Abstract
NADH cytochrome b(5) reductase mediates electron transfer from NADH to cytochrome b(5) utilizing flavin adenine dinucleotide as a redox cofactor. Reduced cytochrome b(5) 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 b(5) by cytochrome b(5) reductase, as measured by heme spectral changes in cytochrome b(5). Under anaerobic conditions where redox cycling is inhibited, menadione had no effect on the reduction of cytochrome b(5). Chemical redox cycling by cytochrome b(5) reductase may be important in generating cytotoxic reactive oxygen species in target tissues. This activity, together with the inhibition of cytochrome b(5) reduction by redox-active chemicals and consequent deficiencies in available cellular cytochrome b(5), are likely to contribute to tissue injury following exposure to quinones and related redox active chemicals.