Figure 3.

[ATP] dependence of PfATP4-associated ATPase activity under high-[Na⁺] (150–153 mm) conditions. The PfATP4-associated ATPase activity (white circles) was calculated by subtracting the total ATPase activity measured in the presence of 500 nm cipargamin (inset, black triangles) from that measured in the absence of cipargamin (inset, black circles). The data were obtained with Dd2 parasites and are shown as the mean (± S.D.) from four independent experiments, each performed on different days with different membrane preparations. For reasons of clarity, only upward error bars are shown for the data obtained in the absence of cipargamin (inset), and only downward error bars are shown for the data obtained in the presence of cipargamin (inset). Where not shown, the error bars fall within the symbols. The addition of ATP to the reaction mixtures was found to give rise to minor effects on pH, with the pH varying from 7.4 in the absence of ATP to 7.2 in the presence of the highest [ATP] tested. The PfATP4-associated ATPase activity data were fitted with the Michaelis-Menten equation (PfATP4-associated ATPase activity = V_max × [ATP]/([ATP] + K_m(ATP))), yielding a K_m(ATP) of 0.23 ± 0.06 mm and a V_max of 24 ± 5 nmol/min/mg of protein (mean ± S.D.).