Vitamin E kinetics and the function of tocopherol regulatory proteins

Plasma and tissue alpha -tocopherol concentrations are remarkably stable, w hich suggests that they are regulated. alpha -Tocopherol transfer protein, tocopherol-associated protein, and tocopherol-binding protein bind alpha -t ocopherol. These proteins might function as tocopherol regulatory proteins, although only tocopherol transfer protein has been shown to influence plas ma and tissue alpha -tocopherol concentrations. Tissue alpha -tocopherol co ncentrations likely depend on tocopherol. regulatory protein function and t issue lipid content, vitamin E uptake and efflux, oxidative stress, and int eractions between vitamin E and other antioxidants. Pharmacokinetic models often divide tissues into rapidly perfused, slowly perfused, and very slowl y perfused compartments. Tissue vitamin E concentrations might equilibrate more rapidly in tissues with greater perfusion, greater vitamin E uptake, i ncreased amounts or activities of tocopherol regulatory protein, and lower lipid contents. The rate at which tissue concentrations approach equilibriu m, however, does not predict the final equilibrium concentrations because o f redistribution among tissues. Redistribution of vitamin E to adipose tiss ue from other tissues may be significant. Intracellular trafficking of vita min E might occur in conjunction with membrane recycling because membrane c onstituents rapidly recycle between the plasma membrane and intracellular e ndocytic compartments, Thus, tocopherol regulatory proteins may modulate ra ther than directly regulate vitamin E tissue distribution and intracellular trafficking. Nutrition 2001;17:799-805. (C) Elsevier Science Inc. 2001.