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.
Keywords: coenzyme Q10; saposin B; α-tocopherol; γ-tocopherol.