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Papers In Press, published online ahead of print June 21, 2002
microbiology, university of Illinois Urbana-Champaign, Urbana, IL 61801
Corresponding Author: jimlay{at}uiuc.edu
When cells are exposed to external H2O2, the H2O2 diffuses inside and oxidizes ferrous iron, forming hydroxyl radicals that damage DNA. The process of oxidative DNA damage requires only H2O2, free iron, and an as-yet unidentified electron donor that reduces ferric iron to ferrous iron. Previous work showed that H2O2 kills E. coli rapidly when respiration is inhibited either by cyanide or by genetic defects in respiratory enzymes. Here, we established that these respiratory blocks accelerate the rate of DNA damage. The respiratory blocks did not substantially affect the amounts of intracellular free iron or H2O2, indicating that that they accelerated damage because they increased the availability of the electron donor. The goal of this work was to identify that donor. As expected, respiratory inhibitors increased the amount of intracellular NADH. However, NADH itself was a poor reductant of free iron in vitro, suggesting that in non-respiring cells electrons are transferred from NADH to another carrier that directly reduces the iron. Genetic manipulations of intracellular amounts of glutathione, NADPH, a-ketoacids, ferredoxin, and thioredoxin indicated that none of these was the direct electron donor. However, cells were protected from cyanide-stimulated DNA damage if they lacked flavin reductase, an enzyme that transfers electrons from NADH to free FAD. The Km of this enzyme for NADH is higher than the usual intracellular NADH concentration, which explains why its flux increased when NADH levels rose during respiratory inhibition. Flavins reduced by purified flavin reductase rapidly transferred electrons to free iron and drove a DNA-damaging Fenton system in vitro. Thus the rate of oxidative DNA damage can be limited by the rate at which electron donors reduce free iron, and reduced flavins become the predominant donors in E. coli when respiration is blocked. It remains unclear what reductants drive Fenton chemistry in respiring cells.
J. Biol. Chem, 10.1074/jbc.M203977200
Submitted on April 24, 2002
Revised on June 14, 2002
Accepted on June 21, 2002
Reduced flavins promote oxidative DNA damage in non-respiring E. coli by delivering electrons to intracellular free iron
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