Is thymoquinone an antioxidant?

Background Thymoquinone is one of the active ingredients of black cumin (Nigella sativa L.) essential oil possessing antiinflammatory, antineoplastic, neuroand hepatoprotective properties. Some of these properties were attributed to an antioxidant activity of thymoquinone, which seems to be unlikely from its structure. Because the lipophilic thymoquinone exhibits a structural similarity with the natural mitochondrial electron carrier, ubiquinone, it was of interest whether the suggested antioxidant effect of thymoquinone in cells can be explained by its interaction with the mitochondrial respiratory chain.