Figure 1.

ATPase activity of P-gp nanodiscs in the presence of the two drugs. Average and standard deviation of duplicate samples for two separate preparations of P-gp nanodisc from two separate P-gp preparations are shown. Samples included 2.5 mM ATP. The ATP hydrolysis rate is shown as fold stimulation of basal (at 2.5 mM) activity, which averaged 38 nmol Pi/min/mg P-gp. The presence of vinblastine stimulates ATP hydrolysis at low concentrations but inhibits hydrolysis at higher concentrations. Zosuquidar stimulates ATP hydrolysis at all concentrations probed but is not completely soluble above 100 μM. Arrows indicate drug concentrations used for HDX-MS (8 or 80 μM vinblastine and 40 μM zosuquidar). Fits to the classical two-binding site (stimulatory/inhibitory) equation are for visualization only as standard errors of fits are too large for interpretation.