Tetrahydroxy-1,4-benzoquinone (THQ) is a highly redox-active substance that generates reactive oxygen species (ROS), which can induce apoptosis in cell culture experiments. The underlying mechanism for ROS production has previously been postulated to be the autoxidation of THQ to rhodizonic acid (RhA). However, our results suggest that the cells detoxify THQ by reducing it to hexahydroxybenzene (HHB), catalyzed by the NADPH-quinone-oxidoreductase (NQO1). Then, HHB undergoes autoxidation back to THQ, closing a redox cycle that continuously generates ROS. Only this continuous mechanism produces enough ROS to trigger apoptosis. The cell's protective measures can effectively eliminate the ROS generated by a single autoxidation of THQ to RhA because RhA is not reduced back to THQ and thus does not close a redox cycle. This also explains why only fresh THQ solutions are cytotoxic, whereas older THQ solutions, which are readily autoxidized to RhA, are not.
Keywords: ROS; THQ; Tetrahydroxy-1,4-benzoquinone; apoptosis; redox cycle.