Two transport binding sites of P-glycoprotein are unequal yet contingent: initial rate kinetic analysis by ATP hydrolysis demonstrates intersite dependence.

The ATP-dependent transport enzyme known as P-glycoprotein (P-gp) confers multidrug resistance (MDR) against many unrelated drugs and xenobiotics. To understand better the broad substrate specificity of the enzyme as well as the mechanism of substrate transport out of the cell, it is critical to characterize the substrate binding sites. Since approximately 1 ATP is hydrolyzed per transport event, phosphate release rate provides a steady-state kinetics assay. Notably, the substrate H33342 causes a decrease in the baseline hydrolysis of ATP (probably due to competition for transport with an endogenous membrane lipid substrate) providing an excellent tool for a comprehensive graphical kinetic analysis of the interaction of substrate pairs at the transport site(s) allowing the determination of inhibition type and hence characterization of transport binding sites. The substrate H33342 interacted with quinidine, progesterone, and propranolol in a non-competitive manner, indicating that binding of H33342 precludes active transport of these other substrates at a distinct site. Compounds such as TPP+ and verapamil, and perhaps also nicardipine, interacted with H33342 as mixed-type inhibitors. This type of interaction results from a reduced affinity at the opposing active site by a factor of alpha and sometimes a partial activity of a fraction beta. Indeed, H33342 binding caused a roughly four-fold reduced affinity for TPP+. Using this definitive approach to inhibition kinetics, we were able to establish traits of a second transport site in P-gp. Therefore, the sites are unequal; however, the performance at one site is contingent on the other being unoccupied, and transport is also sometimes mitigated when the other site is occupied.