Tocopherol-Binding Proteins: Their Function and Physiological Significance

The present review is a continuation of earlier essays on the uptake mechanisms and the biological function of vitamin E. There are eight naturally occurring homologues of vitamin E, which differ in their structure and in biological activity in vivo and in vitro. Various studies have suggested that after normal gastrointestinal absorption of dietary vitamin E specific mechanisms favor the preferential accumulation of one of its homologues, α-tocopherol, in the human body. This process is thought to be mediated in part by the α-tocopherol transfer protein (α-TTP) in the liver cytoplasm. The mechanism and pathway by which α-TTP specifically incorporates α-tocopherol into plasma lipoproteins is not yet fully understood. Because α-tocopherol is widely distributed in tissues in various concentrations but α-TTP resides only in liver, its role as intracellular carrier of α-tocopherol seems unlikely. However, recent data indicate that a system of α-tocopherol-binding proteins is involved in these processes that favor the localization of α-tocopherol at the sites where it is required. The current status of the evidence for the regulation of α-tocopherol levels and their impact on cellular signaling is discussed.