Effects of various tocopherol-containing diets on tocopherol secretion into bile

Gamma-tocopherol is abundant in common vegetable oil, but its concentration in plasma and liver is much lower than that of alpha-tocopherol. Discrimination between different forms of tocopherol is thought to take place via the hepatic alpha-tocopherol transfer protein (alpha-TTP). Gamma-tocopherol, with a low binding capacity to alpha-TTP, is thought to be excreted via the bile. Our previous studies showed that gamma-tocopherol administered with sesame seed exhibits significantly higher concentrations in the plasma and liver of rats than gamma-tocopherol alone. Thus, we attempted to confirm whether a much higher amount of gamma-tocopherol rather than alpha-tocopherol would be secreted in the bile, and whether sesame seed would suppress the secretion of gamma-tocopherol. In one experiment, we examined the concentrations of alpha- or gamma-tocopherol in the plasma, liver, and bile of rats fed diets containing 300 mg/kg of alpha-tocopherol, 300 mg/kg of gamma-tocopherol, or 300 mg/kg each of alpha-tocopherol + gamma-tocopherol, and in the other experiment, we compared the gamma-tocopherol concentrations of rats fed a diet of gamma-tocopherol alone to those of rats fed a gamma-tocopherol + sesame seed diet (each diet contained 300 mg/kg gamma-tocopherol). The bile collection was done over 6 h. The gamma-tocopherol concentration in the bile was markedly lower than that of alpha-tocopherol, paralleling the concentrations in the plasma and liver. Intake of alpha-tocopherol and gamma-tocopherol together further lowered the concentration of gamma-tocopherol in the bile as well as in the plasma and liver, compared to the intake of gamma-tocopherol alone. The gamma-tocopherol concentration in the bile, as well as in the plasma and liver, was markedly higher in the sesame seed-fed group than in the gamma-tocopherol alone group. We found that the concentrations of alpha- or gamma-tocopherol in the bile showed a good correlation with the concentrations of alpha- or gamma-tocopherol in the liver, though the concentrations in the bile were substantially lower than those in the liver. These findings suggest that secretion into the bile is not a major metabolic route of alpha- or gamma-tocopherol.