Inhibitory effects of isomers of tocopherol on lipid peroxidation of microsomes from vitamin E-deficient rats.

NADPH-induced lipid peroxidation of hepatic microsomes from vitamin E-deficient rats has been used to assess the antioxidant effectiveness of dl alpha, d alpha- and gamma-tocopherol. When the tocopherols were added in ethanol to microsomes, the degree of inhibition of formation of thiobarbituric acid reactive substances (TBARS) decreased in the order dl alpha- greater than d alpha- greater than gamma-tocopherol. This reflected the difference in the solubility of the tocopherols in the microsomes, dl alpha-tocopherol being the most soluble and gamma-tocopherol the least. Using inhibition of TBARS produced per tocopherol content in microsome as a measure of antioxidant potency, the effectiveness of the isomers was gamma- greater than d alpha- greater than dl alpha. Despite addition of pharmacological concentrations of the isomers, it was not possible to inhibit lipid peroxidation to the same levels as were found in microsomes from vitamin E sufficient animals. Use of ethanol as a vehicle may not allow optimum orientation of the tocopherols into the lipid bilayer.