Fig. 4.

Effects of sulfa drugs on enzymes known to mediate chemical redox cycling. Human sepiapterin reductase (hSPR; 60 nM), human cytochrome P450 reductase (hOR; 1 nM), rat cytosolic TrxR (rTrxR1; 30 nM), or human glutathione reductase (hGR; 70 nM) was incubated in 50 mM potassium phosphate buffer (pH 7.8) with menadione (MD; 200 μM) in the absence or presence of sulfasalazine (SSZ; 100 μM) or sulfamethoxazole (SMX; 100 μM). The reactions were initiated by the addition of NADPH (200 μM) and H₂O₂ production was measured by the Amplex Red assay. Data are presented as percentage of activity of control enzyme reactions run in the absence of inhibitors. Data are the mean ± S.E. (n = 3). *Significantly different (P < 0.05) from menadione-treated samples in the absence of inhibitors.