Coenzyme Q10-binding/transfer protein saposin B also binds γ-tocopherol

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Abstract

γ-Tocopherol, the major form of dietary vitamin E, is absorbed in the intestine and is secrete in chylomicrons, which are then transferred to liver lysosomes. Most γ-tocopherol is transferred to liver microsomes and is catabolized by cytochrome p450. Due to the hydrophobicity of γ-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 γ-tocopherol, but not α-tocopherol, as efficiently as CoQ10 at pH 7.4. At acidic pH, saposin B binds γ-tocopherol preferentially to CoQ10 and α-tocopherol. Furthermore, we confirmed that saposin B selectively binds γ-tocopherol instead of CoQ10 and α-tocopherol at every pH between 5.4 and 8.0 when all three lipids are competing for binding. We detected γ-tocopherol in human saposin B monoclonal antibody-induced immunoprecipitates from human urine, although the amount of γ-tocopherol was much smaller than that of CoQ10. These results suggest that saposin B binds and transports γ-tocopherol in human cells.

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Jin, G. Z., Horinouchi, R., Sagawa, T., Orimo, N., Kubo, H., Yoshimura, S., … Yamamoto, Y. (2008). Coenzyme Q10-binding/transfer protein saposin B also binds γ-tocopherol. Journal of Clinical Biochemistry and Nutrition, 43(2), 95–100. https://doi.org/10.3164/jcbn.2008052

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