FIGURE 4.

Characterization of redox cycling by human recombinant sepiapterin reductase. Panel A, redox cycling of different quinones by sepiapterin reductase. H₂O₂ formation was measured in the absence or presence of 5 μm 9,10-phenanthrenequinone, 500 μm menadione, or 500 μm dimethoxy-1,4-naphthoquinone. Data are the average of three independent measurements. SPR was added at the indicated time. Panel B, effects of increasing concentrations of NADPH on menadione redox cycling; inset, effects of increasing concentrations of menadione on redox cycling activity. Panel C, inhibition of menadione redox cycling by 100 μm benzoquinone or phenylquinone; inset, inability of benzoquinone and phenylquinone to redox cycle with sepiapterin reductase. Panel D, effects of benzoquinone and phenylquinone on sepiapterin reductase activity. Enzyme activity was analyzed by changes in absorbance of sepiapterin at 420 nm.