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The Cell-based L-Glutathione Protection Assays to Study Endocytosis and Recycling of Plasma Membrane Proteins

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Published on: December 13, 2013

Coenzyme Q10-Binding/Transfer Protein Saposin B also Binds gamma-Tocopherol.

Guangzhi Jin¹, Ryo Horinouchi, Tomofumi Sagawa

¹School of Bionics, Tokyo University of Technology, 1404-1 Katakura-cho, Hachioji, Tokyo 192-0982, Japan.

Journal of Clinical Biochemistry and Nutrition

|September 27, 2008

Summary

Saposin B, a protein, binds gamma-tocopherol (a form of vitamin E) and may transport it within human cells. This finding suggests a new role for saposin B in vitamin E transport.

Keywords:

coenzyme Q10 saposin B α-tocopherol γ-tocopherol

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Area of Science:

Biochemistry
Cell Biology
Nutritional Science

Background:

Gamma-tocopherol, the primary dietary form of vitamin E, undergoes absorption and processing in the liver.
Its hydrophobicity suggests a need for a binding and transfer protein, which has not yet been identified.
Saposin B is known to bind and transfer coenzyme Q10 (CoQ10).

Purpose of the Study:

To investigate whether saposin B binds gamma-tocopherol.
To determine the selectivity of saposin B binding for gamma-tocopherol compared to other lipids like alpha-tocopherol and CoQ10 across different pH levels.
To explore the potential role of saposin B in gamma-tocopherol transport in human cells.

Main Methods:

In vitro binding assays were performed to assess saposin B's affinity for gamma-tocopherol, alpha-tocopherol, and CoQ10 at various pH conditions.
Competitive binding experiments were conducted with all three lipids present.
Immunoprecipitation using human saposin B monoclonal antibodies was employed to detect gamma-tocopherol in human urine samples.

Main Results:

Saposin B demonstrated binding to gamma-tocopherol, but less efficiently than CoQ10 at neutral pH (7.4).
At acidic pH, saposin B preferentially bound gamma-tocopherol over CoQ10 and alpha-tocopherol.
Selective binding of gamma-tocopherol was observed across a pH range of 5.4 to 8.0 when competing with CoQ10 and alpha-tocopherol.
Gamma-tocopherol was detected in immunoprecipitates from human urine, indicating its presence in conjunction with saposin B.

Conclusions:

Saposin B binds gamma-tocopherol, with preferential binding observed at acidic pH.
The findings suggest that saposin B plays a role in the binding and transport of gamma-tocopherol within human cells.
This study identifies a potential mechanism for gamma-tocopherol cellular trafficking mediated by saposin B.