title: Coenzyme Q10-Binding/Transfer Protein Saposin B also Binds gamma-Tocopherol.
abstract: gamma-Tocopherol, the major form of dietary vitamin E, is absorbed in the intestine and is secreted in chylomicrons, which are then transferred to liver lysosomes. Most gamma-tocopherol is transferred to liver microsomes and is catabolized by cytochrome p450. Due to the hydrophobicity of gamma-tocopherol, a binding and transfer protein is plausible, but none have yet been isolated and characterized. We recently found that a ubiquitous cytosolic protein, saposin B, binds and transfers coenzyme Q10 (CoQ10), which is an essential factor for ATP production and an important antioxidant. Here, we report that saposin B also binds gamma-tocopherol, but not alpha-tocopherol, as efficiently as CoQ10 at pH 7.4. At acidic pH, saposin B binds gamma-tocopherol preferentially to CoQ10 and alpha-tocopherol. Furthermore, we confirmed that saposin B selectively binds gamma-tocopherol instead of CoQ10 and alpha-tocopherol at every pH between 5.4 and 8.0 when all three lipids are competing for binding. We detected gamma-tocopherol in human saposin B monoclonal antibody-induced immunoprecipitates from human urine, although the amount of gamma-tocopherol was much smaller than that of CoQ10. These results suggest that saposin B binds and transports gamma-tocopherol in human cells.
authors.affiliation: School of Bionics, Tokyo University of Technology, 1404-1 Katakura-cho, Hachioji, Tokyo 192-0982, Japan.
authors.forename: Guangzhi
authors.initials: G
authors.lastname: Jin
authors.forename: Ryo
authors.initials: R
authors.lastname: Horinouchi
authors.forename: Tomofumi
authors.initials: T
authors.lastname: Sagawa
authors.forename: Nobutsune
authors.initials: N
authors.lastname: Orimo
authors.forename: Hiroshi
authors.initials: H
authors.lastname: Kubo
authors.forename: Shinichi
authors.initials: S
authors.lastname: Yoshimura
authors.forename: Akio
authors.initials: A
authors.lastname: Fujisawa
authors.forename: Misato
authors.initials: M
authors.lastname: Kashiba
authors.forename: Yorihiro
authors.initials: Y
authors.lastname: Yamamoto