Differential effects of redox-cycling and arylating quinones on trans-plasma membrane electron transport

Cytotoxicity of quinones has been attributed to free radical generation and to arylation of cellular nucleophiles. For redox-cycling quinones, cell injury is associated with mitochondrial permeability transition, whereas arylating quinones directly depolarise the mitochondrial membrane and deplete ATP. Like mitochondrial electron transport, plasma membrane electron transport (PMET), plays a multifaceted role in cellular redox homeostasis but the effects of quinones on PMET are unknown. Here we investigate the effects of redox-cycling 2,3-dimethoxy-1,4-naphthoquinone (DMNQ), arylating 1,4-benzoquinone (BQ) and mixed mechanism 2-methyl-1,4-naphthoquinone (MNQ) on PMET, viability and growth of P815 mouse mastocytoma cells. BQ and MNQ rapidly and extensively inhibited PMET as determined by WST-1/mPMS reduction (IC50 3.5-5 mu M at 30 min) whereas the effects of DMNQ were less pronounced. In contrast, MTT reduction (cytosolic NADH dehydrogenase activity over 30 min) was weakly inhibited by BQ (IC50 20 mu M) but not by MNQ or DMNQ and cell viability was unaffected. Inhibition of WST-1/mPMS reduction by BQ and MNQ but not DMNQ was fully reversed by NAC. Treatment with DMNQ, MNQ and to a lesser extent BQ inhibited cell proliferation as determined by MTT reduction at 48 h. The effects of BQ and MNQ were reversed by NAC through covalent bonding to BQ and MNQ, but not DMNQ. These results show that arylating quinones are more potent inhibitors of PMET than pure redox-cycling quinones, but that redox-cycling quinones are more cytotoxic.