Distinct trans-plasma membrane redox pathways reduce cell-impermeable dyes in HeLa cells.

Trans-plasma membrane electron transport (tPMET) in mammalian cells has been demonstrated using artificial cell-impermeable dyes, but the extent to which reduction of these dyes involves distinct pathways remains unclear. Here we compare the properties of three commonly used dyes, WST-1, FeCN and DCIP. The presence of an intermediate electron carrier (mPMS or CoQ(1)) was obligatory for WST-1 reduction, whereas FeCN and DCIP were reduced directly. FeCN reduction was, however, greatly enhanced by CoQ(1), whereas DCIP was unaffected. Superoxide dismutase (SOD) and aminooxyacetate (AOA), a malate/aspartate shuttle inhibitor, strongly inhibited WST-1 reduction and reduced DCIP reduction by 40-60%, but failed to affect FeCN reduction, indicating involvement of mitochondrial TCA cycle-derived NADH and a possible role for superoxide in WST-1 but not FeCN reduction. Reduction of all three substrates was similarly inhibited by dicoumarol, diphenyleneiodonium and capsaicin. These results demonstrate that WST-1 FeCN and DCIP are reduced by distinct tPMET pathways.