secretion is required for vitamin E absorption and was suggested by Muller et al. (1974) to be the most important factor for efficient vitamin E absorption. All of the various vitamin E forms studied, including α- and γ-tocopherols (Meydani et al., 1989; Traber and Kayden, 1989; Traber et al., 1992), RRR- and SRR-α-tocopherols (Kiyose et al., 1997; Traber et al., 1990a, 1992), or RRR- and all racα-tocopherols (Traber et al., 1994a), showed similar apparent efficiencies of intestinal absorption and subsequent secretion in chylomicrons. During chylomicron catabolism, some vitamin E is distributed to all of the circulating lipoproteins (Figure 6-4).
Preferential Secretion of α-Tocopherol from the Liver. Chylomicron remnants, containing newly absorbed vitamin E, are taken up by the liver. Vitamin E is secreted from the liver in very low density lipoproteins (VLDLs), as demonstrated in rats (Cohn et al., 1988), isolated rat hepatocytes (Bjørneboe et al., 1987; Cohn et al., 1988), and perfused monkey livers (Traber et al., 1990b). Plasma vitamin E concentrations depend upon the secretion of vitamin E from the liver, and only one form of vitamin E, α-tocopherol, is preferentially resecreted by the liver (Figure 6-5) (Traber, 1999). Thus, the liver, not the intestine, discriminates between tocopherols and is responsible for the preferential plasma enrichment with α-tocopherol, α-TTP is a likely candidate for this discriminatory function (see below).
Hepatic α-Tocopherol Transfer Protein (α-TTP). α-TTP was first identified (Catignani and Bieri, 1977), purified, and characterized from rat liver cytosol (Sato et al., 1991; Yoshida et al., 1992). It has also been isolated from human liver cytosol (Kuhlenkamp et al., 1993), and the human complementary deoxyribonucleic acid (cDNA) sequence has been reported (Arita et al., 1995). The human cDNA sequence (encoding 238 amino acids) has 94 percent homology to the rat sequence, and the some similarity to sequences for the retinaldehyde binding protein in the retina and to sec14, a phospholipid transfer protein (Arita et al., 1995).
In vitro, α-TTP transfers α-tocopherol between liposomes and microsomes (Hosomi et al., 1997; Sato et al., 1991). The relative affinities of α-TTP toward the various forms of vitamin E (calculated from the degree of competition with RRR-α-tocopherol) are RRR-α-tocopherol = 100 percent; RRR-β-tocopherol = 38 percent; RRR-γ-tocopherol = 9 percent; RRR-δ-tocopherol = 2 percent; α-tocopheryl acetate = 2 percent; α-tocopheryl quinone = 2 percent; SRR-α-tocopherol = 11 percent; α-tocotrienol = 12 percent; and Trolox = 9