The optimal concentrations of substrate, enzyme and co-factor NADPH were determined. (A) The HTRF signals generated by increasing amount of testosterone in the presence and absence of 15 nmol/L aromatase. (B) Michaelis-Menton plots of aromatase activity as a function of the concentration of the substrate testosterone. (C) Time course at various concentrations of aromatase. The detection time was set at 0, 15, 30, 45, 60, 120, and 240 min. (D) Activity of aromatase was raised with the increasing concentration of NADPH (0.3 nmol/L to 10 000 nmol/L). Values are represented as mean±SEM of three independent experiments (n=3).