Residues in P-glycoprotein catalytic sites that react with the inhibitor 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole

The Switch and Reciprocating Models for the Function of ABC Multidrug Exporters: Perspectives on Recent Research

ATP-binding cassette (ABC) transporters comprise a large superfamily of primary active transporters, which are integral membrane proteins that couple energy to the uphill vectorial transport of substrates across cellular membranes, with concomitant hydrolysis of ATP. ABC transporters are found in all living organisms, coordinating mostly import in prokaryotes and export in eukaryotes. Unlike the highly conserved nucleotide binding domains (NBDs), sequence conservation in the transmembrane domains (TMDs) is low, with their divergent nature likely reflecting a need to accommodate a wide range of substrate types in terms of mass and polarity. An explosion in high resolution structural analysis over the past decade and a half has produced a wealth of structural information for ABCs. Based on the structures, a general mechanism for ABC transporters has been proposed, known as the Switch or Alternating Access Model, which holds that the NBDs are widely separated, with the TMDs and NBDs together forming an intracellular-facing inverted "V" shape. Binding of two ATPs and the substrate to the inward-facing conformation induces a transition to an outward conformation. Despite this apparent progress, certainty around the transport mechanism for any given ABC remains elusive. How substrate binding and transport is coupled to ATP binding and hydrolysis is not known, and there is a large body of biochemical and biophysical data that is at odds with the widely separated NBDs being a functional physiological state. An alternative Constant Contact model has been proposed in which the two NBSs operate 180 degrees out of phase with respect to ATP hydrolysis, with the NBDs remaining in close proximity throughout the transport cycle and operating in an asymmetric allosteric manner. The two models are discussed in the light of recent nuclear magnetic resonance and hydrogen-deuterium exchange mass spectrometry analyses of three ABC exporters.

Measurement of protein sulfenic acid content

Protein sulfenic acids are reactive, reversibly oxidized cysteinyl residues with roles in redox catalysis and regulation. Detection and quantification of these species in proteins is accomplished through chemical modification by reagents such as 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD chloride), 2-nitro-5-thiobenzoate (TNB), dimedone, or derivatives of dimedone, followed by UV-visible spectroscopy or mass spectrometric analysis.

Enhanced transport of P-glycoprotein substrate saquinavir in presence of thiolated chitosan

It was the aim of this study to investigate the effect of chitosan-4-thiobutylamidine (Ch-TBA) and reduced glutathione (GSH) on the absorption of P-glycoprotein (P-gp) and multidrug resistance protein (MRP) substrate saquinavir in vitro and in vivo. Bidirectional transport studies were performed with Caco-2 cell monolayers and additionally with freshly excised rat small intestinal mucosa mounted in Ussing type chambers. Furthermore, a delivery system based on Ch-TBA and GSH was evaluated in vivo in rats. The functional activity of the efflux pumps in Caco-2 cells and rat intestinal mucosa during the experiment was proven by the efflux ratio of saquinavir, which was 6.4 for Caco-2 cells and 2.1 for rat intestinal mucosa, respectively. Ch-TBA and particularly the combination of Ch-TBA with GSH enhanced apical (AP) absorption and decreased the secretory transport of saquinavir. In presence of 0.5% Ch-TBA and 0.5% GSH, the uptake of saquinavir was 1.6-fold improved in Caco-2 monolayer and 2.1-fold improved in rat intestinal mucosa. In vivo, the area under the plasma concentration time curve (AUC) of saquinavir was 1.4-fold and Cmax 1.6-fold increased, in comparison with control. Results of this study showed that Ch-TBA in combination with GSH can be an interesting tool for increasing the oral bioavailability of actively secreted compounds.

Glutathione and thiolated chitosan inhibit multidrug resistance P-glycoprotein activity in excised small intestine

The aim of the present study was to evaluate the influence of glutathione (GSH), the thiomer chitosan-4-thiobutylamidine (chitosan-TBA) and a combination of both compounds on P-glycoprotein (P-gp) activity. Permeation studies were performed with freshly excised guinea pig ileum mounted in Ussing chambers using the fluorescent dye rhodamine-123 (Rho-123) as P-gp substrate. Apparent permeability coefficients (Papp) as well as efflux ratios (secretory Papp/absorptive Papp) were calculated and compared with values gained from experiments with the well-established P-gp inhibitors terfenadine and verapamil. In the presence of terfenadine, verapamil as well as GSH, the absorptive transport of Rho-123 across intestinal tissue increased, while the secretory decreased with efflux ratios around 1.0. Chitosan-TBA and especially chitosan-TBA/GSH not only enhanced absorption of Rho-123, but also reduced the basolateral to apical secretion of Rho-123 resulting in efflux ratios of 1.1, 0.8 and 0.5. The study indicates that chitosan-TBA/GSH is a potentially valuable tool for inhibiting the ATPase activity of P-gp in the intestine.

Location of the rhodamine-binding site in the human multidrug resistance P-glycoprotein

The human multidrug resistance P-glycoprotein (P-gp) pumps a wide variety of structurally diverse compounds out of the cell. It is an ATP-binding cassette transporter with two nucleotide-binding domains and two transmembrane (TM) domains. One class of compounds transported by P-gp is the rhodamine dyes. A P-gp deletion mutant (residues 1-379 plus 681-1025) with only the TM domains retained the ability to bind rhodamine. Therefore, to identify the residues involved in rhodamine binding, 252 mutants containing a cysteine in the predicted TM segments were generated and reacted with a thiol-reactive analog of rhodamine, methanethiosulfonate (MTS)-rhodamine. The activities of 28 mutants (in TMs 2-12) were inhibited by at least 50% after reaction with MTS-rhodamine. The activities of five mutants, I340C(TM6), A841C(TM9), L975C(TM12), V981C(TM12), and V982C(TM12), however, were significantly protected from inhibition by MTS-rhodamine by pretreatment with rhodamine B, indicating that residues in TMs 6, 9, and 12 contribute to the binding of rhodamine dyes. These results, together with those from previous labeling studies with other thiol-reactive compounds, dibromobimane, MTS-verapamil, and MTS-cross-linker substrates, indicate that common residues are involved in the binding of structurally different drug substrates and that P-gp has a common drug-binding site. The results support the "substrate-induced fit" hypothesis for drug binding.

Structure of the ArsA ATPase: the catalytic subunit of a heavy metal resistance pump

Active extrusion is a common mechanism underlying detoxification of heavy metals, drugs and antibiotics in bacteria, protozoa and mammals. In Escherichia coli, the ArsAB pump provides resistance to arsenite and antimonite. This pump consists of a soluble ATPase (ArsA) and a membrane channel (ArsB). ArsA contains two nucleotide-binding sites (NBSs) and a binding site for arsenic or antimony. Binding of metalloids stimulates ATPase activity. The crystal structure of ArsA reveals that both NBSs and the metal-binding site are located at the interface between two homologous domains. A short stretch of residues connecting the metal-binding site to the NBSs provides a signal transduction pathway that conveys information on metal occupancy to the ATP hydrolysis sites. Based on these structural features, we propose that the metal-binding site is involved directly in the process of vectorial translocation of arsenite or antimonite across the membrane. The relative positions of the NBS and the inferred mechanism of allosteric activation of ArsA provide a useful model for the interaction of the catalytic domains in other transport ATPases.

Large scale purification of detergent-soluble P-glycoprotein from Pichia pastoris cells and characterization of nucleotide binding properties of wild-type, Walker A, and Walker B mutant proteins

P-glycoprotein (Pgp; mouse MDR3) was expressed in Pichia pastoris, grown in fermentor culture, and purified. The final pure product is of high specific ATPase activity and is soluble at low detergent concentration. 120 g of cells yielded 6 mg of pure Pgp; >4 kg of cells were obtained from a single fermentor run. Properties of the pure protein were similar to those of previous preparations, except there was significant ATPase activity in absence of added lipid. Mutant mouse MDR3 P-glycoproteins were purified by the same procedure after growth of cells in flask culture, with similar yields and purity. This procedure should open up new avenues of structural, biophysical, and biochemical studies of Pgp. Equilibrium nucleotide-binding parameters of wild-type mouse MDR3 Pgp were studied using 2'-(3')-O-(2,4,6-trinitrophenyl)adenosine tri- and diphosphate. Both analogs were found to bind with K(d) in the low micromolar range, to a single class of site, with no evidence of cooperativity. ATP displacement of the analogs was seen. Similar binding was seen with K429R/K1072R and D551N/D1196N mutant mouse MDR3 Pgp, showing that these Walker A and B mutations had no significant effect on affinity or stoichiometry of nucleotide binding. These residues, known to be critical for catalysis, are concluded to be involved primarily in stabilization of the catalytic transition state in Pgp.