid and reassembled into chylomicrons. 2: Exchange between apolipoproteins of the chylomicrons (types AI, AII, and B48) and high-density lipoprotein (HDL) (types C and E) occurs. 3: Chylomicrons are degraded to remnants by lipoprotein lipase (LPL) and some a-T and 7-T are transported to peripheral tissues. 4: The resulting chylomicron remnants are then taken up by the liver. 5: In the liver, most of the remaining a-T, but only a small fraction of 7-T, is reincorporated in nascent very low-density lipoproteins (VLDLs) by a-tocopherol transfer protein (a-TTP). 6: Plasma phospholipid transfer protein (PLTP) facilitates the exchange of tocopherol between HDL and LDL for delivery to tissues. 7: Plasma tocopherols are delivered to tissues by LDL and HDL. 8: Tocopherol-associated proteins (TAPs) probably facilitate intracellular tocopherol transfer between membrane compartments. 9: Substantial amounts of 7-T are degraded by a cytochrome P450-mediated reaction to 2,7,8-trimethyl-2-(^-carboxyethyl-6-hydroxychroman (7-CEHC). 10: 7-CEHC is excreted into urine. Adapted from Azzi A and Stocker A (2000) Vitamin E: Non-antioxidant roles. Progress in Lipid Research 39: 231-255; and from Jiang Q, Christen S, Shigenaga MK and Ames BN (2001) 7-Tocopherol, the major form of vitamin E in the US diet, deserves more attention. American Journal of Clinical Nutrition 74: 714-722.

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