Binding of α-tocopherylquinone, an oxidized form of α-tocopherol, to glutathione-S-transferase in the liver cytosol

α-Tocopherol (vitamin E) is an important fat-soluble antioxidant in biological systems and, as a result of scavenging reactive oxygen, it is converted to α-tocopherylquinone. α-Tocopherol binds to α-tocopherol transfer protein (αTTP) in the liver cytosol, whereas α-tocopherylquinone does not. We found that α-tocopherylquinone binds to a liver protein with a molecular mass of about 40 kDa that is distinct from αTTP. This α-tocopherylquinone binding protein was purified further by multiple-step column chromatography. Sodium dodecylsulfate-polyacrylamide gel electrophoresis of the final preparation yielded a single band with an apparent molecular mass of 25 kDa, which microsequencing revealed was identical to glutathione-S-transferase (GST). The GST activity was inhibited in the presence of α-tocopherylquinone, as it is by other non-substrate ligands for GST, confirming that GST and α-tocopherylquinone interact directly. α-Tocopherylquinone binds to GST and may be transported to the site of metabolism or excreted in the bile as other non-substra.