Effect of modulators on the ATPase activity and vanadate nucleotide trapping of human P-glycoprotein

Biochem Pharmacol. 1998 Sep 15;56(6):719-27. doi: 10.1016/s0006-2952(98)00212-3.


P-Glycoprotein (Pgp) is responsible for the energy-dependent efflux of many natural product oncolytics. Overexpression of Pgp may result in multidrug resistance (MDR). Modulators can block Pgp efflux and sensitize multidrug resistant cells to these oncolytics. To study the interaction of modulators with Pgp, Pgp-ATPase activity was examined, using plasma membranes isolated from the multidrug-resistant cell line CEM/VLB100. A survey of modulators indicated that verapamil, trifluoperazine, and nicardipine stimulated ATPase activity by 1.3- to 1.8-fold, whereas two others, trimethoxybenzoylyohimbine (TMBY) and vindoline, had no effect. Further evaluation showed that TMBY completely blocked the stimulation by verapamil of ATPase activity by competitive inhibition, with a Ki of 2.1 microM. When the effects of these two modulators on the formation of the enzyme-nucleotide complex important in the catalytic cycle were examined, verapamil increased the amount of vanadate-trapped 8-azido-[alpha-32P]ATP bound to Pgp by two-fold, whereas TMBY had no effect. Moreover, TMBY blocked the verapamil stimulation of vanadate-8-azido-[alpha-32P]ATP. Together, these data indicate that verapamil and TMBY bind to Pgp at a common site or overlapping sites, but only verapamil results in enhanced Pgp-ATP hydrolysis and formation of the vanadate-nucleotide-enzyme complex.

MeSH terms

  • ATP Binding Cassette Transporter, Subfamily B, Member 1 / drug effects*
  • ATP Binding Cassette Transporter, Subfamily B, Member 1 / metabolism*
  • Adenosine Triphosphatases / drug effects*
  • Adenosine Triphosphatases / metabolism*
  • Adenosine Triphosphate / analogs & derivatives*
  • Adenosine Triphosphate / metabolism
  • Antineoplastic Agents, Phytogenic / pharmacology
  • Azides / metabolism*
  • Binding Sites
  • Calcium Channel Blockers / pharmacology
  • Cell Membrane / enzymology
  • Cell Membrane / metabolism
  • Drug Interactions
  • Drug Resistance, Multiple
  • Humans
  • Hydrolysis
  • Leukemia, Lymphoid / metabolism
  • Phosphorus Radioisotopes
  • Protein Conformation
  • Stimulation, Chemical
  • Tumor Cells, Cultured
  • Vanadates / pharmacology*
  • Verapamil / pharmacology
  • Vinblastine / pharmacology
  • Yohimbine / analogs & derivatives
  • Yohimbine / pharmacology


  • ATP Binding Cassette Transporter, Subfamily B, Member 1
  • Antineoplastic Agents, Phytogenic
  • Azides
  • Calcium Channel Blockers
  • Phosphorus Radioisotopes
  • trimethoxybenzoylyohimbine
  • Yohimbine
  • Vanadates
  • 8-azidoadenosine 5'-triphosphate
  • Vinblastine
  • Adenosine Triphosphate
  • Verapamil
  • Adenosine Triphosphatases