Pulmonary fibrosis is one of the most severe consequences of exposure to paraquat, an herbicide that causes rapid alveolar inflammation and epithelial cell damage. Paraquat is known to induce toxicity in cells by stimulating oxygen utilization via redox cycling and the generation of reactive oxygen intermediates. However, the enzymatic activity mediating this reaction in lung cells is not completely understood. Using self-referencing microsensors, we measured the effects of paraquat on oxygen flux into murine lung epithelial cells. Paraquat (10-100 microm) was found to cause a 2-4-fold increase in cellular oxygen flux. The mitochondrial poisons cyanide, rotenone, and antimycin A prevented mitochondrial- but not paraquat-mediated oxygen flux into cells. In contrast, diphenyleneiodonium (10 microm), an NADPH oxidase inhibitor, blocked the effects of paraquat without altering mitochondrial respiration. NADPH oxidases, enzymes that are highly expressed in lung epithelial cells, utilize molecular oxygen to generate superoxide anion. We discovered that lung epithelial cells possess a distinct cytoplasmic diphenyleneiodonium-sensitive NAD(P)H:paraquat oxidoreductase. This enzyme utilizes oxygen, requires NADH or NADPH, and readily generates the reduced paraquat radical. Purification and sequence analysis identified this enzyme activity as thioredoxin reductase. Purified paraquat reductase from the cells contained thioredoxin reductase activity, and purified rat liver thioredoxin reductase or recombinant enzyme possessed paraquat reductase activity. Reactive oxygen intermediates and subsequent oxidative stress generated from this enzyme are likely to contribute to paraquat-induced lung toxicit
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