Evidence for NAD(P)H:quinone oxidoreductase 1 (NQO1)-mediated quinone-dependent redox cycling via plasma membrane electron transport: A sensitive cellular assay for NQO1

2,3-Dimethoxy 1,4-naphthoquinone (DMNQ), which redox cycles via two-electron reduction, mediates reduction of the cell-impermeative tetrazolium dye WST-1 in kidney epithelial cells (MDCK), which express high levels of NQO1, but not in HL60 or CHO cells, which are NQO1 deficient. DMNQ-dependent WST-1 reduction by MDCK cells was strongly inhibited by low concentrations of the NQO1 inhibitor dicoumarol and was also inhibited by diphenyleneiodonium, capsaicin, and superoxide dismutase (SOD), but not by the uncoupler FCCP or the complex IV inhibitor cyanide. This suggests that DMNQ-dependent WST-1 reduction by MDCK cells is catalyzed by NQO1 via redox cycling and plasma membrane electron transport (PMET). Interestingly, we observed an association between DMNQ/WST-1 reduction and extracellular H2O2 production as determined by Amplex red. Exposure of MDCK cells to DMNQ for 48 h caused cellular toxicity that was extensively reversed by co-incubation with dicoumarol or exogenous SOD, catalase, or N-acetylcysteine. No effects were observed in NQO1-deficient CHO and HL60 cells. In conclusion, we have developed a simple real-time cellular assay for NQO1 and show that PMET plays a significant role in DMNQ redox cycling via NQO1, leading to cellular toxicity in cells with high NQO1 levels.