Abstract

α-Tocopherolquinone (TQ), a product of α-tocopherol oxidation, can function as an antioxidant after reduction to α-tocopherolhydroquinone (TQH₂). We examined the ability of human NAD(P)H:quinone oxidoreductase (NQO1) to catalyze the reduction of TQ to TQH₂ in cell-free and cellular systems. In reactions with purified human NQO1, TQ was reduced to TQH₂. Kinetic parameters for the reduction of TQ by NQO1 (K _m = 370 μm;k _cat = 5.6 × 10^{3 min −1};k _cat/ K_m = 15 min⁻¹ · μm ⁻¹) indicate that NQO1 can efficiently reduce TQ to TQH₂. A comparison of the rate of reduction of TQ and coenzyme Q₁₀ by NQO1 showed that TQ is reduced more efficiently than coenzyme Q₁₀. Experiments with either Chinese hamster ovary (CHO) cells stably transfected with human NQO1 or CHO cell sonicates demonstrated a correlation between NQO1 activity and TQ reduction to TQH₂. CHO cells with elevated NQO1 generated and maintained higher levels of TQH₂ after treatment with TQ relative to NQO1-deficient CHO cells. TQH₂generated from NQO1-mediated reduction of TQ prevented cumene hydroperoxide-induced lipid peroxidation in rat liver microsomes. In addition, cumene hydroperoxide-induced lipid peroxidation was inhibited more efficiently by TQ in CHO cell lines with elevated NQO1 activity. These data demonstrate that NQO1 can reduce TQ to TQH₂ and that TQH₂ can function as an efficient antioxidant. This work suggests that one of the physiological functions of NQO1 may be to regenerate antioxidant forms of α-tocopherol.