Electron transport-mediated wasteful consumption of NADH promotes the lethal response of U937 cells to tert-butylhydroperoxide.

The toxicity of a short-term exposure to tert-butylhydroperoxide in U937 cells was markedly reduced by chemically or experimentally induced respiratory deficiency. Rotenone mitigated the lethal effects of the hydroperoxide over the same concentration-range in which the complex I inhibitor inhibited oxygen utilization. U937 cells that were made respiration deficient by growing them in the presence of either chloramphenicol or ethidium bromide, were in both circumstances highly resistant to tert-butylhydroperoxide. The improved survival was not a direct consequence of the absence of electron transport, but rather was attributable to the large amounts of NADH which accumulate in the mitochondria of chemically hypoxic or respiration-deficient cells. Indeed, the toxicity elicited by tert-butylhydroperoxide was also abolished by supplementation with either of two different NADH-linked substrates, namely pyruvate or beta-hydroxybutyrate. Accumulation of intramitochondrial NADH, and the resulting cytoprotective effects, was associated with prevention of the loss of nonprotein sulphydryls and mitochondrial membrane potential. Neither rotenone nor pyruvate reduced the toxicity of tert-butylhydroperoxide in thiol-depleted cells. Taken together, these results indicate that depletion of mitochondrial NADH is a critical event in the cytotoxic response to tert-butylhydroperoxide since this pyridine nucleotide prevents mitochondrial dysfunction and cell death caused by the hydroperoxide. As a consequence, in hydroperoxide-treated cells electron transport is highly detrimental since it consumes mitochondrial NADH.