title: Functionally similar vanadate-induced 8-azidoadenosine 5'-[alpha-(32)P]Diphosphate-trapped transition state intermediates of human P-glycoprotin are generated in the absence and presence of ATP hydrolysis.
abstract: P-glycoprotein (Pgp) is an ATP-dependent drug efflux pump whose overexpression confers multidrug resistance to cancer cells. Pgp exhibits a robust drug substrate-stimulable ATPase activity, and vanadate (Vi) blocks this activity effectively by trapping Pgp nucleotide in a non-covalent stable transition state conformation. In this study we compare Vi-induced [alpha-(32)P]8-azido-ADP trapping into Pgp in the presence of [alpha-(32)P]8-azido-ATP (with ATP hydrolysis) or [alpha-(32)P]8-azido-ADP (without ATP hydrolysis). Vi mimics P(i) to trap the nucleotide tenaciously in the Pgp.[alpha-(32)P]8-azido-ADP.Vi conformation in either condition. Thus, by using [alpha-(32)P]8-azido-ADP we show that the Vi-induced transition state of Pgp can be generated even in the absence of ATP hydrolysis. Furthermore, half-maximal trapping of nucleotide into Pgp in the presence of Vi occurs at similar concentrations of [alpha-(32)P]8-azido-ATP or [alpha-(32)P]8-azido-ADP. The trapped [alpha-(32)P]8-azido-ADP is almost equally distributed between the N- and the C-terminal ATP sites of Pgp in both conditions. Additionally, point mutations in the Walker B domain of either the N- (D555N) or C (D1200N)-terminal ATP sites that arrest ATP hydrolysis and Vi-induced trapping also show abrogation of [alpha-(32)P]8-azido-ADP trapping into Pgp in the absence of hydrolysis. These data suggest that both ATP sites are dependent on each other for function and that each site exhibits similar affinity for 8-azido-ATP (ATP) or 8-azido-ADP (ADP). Similarly, Pgp in the transition state conformation generated with either ADP or ATP exhibits drastically reduced affinity for the binding of analogues of drug substrate ([(125)I]iodoarylazidoprazosin) as well as nucleotide (2'(3')-O-(2,4,6-trinitrophenyl)adenosine 5'-triphosphate). Analyses of Arrhenius plots show that trapping of Pgp with [alpha-(32)P]8-azido-ADP (in the absence of hydrolysis) displays an approximately 2.5-fold higher energy of activation (152 kJ/mol) compared with that observed when the transition state intermediate is generated through hydrolysis of [alpha-(32)P]8-azido-ATP (62 kJ/mol). In aggregate, these results demonstrate that the Pgp.[alpha-(32)P]8-azido-ADP (or ADP).Vi transition state complexes generated either in the absence of or accompanying [alpha-(32)P]8-azido-ATP hydrolysis are functionally indistinguishable.
authors: Laboratory of Cell Biology, Center for Cancer Research, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20892-4255, USA.
authors: Z E
authors: ZE
authors: Sauna
authors: M M
authors: MM
authors: Smith
authors: M
authors: M
authors: Muller
authors: S V
authors: SV
authors: Ambudkar
mesh_terms: D018435:ATP Binding Cassette Transporter, Subfamily B / Q000737:chemistry / Q000378:metabolism*; D000244:Adenosine Diphosphate / Q000031:analogs & derivatives* / Q000493:pharmacokinetics*; D000255:Adenosine Triphosphate / Q000031:analogs & derivatives* / Q000378:metabolism* / Q000493:pharmacokinetics*; D000345:Affinity Labels / Q000493:pharmacokinetics; D000818:Animals; D001386:Azides / Q000493:pharmacokinetics*; D001665:Binding Sites; D002460:Cell Line; D002462:Cell Membrane / Q000378:metabolism; D006367:HeLa Cells; D006801:Humans; D006868:Hydrolysis; D007313:Insecta; D007700:Kinetics; D010761:Phosphorus Radioisotopes; D011994:Recombinant Proteins / Q000378:metabolism; D013816:Thermodynamics; D014162:Transfection; D014638:Vanadates / Q000494:pharmacology*
chemical_list: D018435:ATP Binding Cassette Transporter, Subfamily B; D000345:Affinity Labels; D001386:Azides; D010761:Phosphorus Radioisotopes; D011994:Recombinant Proteins; D014638:Vanadates; C017040:8-azidoadenosine 5'-triphosphate; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; C021622:8-azidoadenosine diphosphate