title: Mechanisms of ligand transfer by the hepatic tocopherol transfer protein.
abstract: alpha-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 alpha-tocopherol transfer protein (alphaTTP), which stimulates secretion of the vitamin from hepatocytes to circulating lipoproteins. This biological activity of alphaTTP 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 alphaTTP increases with increasing alphaTTP concentration. This concentration dependence indicates that ligand transfer by alphaTTP 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 alphaTTP-bilayer complex. The physical association of alphaTTP with membranes is markedly sensitive to the presence of vitamin E in the bilayer. Some naturally occurring mutations in alphaTTP 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.
authors.affiliation: Cornell University, Ithaca, NY 14853, USA.
authors.forename: Samantha
authors.initials: S
authors.lastname: Morley
authors.forename: Matt
authors.initials: M
authors.lastname: Cecchini
authors.forename: Wendy
authors.initials: W
authors.lastname: Zhang
authors.forename: Alessandro
authors.initials: A
authors.lastname: Virgulti
authors.forename: Noa
authors.initials: N
authors.lastname: Noy
authors.forename: Jeffrey
authors.initials: J
authors.lastname: Atkinson
authors.forename: Danny
authors.initials: D
authors.lastname: Manor
mesh_terms: D000818:Animals; D002352:Carrier Proteins / Q000737:chemistry*; D004530:Egg Yolk / Q000378:metabolism; D031541:Fluorescence Resonance Energy Transfer; D006801:Humans; D007368:Interferometry; D007700:Kinetics; D008024:Ligands; D008051:Lipid Bilayers / Q000737:chemistry; D008074:Lipoproteins / Q000737:chemistry; D008099:Liver / Q000378:metabolism*; D008958:Models, Molecular; D011994:Recombinant Proteins / Q000737:chemistry; D013050:Spectrometry, Fluorescence / Q000379:methods; D014364:Tryptophan / Q000737:chemistry; D014810:Vitamin E / Q000737:chemistry
chemical_list: D002352:Carrier Proteins; D008024:Ligands; D008051:Lipid Bilayers; D008074:Lipoproteins; D011994:Recombinant Proteins; C017763:alpha-tocopherol transfer protein; D014810:Vitamin E; D014364:Tryptophan