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Toxicol Appl Pharmacol. 2018 Nov 15;359:102-107. doi: 10.1016/j.taap.2018.09.011. Epub 2018 Sep 15.

Quinone and nitrofurantoin redox cycling by recombinant cytochrome b5 reductase.

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Department of Environmental and Occupational Health, Rutgers University School of Public Health, Piscataway, NJ 08854, United States.
Department of Pharmacology and Toxicology, Ernest Mario School of Pharmacy, Rutgers University, Piscataway, NJ 08854, United States.
Department of Pharmaceutical Sciences, Northeast Ohio Medical University, Rootstown, OH 44272, United States.
Department of Environmental Health Science, New York Medical College, Valhalla, NY 10595, United States.
Department of Environmental and Occupational Health, Rutgers University School of Public Health, Piscataway, NJ 08854, United States. Electronic address:


NADH cytochrome b5 reductase mediates electron transfer from NADH to cytochrome b5 utilizing flavin adenine dinucleotide as a redox cofactor. Reduced cytochrome b5 is an important cofactor in many metabolic reactions including cytochrome P450-mediated xenobiotic metabolism, steroid biosynthesis and fatty acid metabolism, hemoglobin reduction, and methionine and plasmalogen synthesis. Using recombinant human enzyme, we discovered that cytochrome b5 reductase mediates redox cycling of a variety of quinones generating superoxide anion, hydrogen peroxide, and, in the presence of transition metals, hydroxyl radicals. Redox cycling activity was oxygen-dependent and preferentially utilized NADH as a co-substrate; NADH was 5-10 times more active than NADPH in supporting redox cycling. Redox cycling activity was greatest for 9,10-phenanthrenequinone and 1,2-naphthoquinone, followed by 1,4-naphthoquinone and 2-methyl-1,4-naphthoquinone (menadione), nitrofurantoin and 2-hydroxyestradiol. Using menadione as the substrate, quinone redox cycling was found to inhibit reduction of cytochrome b5 by cytochrome b5 reductase, as measured by heme spectral changes in cytochrome b5. Under anaerobic conditions where redox cycling is inhibited, menadione had no effect on the reduction of cytochrome b5. Chemical redox cycling by cytochrome b5 reductase may be important in generating cytotoxic reactive oxygen species in target tissues. This activity, together with the inhibition of cytochrome b5 reduction by redox-active chemicals and consequent deficiencies in available cellular cytochrome b5, are likely to contribute to tissue injury following exposure to quinones and related redox active chemicals.


Cytochrome b5; Free radicals; Reactive oxygen species; Redox cycling


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