Tocopherols are metabolized in HepG2 cells by side chain ω-oxidation and consecutive β-oxidation

The metabolism of tocopherols by ω- and β-oxidation of the phytyl side chain has been inferred from the identification of the final products carboxyethyl-hydroxychromans (CEHC) and immediate precursors, α- and γ-carboxymethylbutyl-hydroxychromans (CMBHCs). This hypothesis is here corroborated by the identification of a further α-tocopherol metabolite, α-carboxymethylhexyl-hydroxychroman (α-CMHHC), and evidence for the involvement of a P450-type cytochrome. HepG2 cells, when exposed to 100 μM all-rac-α-tocopherol, released α-CEHC, α-CMBHC, and α-CMHHC into the medium. The detection of those metabolites required pretreatment of the cells with α-tocopherol for 10 d. In contrast, analogous metabolites of γ and δ-tocopherol were detectable without any preconditioning, while corresponding metabolites of RRR-α-tocopherol could not be detected at all. The formation of α-CEHC from all-rac-α-tocopherol was enhanced up to 5-fold by pretreatment of the HepG2 cells with rifampicin, known to induce CYP3A-type cytochromes with the capability of catalyzing ω-oxidation. In contrast, clofibrate did not reveal any effect. This observation suggests that a CYP3A-type cytochrome initiates tocopherol metabolism by ω-oxidation. It further reveals that inducible ω-oxidation is the rate-limiting step in tocopherol metabolism. It is discussed that competition of microsomal ω-oxidation with specific binding by the α-tocopherol transfer protein (α-TTP) determines the metabolic fate of the individual tocopherols.