α-Tocopherol does not Accelerate Depletion of γ-Tocopherol and Tocotrienol or Excretion of their Metabolites in Rats

From an enzyme kinetic study using rat liver microsomes, alpha-tocopherol has been suggested to accelerate the other vitamin E catabolism by stimulating vitamin E omega-hydroxylation, the late limiting reaction of the vitamin E catabolic pathway. To test the effect of alpha-tocopherol on catabolism of the other vitamin E isoforms in vivo, we determined whether alpha-tocopherol accelerates depletion of gamma-tocopherol and tocotrienol and excretion of their metabolites in rats. Male Wistar rats were fed a gamma-tocopherol-rich diet for 6 weeks followed by a gamma-tocopherol-free diet with or without alpha-tocopherol for 7 days. Intake of gamma-tocopherol-free diets lowered gamma-tocopherol concentrations in serum, liver, adrenal gland, small intestine, and heart, but there was no effect of dietary alpha-tocopherol on gamma-tocopherol concentrations. The level of urinary excretion of gamma-tocopherol metabolite was not affected by dietary alpha-tocopherol. Next, the effect of alpha-tocopherol on tocotrienol depletion was examined using rats fed a tocotrienol-rich diet for 6 weeks. Subsequent intake of a tocotrienol-free diet with or without alpha-tocopherol for 7 days depleted concentrations of alpha- and gamma-tocotrienol in serum and tissues, which was accompanied by a decrease in the excretion of gamma-tocotrienol metabolite. However, neither the tocotrienol concentration nor gamma-tocotrienol metabolite excretion was affected by dietary alpha-tocopherol. These data showed that dietary alpha-tocopherol did not accelerate the depletion of gamma-tocopherol and tocotrienol and their metabolite excretions, suggesting that the positive effect of alpha-tocopherol on vitamin E omega-hydroxylase is not sufficient to affect the other isoform concentrations in tissues.