Evidence for redox cycling of lawsone (2‐hydroxy‐1,4‐naphthoquinone) in the presence of the hypoxanthine/xanthine oxidase system

This study reports that lawsone (2‐hydroxy‐1,4‐naphthoquinone) undergoes redox cycling in the presence of the hypoxanthine/xanthine oxidase system. The rate of cytochrome c reduction obtained in the presence of 80 µM lawsone was almost three times the rate of cytochrome c reduction measured in its absence. This increase in the rate of cytochrome c reduction was partially inhibited by superoxide dismutase, suggesting the involvement of O 2 •− in this process. It is remarkable to note that, even though lawsone is considered to be a non‐redox‐cycling quinone in vitro , this quinone was shown to be more toxic in vivo in rats than menadione, causing haemolytic anemia of an oxidative nature and renal damage. The view that this quinone is a non‐redox‐cycling quinone was based on the inability of one‐electron‐transferring avoenzymes such as NADPH‐cytochrome c reductase to reduce this naphthoquinone. Our nding that lawsone, like menadione, undergoes redox cycling in the presence of the hypoxanthine/xanthine oxidase system could explain the observed oxidative damage of tissues inicted by this quinone in rats in vivo . Such an observation therefore reconciles the in vivo toxicity results of this naphthoquinone with those of in vitro experiments. Copyright © 2003 John Wiley & Sons, Ltd.