P-glycoprotein (Pgp) mediates drug transport utilizing the energy released from ATP hydrolysis. However, the mechanism by which Pgp couples these two reactions remains unclear. The present work is undertaken to describe kinetically the first step, which is the interdependence of nucleotide and drug binding to the Pgp by the use of vanadate. Preincubation of human Pgp expressed in Sf9 insect cells with vanadate in the presence of Mg2+, ATP, and verapamil resulted in nearly complete and stable inhibition of the drug-stimulated ATPase function. In contrast, the Pgp ATPase function was nearly unaffected when Mg2+, ATP, or verapamil was omitted. Inhibition was highly specific for divalent cations that support ATP hydrolysis, for nucleotides that serve as substrates of hydrolysis, and for those drugs/compounds that interact with the drug-binding/transport sites of the Pgp. Kinetic analysis indicated that vanadate inhibition was MgATP concentration-dependent with an apparent Ki value similar to the apparent Km, suggesting that MgATP was bound to a similar ATP-binding site in both the ATPase inhibition and activation reactions. In support of this conclusion, vanadate, in the presence of Mg2+ and verapamil, caused selective trapping of 8-azido [alpha-32P] ATP and covalent labeling of ATP-binding site in the Pgp. Differences were observed in the vanadate-induced inhibition of wild-type and Val185 mutant Pgp's with different drug/compounds. These results suggested that the affinity of the interacting drug/compound is a constant and influences the overall stability of the inhibited Pgp species. Possible implications of these observations for the coupling of ATP hydrolysis to drug transport are discussed.