Fatty acid metabolism and cell proliferation. VII. Antioxidant effects of tocopherols and their quinones.

The antioxidant capacities of alpha- and gamma-tocopherols (alpha-E and gamma-E) and their quinones (alpha-EQ and gamma-EQ) were determined in non-biological and biological systems. The non-biological system consisted of arachidonic acid [20:4 (n-6)], the oxidant cumene hydroperoxide, and a Fe3+ catalyst to facilitate malondialdehyde (MDA) formation from lipid peroxides. alpha-E and gamma-E had similar antioxidant capacities in this system. alpha-EQ also functioned as an antioxidant, while gamma-EQ exhibited a crossover effect by functioning as an antioxidant at low concentrations and a prooxidant at high concentrations. Biological lipid peroxidation in smooth muscle cells challenged with 20:4 (n-6) was measured both by MDA formation in confluent cultures and by cell growth in proliferating cultures. alpha-E, gamma-E and alpha-EQ had similar antioxidant capacities, but gamma-EQ was highly cytotoxic for cells in both confluent and proliferating cultures. Cellular retention of antioxidants was estimated indirectly from MDA formation when cells were loaded with an antioxidant (preincubation) and then incubated for varying periods of time in fresh media containing 20:4 (n-6). Cellular retention also was measured directly with tritiated alpha-E and tritiated alpha-EQ. These studies showed that cellular retention decreased in the sequence gamma-E greater than alpha-E greater than alpha-EQ. Thus, cellular retention does not explain the enhanced antioxidant capacity of alpha-E compared to gamma-E that has been reported for animal systems. The antioxidant capacity of alpha-E evidently is enhanced by its metabolism to a quinone which, unlike the quinone from gamma-E, functions as a biological antioxidant.