Figure 3. Michaelis-Menten enzyme kinetics for Drosophila melanogaster soluble guanylyl cyclase subunits. Extracts from cells transiently transfected with the subunits shown were assayed for guanylyl cyclase activity with GTP concentrations of 0.1 mM to 5 mM in the presence of Mg2+ (A & C) or Mn2+ (B & D). Values for the Vmax and Km were calculated from the untransformed data using the following equation: Rate = (Vmax × [GTP])/(Km + [GTP]) using GraphPad Prism 4.0 and are shown in Table 2. Each point represents mean ± SEM. of 2–3 determinations.