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J. Biol. Chem., Vol. 279, Issue 47, 48742-48750, November 19, 2004

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Are Respiratory Enzymes the Primary Sources of Intracellular Hydrogen Peroxide?^*

Lauren Costa Seaver and James A. Imlay

From the Department of Microbiology, University of Illinois, Urbana, Illinois 61801

Endogenous H₂O₂ is believed to be a source of chronic damage in aerobic organisms. To quantify H₂O₂ formation, we have generated strains of Escherichia colithat lack intracellular scavenging enzymes. The H₂O₂ that is formed within these mutants diffuses out into the medium, where it can be measured. We sought to test the prevailing hypothesis that this H₂O₂ is primarily generated by the autoxidation of redox enzymes within the respiratory chain. The rate of H₂O₂ production increased when oxygen levels were raised, confirming that H₂O₂ is formed by an adventitious chemical process. However, mutants that lacked NADH dehydrogenase II and fumarate reductase, the most oxidizable components of the respiratory chain in vitro, continued to form H₂O₂ at normal rates. NADH dehydrogenase II did generate substantial H₂O₂ when it was when overproduced or quinones were absent, forcing electrons to accumulate on the enzyme. Mutants that lacked both NADH dehydrogenases respired very slowly, as expected; however, these mutants showed no diminution of H₂O₂ excretion, suggesting that H₂O₂ is primarily formed by a source outside the respiratory chain. That source has not yet been identified. In respiring cells the rate of H₂O₂ production was 0.5% the rate of total oxygen consumption, with only modest changes when cells used different carbon sources.

Received for publication, August 2, 2004 , and in revised form, September 9, 2004.

^* This study was supported by National Institutes of Health Grant GM49640. The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

Recipient of National Institutes of Health Training Grant GM07283.

To whom correspondence should be addressed. Tel.: 217-333-5812; Fax: 217-244-6697; E-mail: jimlay{at}uiuc.edu.

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