title: Saposin B is a human coenzyme q10-binding/transfer protein.
abstract: Coenzyme Q10 (CoQ10) is essential for ATP production in the mitochondria, and is an important antioxidant in every biomembrane and lipoprotein. Due to its hydrophobicity, a binding and transfer protein for CoQ10 is plausible, but none have yet been isolated and characterized. Here we purified a CoQ10-binding protein from human urine and identified it to be saposin B, a housekeeping protein necessary for sphingolipid hydrolysis in lysosomes. We confirmed that cellular saposin B binds CoQ10 in human sperm and the hepatoma cell line HepG2 by using saposin B monoclonal antibody. The molar ratios of CoQ10 to saposin B were estimated to be 0.22 in urine, 0.003 in HepG2, and 0.12 in sperm. We then confirmed that aqueous saposin B extracts CoQ10 from hexane to form a saposin B-CoQ10 complex. Lipid binding affinity to saposin B decreased in the following order: CoQ10>CoQ9>CoQ7>>alpha-tocopherol>>cholesterol (no binding). The CoQ10-binding affinity to saposin B increased with pH, with maximal binding seen at pH 7.4. On the other hand, the CoQ10-donating activity of the saposin B-CoQ10 complex to erythrocyte ghost membranes increased with decreasing pH. These results suggest that saposin B binds and transports CoQ10 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: Hiroshi
authors.initials: H
authors.lastname: Kubo
authors.forename: Misato
authors.initials: M
authors.lastname: Kashiba
authors.forename: Ryo
authors.initials: R
authors.lastname: Horinouchi
authors.forename: Makoto
authors.initials: M
authors.lastname: Hasegawa
authors.forename: Masaru
authors.initials: M
authors.lastname: Suzuki
authors.forename: Tomofumi
authors.initials: T
authors.lastname: Sagawa
authors.forename: Mikiko
authors.initials: M
authors.lastname: Oizumi
authors.forename: Akio
authors.initials: A
authors.lastname: Fujisawa
authors.forename: Hideo
authors.initials: H
authors.lastname: Tsukamoto
authors.forename: Shinichi
authors.initials: S
authors.lastname: Yoshimura
authors.forename: Yorihiro
authors.initials: Y
authors.lastname: Yamamoto