P-glycoprotein functions as an active efflux pump for lipophilic compounds and plays an important role in the resistance of human cancers to chemotherapeutic drugs. Drug transport is powered by ATP hydrolysis at two highly conserved nucleotide-binding domains, which are proposed to be located at the cytosolic face of the protein. The ATPase activity of P-glycoprotein depends on the presence of phospholipids, and various lipids affect both basal ATPase activity and its stimulation or inhibition by drug substrates. The modulating effects of the lipid-phase state and effects on the function of the nucleotide-binding domains of P-glycoprotein have been studied in reconstituted vesicles of the synthetic phospholipids 1-palmitoyl-2-myristoylphosphatidylcholine (PamMyrGroPCho) and dimyristoylphosphatidylcholine (Myr2GroPCho). The kinetic parameters for P-glycoprotein ATPase activity were determined, and a fluorescence-quenching technique was used to measure the Kd for ATP binding. The values of both the Km for ATP hydrolysis and Kd for ATP binding were significantly different above and below the gel/liquid-crystalline phase transition temperature (tm) of PamMyrGroPCho and Myr2GroPCho, whereas they were similar at the same temperatures for P-glycoprotein in detergent solution. A discontinuity at 21-24 degrees C was observed in the Arrhenius plots of P-glycoprotein ATPase activity in a membrane environment, but not in detergent solution. In addition, the activation energies for ATP hydrolysis in the gel and liquid-crystalline phases of the lipid bilayer were significantly different. P-glycoprotein in PamMyrGroPCho bilayers displayed an unusually low activation energy just below the melting transition. These results indicate that both ATP binding and ATP hydrolysis by P-glycoprotein are affected by the phase state of the host lipids in which it is reconstituted. Lipids may modulate the function of the nucleotide-binding domains of P-glycoprotein by interacting with the transmembrane regions of the protein, or the nucleotide-binding domains themselves may interact with the surface of the bilayer.