Mechanisms of ligand transfer by the hepatic tocopherol transfer protein

Abstract

α-Tocopherol is a member of the vitamin E family that functions as the principal fat-soluble antioxidant in vertebrates. Body-wide distribution of tocopherol is regulated by the hepatic α-tocopherol transfer protein (αTTP), which stimulates secretion of the vitamin from hepatocytes to circulating lipoproteins. This biological activity of αTTP is thought to stem from its ability to facilitate the transfer of vitamin E between membranes, but the mechanism by which the protein exerts this activity remains poorly understood. Using a fluorescence energy transfer methodology, we found that the rate of tocopherol transfer from lipid vesicles to αTTP increases with increasing αTTP concentration. This concentration dependence indicates that ligand transfer by αTTP involves direct protein-membrane interaction. In support of this notion, equilibrium analyses employing filtration, dual polarization interferometry, and tryptophan fluorescence demonstrated the presence of a stable αTTP-bilayer complex. The physical association of αTTP with membranes is markedly sensitive to the presence of vitamin E in the bilayer. Some naturally occurring mutations in αTTP that cause the hereditary disorder ataxia with vitamin E deficiency diminish the effect of tocopherol on the protein-membrane association, suggesting a possible mechanism for the accompanying pathology. © 2008 by The American Society for Biochemistry and Molecular Biology, Inc.