The Antioxidant Activity of α-Tocopherol Metabolites in Comparison with the Parent Compound in Hepatocytes

α-Tocopherol (αT) is the major form of vitamin E. Although it is metabolized in the liver when high levels of αT are present, the function of its metabolites has not yet been fully elucidated. This study aimed to evaluate the antioxidant activities of the αT metabolites, α-carboxyethyl hydroxychroman (αCEHC) and α-carboxymethylbutyl hydroxychroman (αCMBHC), in comparison with their parent compound. The pretreatment with αT and its metabolites showed a cytoprotective effect on the cytotoxicity induced by hydrogen peroxide in the mouse hepatoma Hepa1c1c7 cells. To elucidate the mechanism underlying the effect, in vitro assays and assessments of antioxidant-related genes and proteins were conducted. The in vitro assays showed that these compounds showed little scavenging activity against hydrogen peroxide but exhibited scavenging activity against DPPH radicals. On the other hand, assessments of antioxidant-related genes and proteins showed that αT and its metabolites significantly upregulated the expression of heme oxygenase-1 (HO-1) possibly through activation of the transcript factor Nrf2. Moreover, the cytoprotective effects of αT and its metabolites against hydrogen peroxide were counteracted by HO-1 inhibitors. These results suggest that not only αT but also its metabolites are potential cytoprotective factors against oxidative stress-induced cytotoxicity, and that their effects are exerted not through a direct action of scavenging hydrogen peroxide in the body, but rather through the regulation of gene expression.