ATP occlusion by P-glycoprotein as a surrogate measure for drug coupling.

The multidrug efflux pump P-glycoprotein (Pgp) couples drug transport to ATP hydrolysis. Previously, using a synthetic library of tetramethylrosamine (TMR) analogues, we observed significant variation in ATPase stimulation (V-m(D)). Concentrations required for half-maximal ATPase stimulation (K-m(D)) correlated with ATP hydrolysis transition-state stabilization and ATP occlusion (EC50D) at a single site. Herein, we characterize several TMR analogues that elicit modest turnover (k(cat) <= 1-2 s(-1)) compared to verapamil (VER) (k(cat) similar to 10 s(-1)). Apparent ATPase activities manifest as nearly equivalent to basal values. In some cases, K I parameters for drug stimulation of ATPase could not be accurately determined, yet these same TMR analogues promoted ATP occlusion at relatively low concentrations (similar to 0.4-40 mu M). Moreover, the TMR analogues competitively inhibited VER-dependent ATPase activity at concentrations similar to those required for ATP occlusion. Finally, the TMR analogues facilitated uptake of calcein-AM into CR1R12 and MDCK-MDR1 cells and are actively transported by Pgp in monolayers of MDCK-MDR1 cells at similarly low concentrations (similar to 1-20 mu M). ADP center dot V-i release kinetics were identical in the presence of the TMR derivatives, VER, or in the absence of drug, suggesting that slow turnover is not likely due to slow release of the ATP hydrolysis products ADP and Pi. These data support the partition model in which drug site occupancy converts residual basal ATPase activity to a drug-dependent mechanism even in cases where stimulation appears to be exactly compensatory to basal values. It is noteworthy that when compared to previously reported TMR analogues, subtle modification of the TMR scaffold can confer large differences in ATP turnover.