Double reciprocal plots from initial velocity data of the ArsH reactions. Measurements were performed at 30 °C in 0.1 M sodium phosphate buffer (pH 7.0). The reaction was started by the addition of ArsH, and the decrease in absorbance at 340 nm was recorded. Plot (A), NADPH:FMN oxidoreductase. ArsH concentration, 205 nM. The fixed concentrations of NADPH are 5 (circles), 20 (squares), 50 (rhombs) and 150 μM (triangles). Plot (B), NADPH:PhAsO(OH)2 oxidoreductase. ArsH concentration, 1.4 μM. FMN concentration, 50 μM. The fixed concentrations of PhAsO(OH)2 are 10 (squares), 30 (circles), 100 (rhombs) and 1000 μM (triangles).