protein kinase C activity, which is involved in cell proliferation and differentiation, in smooth muscle cells (Boscoboinik et al., 1991; Chatelain et al., 1993; Clement et al., 1997; Stauble et al., 1994; Tasinato et al., 1995), human platelets (Freedman et al., 1996), and monocytes (Cachia et al., 1998; Devaraj et al., 1996). Protein kinase C inhibition by α-tocopherol is in part attributable to its attenuating effect on the generation of membrane-derived diacylglycerol, a lipid that facilitates protein kinase C translocation, thus increasing its activity (Kunisaki et al., 1994; Tran et al., 1994).

Vitamin E enrichment of endothelial cells downregulates the expression of intercellular cell adhesion molecule (ICAM-1) and vascular cell adhesion molecule-1 (VCAM-1), thereby decreasing the adhesion of blood cell components to the endothelium (Cominacini et al., 1997). Vitamin E also upregulates the expression of cytosolic phospholipase A2 (Chan et al., 1998a; Tran et al., 1996) and cyclooxygenase-1 (Chan et al., 1998b). The enhanced expression of these two rate-limiting enzymes in the arachidonic acid cascade explains the observation that vitamin E, in a dose-dependent fashion, enhanced the release of prostacyclin, a potent vasodilator and inhibitor of platelet aggregation in humans (Szczeklik et al., 1985; Tran and Chan, 1990).

Physiology of Absorption, Metabolism, and Excretion
Absorption and Transport

Intestinal Absorption. While the efficiency of vitamin E absorption is low in humans, the precise rate of absorption is not known with certainty. In the early 1970s, vitamin E absorption was estimated to be 51 to 86 percent, measured as fecal radioactivity following ingestion of α-tocopherol (Kelleher and Losowsky, 1970; MacMahon and Neale, 1970). However, when Blomstrand and Forsgren (1968) measured vitamin E absorption in two individuals with gastric carcinoma and lymphatic leukemia, respectively, they found fractional absorption in the lymphatics to be only 21 and 29 percent of label from meals containing α-tocopherol and α-tocopheryl acetate, respectively.

Vitamin E absorption from the intestinal lumen is dependent upon biliary and pancreatic secretions, micelle formation, uptake into enterocytes, and chylomicron secretion. Defects at any step lead to impaired absorption (Gallo-Torres, 1970; Harries and Muller, 1971; Sokol, 1993; Sokol et al., 1983, 1989). Chylomicron



The National Academies | 500 Fifth St. N.W. | Washington, D.C. 20001
Copyright © National Academy of Sciences. All rights reserved.
Terms of Use and Privacy Statement