Mechanisms of Absorption, Transport and Tissue Uptake of RRR-α-Tocopherol and d-γ-Tocopherol in the White Rat

Abstract

The metabolism of α- and γ-tocopherol was studied in three groups of rats that were fed a modified AIN-76 diet containing normal (NE, 0.2 g α-tocopherol/kg), high (HE, 1.0 g α-tocopherol/kg) or low (LE, <0.02 α-tocopherol/kg) vitamin E for 3 mo. After 1, 2 and 3 d of an oral dose of 20 mg of α-tocopherol, γ-tocopherol or both, the levels of the two vitamers were measured in plasma and tissues and in some cases in isolated microsomal and mitochondrial fractions from liver. Twenty-four hours after an oral dose of 20 mg γ-tocopherol the levels of α-tocopherol in plasma and tissues remained constant and higher levels of γ-tocopherol were found in tissues in which low α-tocopherol levels could be found such as in the LE group. In spite of this, it was enabled to remain there, after 2 and 3 d γ-tocopherol had decreased levels in all tissues. When given in combination with α-tocopherol, the levels of γ-tocopherol were lower than when γ-tocopherol was given alone. Microsomes and mitochondria from livers of LE group bound five and nine times more α-tocopherol than γ-tocopherol in rats dosed with equal amount of α- or γ-tocopherol, respectively. These data indicate that the mechanisms that regulate the metabolism of vitamin E are highly specific for α-tocopherol. Moreover, the relative amount of α-tocopherol determined the levels of γ-tocopherol in tissues. However, the retention of γ-tocopherol in tissues did not depend on the presence of α-tocopherol. This observation may be due to preferential binding of α-tocopherol by soluble proteins and cellular membranes such as those of microsomes and mitochrondria and could well explain why α-tocopherol is the vitamer with more vitamin E potency.