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

The metabolism of tocopherols by omega- and beta -oxidation of the phytyl side chain has been inferred from the identification of the final products carboxyethyl-hydroxychromans (CEHC) and immediate precursors, alpha- and gamma -carboxymethylbutyl-hydroxychromans (CMBHCs). This hypothesis is here corroborated by the identification of a further alpha -tocopherol metabolite, alpha -carboxymethylhexyl-hydroxychroman (alpha -CMHHC), and evidence for the involvement of a P-450-type cytochrome. HepG2 cells, when exposed to 100 muM all-rac-alpha -tocopherol, released alpha -CEHC, alpha -CMBHC, and alpha -CMHHC into the medium. The detection of those metabolites required pretreatment of the cells with alpha -tocopherol for 10 d. In contrast, analogous metabolites of gamma and delta -tocopherol were detectable without any preconditioning. while corresponding metabolites of RRR-alpha -tocopherol could not be detected at all. The formation of alpha -CEHC from all-rac-alpha -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 omega -oxidation. In contrast, clofibrate did not reveal any effect. This observation suggests that a CYP3A-type cytochrome, initiates tocopherol metabolism by omega -oxidation. It further reveals that inducible omega -oxidation is the rate-limiting step in tocopherol metabolism. It is discussed that competition of microsomal omega -oxidation with specific binding by the alpha -tocopherol transfer protein (alpha -TTP) determines the metabolic fate of the individual tocopherols. (C) 2001 Elsevier Science Inc.