ATP dependent primary active transport of xenobiotic-glutathione conjugates by human erythrocyte membrane

We have demonstrated the presence of a dinitrophenyl glutathione (Dnp-SG) stimulated ATPase in human erythrocyte membranes. This ATPase mediates the active transport of glutathione — xenobiotic conjugate such as Dnp-SG from erythrocytes into the plasma. It is suggested that this transport system is distinct from the system which actively transports oxidized glutathione from the erythrocytes.     

ATP-dependent S-(2,4-dinitrophenyl)glutathione transport in canalicular plasma membrane vesicles from rat liver

Uptake of the thioether S-(2,4-dinitrophenyl)glutathione (DNPSG) in canalicular plasma membrane vesicles from rat liver is enhanced in the presence of ATP and exhibits an overshoot with a transient 5.5-fold accumulation of DNPSG. Stimulation by ATP is not caused by the generation of a membrane potential, based on responses of the indicator dye oxonol V. ATP-dependent uptake has an apparent Km of 71 microM for DNPSG and a Vmax of 0.34 nmol.min-1.mg of vesicle protein-1. Protein thiol groups are essential for transport activity as indicated by the sensitivity of DNPSG transport to sulfhydryl reagents. There is competitive inhibition with other thioethers, S-hexylglutathione (Ki = 66 microM), the photoaffinity label S-(4-azidophenacyl)glutathione (Ki = 56 microM), as well as with glutathione disulfide (Ki = 0.44 mM) and with the bile acid taurocholate (Ki = 0.61 mM). GSH (2 mM) or cholate (0.4 mM) does not inhibit. Both glutathione disulfide and taurocholate show ATP-dependent transport in the canalicular membrane vesicles which is inhibited by DNPSG. No ATP-dependent transport is found for GSH. Transport of DNPSG is also inhibited competitively by alpha-naphthyl-beta-D-glucuronide (Ki = 0.42 mM) but not by alpha-naphthylsulfate (2 mM), and there is substantial inhibition with the glucuronides from ebselen and p-nitrophenol. The results indicate that the canalicular transport system for DNPSG is directly driven by ATP and that the biliary transport of other classes of compounds may also proceed via this system.     

Hepatic glutathione and glutathione S-conjugate transport mechanisms.

Glutathione (GSH) plays a critical role in many cellular processes, including the metabolism and detoxification of oxidants, metals, and other reactive electrophilic compounds of both endogenous and exogenous origin. Because the liver is a major site of GSH and glutathione S-conjugate biosynthesis and export, significant effort has been devoted to characterizing liver cell sinusoidal and canalicular membrane transporters for these compounds. Glutathione S-conjugates synthesized in the liver are secreted preferentially into bile, and recent studies in isolated canalicular membrane vesicles indicate that there are multiple transport mechanisms for these conjugates, including those that are energized by ATP hydrolysis and those that may be driven by the electrochemical gradient. Glutathione S-conjugates that are relatively hydrophobic or have a bulky S-substituent are good substrates for the canalicular ATP-dependent transporter mrp2 (multidrug resistance-associated protein 2, also called cMOAT, the canalicular multispecific organic anion transporter, or cMrp, the canalicular isoform of mrp). In contrast with the glutathione S-conjugates, hepatic GSH is released into both blood and bile. GSH transport across both of these membrane domains is of low affinity and is energized by the electrochemical potential. Recent reports describe two candidate GSH transport proteins for the canalicular and sinusoidal membranes (RcGshT and RsGshT, respectively); however, some concerns have been raised regarding these studies. Additional work is needed to characterize GSH transporters at the functional and molecular level.     

The mechanism of biliary secretion of reduced glutathione. Analysis of transport process in isolated rat-liver canalicular membrane vesicles

Transport of reduced glutathione (GSH) was studied in isolated rat liver canalicular membrane vesicles by a rapid filtration technique. The membrane vesicles exhibit uptake of [2-3H]glycine--labeled GSH into an osmotically reactive intravesicular space. Although the canalicular membrane vesicles possess gamma-glutamyltransferase and aminopeptidase M, enzymes that hydrolyze glutathione into component amino acids, inactivation of the vesicle-associated transferase by affinity labeling with L-(alpha S,5S)-alpha-amino-3-chloro-4,5-dihydro-5-isoxazoleacetic acid (AT-125) had no effect on the initial rate of GSH transport. Chemical analysis revealed that intact GSH accounted for most of vesicle-associated radioactivity. The initial rate of transport followed saturation kinetics with respect to GSH concentration; an apparent Km of 0.33 mM and V of 1.47 nmol/mg protein in 20 s were calculated. These results indicate that transport of GSH across the canalicular membranes is a carrier-mediated process. Replacement of NaCl in the transport medium by KCl, LiCl or choline chloride had no effect on the transport activity of the vesicles. The rate of GSH uptake by the vesicles was enhanced by valinomycin-induced K+-diffusion potential (vesicle inside-positive) and was inhibited by probenecid, indicating that GSH transport across the canalicular membranes is electrogenic and involves the transfer of negative charge. The transport of GSH was inhibited by oxidized glutathione or S-benzyl-glutathione. This transport system in canalicular plasma membranes may function in biliary secretion of GSH and its derivatives which are synthesized in hepatocytes by oxidative processes or glutathione S-transferase.     

RLIP76 is the major ATP-dependent transporter of glutathione-conjugates and doxorubicin in human erythrocytes.

We have recently demonstrated that RLIP76, a Ral-binding GTPase activating protein mediates ATP-dependent transport of glutathione (GSH) conjugates of electrophiles (GS-E) as well as doxorubicin (DOX), and that it is identical with DNP-SG ATPase, a GS-E transporter previously characterized by us in erythrocyte membranes (Awasthi et al. Biochemistry 39, 9327-9334). Multidrug resistance-associated protein (MRP1) belonging to the family of the ABC-transporters has also been suggested to be a GS-E transporter in human erythrocytes. Using immunological approaches, the present studies were designed to elucidate the relative contributions of RLIP76, MRP1, and P-glycoprotein (Pgp), in the ATP-dependent transport of GS-E and DOX in human erythrocytes. In Western blot analyses using antibodies against RLIP76, a strong expression of RLIP76 was observed in erythrocytes. Immunohistochemical studies using a fluorescent probe showed association of RLIP76 with erythrocyte membrane, which was consistent with its transport function. Neither MRP1 nor Pgp were detected in erythrocytes when the antibodies against MRP1 or Pgp were used. In erythrocyte inside-out vesicles (IOVs) coated with antibodies against RLIP76, a dose-dependent inhibition of the ATP-dependent transport of DOX and GS-E, including S-(dinitrophenyl)glutathione (DNP-SG), leukotriene C(4), and the GSH conjugate of 4-hydroxynonenal, was observed with a maximal inhibition of about 70%. On the contrary, in the IOVs coated with the antibodies against MRP1 or Pgp no significant inhibition of the ATP-dependent transport of these compounds was observed. These findings suggest that RLIP76 is the major ATP-dependent transporter of GS-E and DOX in human erythrocytes.     

Excretion of glutathione conjugates by primary cultured rat hepatocytes

Conjugation of xenobiotics with glutathione occurs commonly within the liver, and these glutathione conjugates are then preferentially excreted into bile. We have characterized this excretory process using primary cultured hepatocytes (24 h). 1-Chloro-2,4-dinitrobenzene rapidly entered the cells and formed a glutathione conjugate, S-(dinitrophenyl)glutathione, irrespective of the temperature of incubation. In contrast, the efflux of the glutathione conjugate was essentially absent in the cold but recovered rapidly upon rewarming of the cells. Therefore, initial rates of efflux of the conjugate at 37 degrees C were measured from cells preloaded biosynthetically at 10 degrees C. Efflux was a saturable process with respect to intracellular S-(dinitrophenyl)glutathione with an apparent Km of 0.58 +/- 0.12 mM and Vmax of 0.15 +/- 0.05 nmol/min/mg of protein. The excretion of S-(dinitrophenyl)glutathione had an energy of activation of 15.3 kcal/mol. The glutathione conjugate of p-nitrobenzylchloride when formed within the hepatocytes acted as a competitive inhibitor of S-(dinitrophenyl)glutathione efflux. Cultured hepatocytes, therefore, appeared to have a specific transport process for the excretion of glutathione conjugates. The addition of S-(dinitrophenyl)glutathione, but not GSH, GSSG, or methionine, to the medium caused a decrease in the rate of efflux of radiolabeled S-(dinitrophenyl)glutathione. The hepatocytes were able, however, to excrete the glutathione conjugate against an excess of extracellular S-(dinitrophenyl)glutathione. This observation suggested that extracellular S-(dinitrophenyl)glutathione, although capable of binding to the carrier, entered the hepatocytes quite slowly relative to rates of efflux. This carrier may function in a manner that would minimize the reuptake by hepatocytes of conjugates that have been excreted into the bile.     

Dinitrophenyl glutathione efflux from human erythrocytes is primary active ATP-dependent transport.

Dinitrophenyl S-glutathione is accumulated by inside-out vesicles made from human erythrocytes in a process totally dependent on ATP and Mg2+. The vesicles were shown to accumulate dinitrophenyl S-glutathione against a concentration gradient. The vesicles were able to concentrate this glutathione derivative even in the absence of membrane potential. This indicated that the ATP-dependent uptake of dinitrophenyl S-glutathione by inside-out vesicles represented an active transport process. Neither extravesicular EGTA nor intravesicular ouabain inhibited the transport process, indicating that neither the Ca2+-ATPase nor the Na+, K+-ATPase were involved. These results indicated that dinitrophenyl S-glutathione uptake by inside-out vesicles probably represented primary active transport. The uptake of dinitrophenyl S-glutathione was a linear function of time (up to 5 h) and vesicle protein. The rate of uptake was optimal between pH 7.0 and 8.0 and at 37 degrees C. The Km values determined for dinitrophenyl S-glutathione and ATP were 0.29 mM and 1 mM, respectively. The transport process was completely inhibited by vanadate and by p-hydroxymercuribenzene sulphonate and inhibited to a lesser extent by N-ethylmaleimide. GTP could efficiently substitute for ATP as an energy source for the transport process, but CTP and UTP were comparatively much less effective.     

Quantitative immunoferritin localization of [Na+,K+]ATPase on canine hepatocyte cell surface

Distribution of [Na+,K+]ATPase on the cell surface of canine hepatocytes was investigated quantitatively by incubating prefixed and dissociated liver cells with ferritin antibody conjugates against canine kidney holo[Na+,K+]ATPase. We found that [Na+,K+]-ATPase exists bilaterally both on the bile canalicular and sinusoid-lateral surfaces. The particle density on the bile canalicular surface was much higher (approximately 2.5 times) than that on the sinusoid-lateral surface. In the latter region, the enzyme was detected almost equally both on the sinusoidal and lateral surfaces. On all the surfaces, the distribution of the enzyme was homogeneous and no clustering of the enzyme was detected. Total number of the enzyme on the sinusoid-lateral surface was, however, approximately three times higher than that on the bile canalicular region, because the sinusoid-lateral surface represents approximately 87% of the total cell surface of a hepatocyte. We suggest that the [Na+, K+]ATPase on the bile canalicular surface is responsible for the bile acid-independent bile flow and the other transport processes on the bile canalicular cell surface, while that on the sinusoid-lateral surface is responsible not only for the active transport of Na+ but also for the secondary active transport of various substances in this region.     

Bicarbonate-activated ATPase activity in renal cortex of chronically acidotic rats.

The effect of chronic NH4Cl-induced acidosis on the activity of a bicarbonate-activated component of ATPase was studied in homogenates of renal tissue from Wistar rats. This particular component of ATPase, which is maximally stimulated by 50 mM bicarbonate, and is insensitive to the action of ouabain, has been implicated in the active transport of bicarbonate in various tissues. The activity of this enzyme in cortical homogenates from an acidotic group of animals was 4.3 +/- 0.4 mumol Pi/mg protein per hour compared with 5.8 +/- 0.3 mumol Pi/mg protein per hour in a control group (p less than 0.02). No significant change in bicarbonate ATPase activity was observed in medullary homogenates, and NaK-ATPase activity remained the same in cortex and medulla of both groups. Subcellular fractionation of the cortical tissue homogenates revealed that bicarbonate ATPase activity in a microsomal fraction from acidotic animals was 6.5 +/- 1.1 mumol Pi/mg protein per hour compared with 9.4 +/- 1.2 mumol Pi/mg protein per hour in control animals (p less than 0.02). Bicarbonate ATPase activity in other subcellular fractions was not different in the two groups of animals. These findings are compatible with the hypothesis that a certain percentage of bicarbonate reabsorption in the nephron is mediated by a bicarbonate-activated component of ATPase.     

Characterization and functional reconstitution of the multidrug transporter

P-Glycoprotein, the multidrug transporter, is isolated from the plasma membrane of CH^RC5 cells using a selective two-step detergent extraction procedure. The partially purified protein displays a high level of ATPase activity, which has a highK _M for ATP, is stimulated by drugs, and can be distinguished from that of other membrane ATPases by its unique inhibition profile. Delipidation completely inactivates ATPase activity, which is restored by the addition of fluid lipid mixtures. P-Glycoprotein was reconstituted into lipid bilayers with retention of both drug transport and ATPase activity. Proteoliposomes containing P-glycoprotein display osmotically sensitive ATP-dependent accumulation of³H-colchicine in the vesicle lumen. Drug transport is active, generating a stable 5.6-fold concentration gradient, and can be blocked by compounds in the multidrug resistance spectrum. Reconstituted P-glycoprotein also exhibits a high level of ATPase activity which is further stimulated by various drugs. P-Glycoprotein therefore functions as an active drug transporter with constitutive ATPase activity.     

Export pumps for glutathione S-conjugates

The release of glutathione S-conjugates from cells is an ATP-dependent process mediated by integral membrane glycoproteins belonging to the recently discovered multidrug-resistance protein (MRP) family. Many lipophilic compounds conjugated with glutathione, glucuronate, or sulfate are substrates for export pumps of the MRP family. In humans six MRP isoforms encoded by different genes have been cloned. Orthologs of MRP have been identified in many species including yeast, plants, and nematodes. Human MRP1 and MRP2 are currently best characterized with respect to substrate specificity by measurements of ATP-dependent transport into inside-out membrane vesicles. High-affinity substrates include the glutathione S-conjugate leukotriene C4, S-(2,4dinitrophenyl)glutathione, bilirubin glucuronosides, and 17beta-glucuronosyl estradiol. In addition, glutathione disulfide is transported by MRP1 and MRP2. Reduced glutathione may be released from cells in a process directly or indirectly mediated by members of the MRP family. Proteins of the MRP family are indispensable for transport of glutathione S-conjugates and glutathione disulfide into the extracellular space and play, therefore, a decisive role in detoxification and defense against oxidative stress.     

Canalicular transport of reduced glutathione in normal and mutant Eisai hyperbilirubinemic rats.

We have characterized the transport of GSH and the mechanism for impaired GSH transport in mutant Eisai hyperbilirubinemic rats (EHBR) using isolated canalicular membrane-enriched vesicles (cLPM). In control animals, the transport of GSH is an electrogenic process and is trans-stimulated by preloading the vesicles with GSH and is not enhanced in the presence of ATP. GSH transport in cLPM is saturable with a single component having a Km of approximately 16 mM and a Vmax of 6.7 nmol/mg/15 s. EHBR is a Sprague-Dawley rat with hyperbilirubinemia due to impaired bile secretion of organic anions by the ATP-dependent organic anion/GSH-conjugate transporter. In cLPM from EHBR we confirmed the defective stimulation by ATP of the transport of LTC4 and GSSG. In the mutant cLPM, the characteristics and kinetics of GSH transport were the same as in the controls. 2,4-(dinitrophenyl)-glutathione (DNP-GSH), which is a substrate for the ATP-dependent canalicular organic anion carrier, in the absence of ATP, cis-inhibited the transport of GSH into cLPM vesicles; however, when the vesicles were preloaded with DNP-GSH, there was a dose-dependent trans-stimulation of GSH transport. In contrast, in the presence of ATP, DNP-GSH enhanced GSH transport in cLPM vesicles; at 0.25 mM DNP-GSH, a concentration which did not cis-inhibit GSH, addition of ATP resulted in accelerated GSH transport; at 1.0 mM DNP-GSH, cis-inhibition was completely reversed by the addition of ATP despite a negligible fall in the medium DNP-GSH. Interestingly, sulfobromophthalein-glutathione (BSP-GSH) neither cis-inhibited nor trans-stimulated GSH transport in cLPM. This contrasts with bLPM where BSP-GSH interacts with the GSH carrier. Therefore, GSH is transported into bile by a multispecific low affinity electrogenic carrier which is distinct from the multispecific high affinity ATP-driven organic anion transporter. Although both carriers have overlapping specificities, BSP-GSH and GSH are uniquely specific for only one of the carriers. The near absence of GSH in the bile of mutant rats can be best explained as a secondary defect due to cis-inhibition from retained substrates for the defective carrier and/or loss of trans-stimulation by these same substrates which normally are concentratively transported into the bile. Other possibilities such as change in GSH carrier activity upon isolation or loss of a negative protein regulator during membrane isolation, although theoretical alternatives are less easily reconciled with the defect in the ATP-driven organic anion transporter.     

ATPase activity of purified and reconstituted multidrug resistance protein MRP1 from drug-selected H69AR cells.

The ATP-binding cassette transporter protein, multidrug resistance protein MRP1, was purified from doxorubicin-selected H69AR lung tumor cells which express high levels of this protein. A purification procedure comprised of a differential two-step solubilization of MRP1 from plasma membranes with 3-(3-cholamidopropyl)dimethylammonio-1-propanesulfonate followed by immunoaffinity chromatography using the MRP1-specific monoclonal antibody QCRL-1 was developed. Approximately 300 microgram of MRP1 was obtained from 6 mg of plasma membranes at 80-90% purity, as indicated by silver staining of protein gels. After reconstitution of purified MRP1 into proteoliposomes, kinetic analyses indicated that its K(m) for ATP hydrolysis was 104+/-22 microM with maximal activity of 5-10 nmol min(-1) mg(-1) MRP1. MRP1 ATPase activity was further characterized with various inhibitors and exhibited an inhibition profile that distinguishes it from P-glycoprotein and other ATPases. The ATPase activity of reconstituted MRP1 was stimulated by the conjugated organic anion substrates leukotriene C(4) (LTC(4)) and 17beta-estradiol 17-(beta-D-glucuronide) with 50% maximal stimulation achieved at concentrations of 150 nM and 1.6 microM, respectively. MRP1 ATPase was also stimulated by glutathione disulfide but not by reduced glutathione or unconjugated chemotherapeutic agents. This purification and reconstitution procedure is the first to be described in which the ATPase activity of the reconstituted MRP1 retains kinetic characteristics with respect to ATP-dependence and substrate stimulation that are very similar to those deduced from transport studies using MRP1-enriched plasma membrane vesicles.     

Cytochemical localization of ouabain-sensitive (K+)-dependent p-nitrophenyl phosphatase (transport ATPase) in human blood platelets

The ultrastructural cytochemical localization of a potassium-dependent oubain-sensitive nitrophenyl phosphatase (transport ATPase) activity in human blood platelets is described. This potassium-dependent nitrophenyl phosphatase activity was not affected by 5 mM levamisole, indicating that the reaction product identified was not due to nonspecific alkaline phosphatase activity. The K+-dependent nitrophenyl phosphatase was strictly localized to the platelet plasma membrane, while the open canalicular system and dense tubular system were devoid of reaction product. In contrast, (Ca2+,Mg2+)-activated ATPase activity was predominantly localized in the open canalicular system and dense tubular system with very little cytochemical activity expressed at the plasma membrane. These data demonstrate a relative segregation of these enzymes into unique membrane compartments of the human platelet. Such data may be useful with regard to identification of purified membrane fractions from platelets and may be significant with regard to the understanding of the function(s) of the different membrane compartments of the human platelet.     

Dinitrophenylation of chicken gizzard myosin: reactivity of the 17 000-dalton light chain

Chicken gizzard myosin rapidly incorporated 3 mol of 1-fluoro-2,4-dinitrobenzene per 4.7 x 10(5) g of protein with little change in the ATPase (ATP phosphohydrolase, EC 3.6.1.3) activity. During an interval when 2 additional mol of the reagent were bound the K+-ATPase activity in the presence of EDTA was inhibited and the Ca2+-ATPase activity was altered to a lesser extent. Cysteine residues were modified in the dinitrophenylated gizzard myosin. The dinitrophenyl group was located mainly in the active proteolytic fragment, subfragment 1. Dinitrophenylation of the heavy and light chains was observed but major changes in the ATPase activity occurred when the 17 000-dalton light chain and some heavy chains were modified as judged by dissociation experiments in sodium dodecyl sulfate. Thiolysis of the dinitrophenylated gizzard myosin with 2-mercaptoethanol restored the ATPase activity and approx. 2 mol of the dinitrophenyl group were removed. The restoration of the enzymic activity, however, occurred when 1 mol of the label was thiolytically cleaved from cysteine residues of the 17 000-dalton light chain. Substrate Mg-ATP(2-) or MgADP did not protect the ATPase activity of modified gizzard myosin. In the presence of nucleotide there was an increase in the incorporation of the reagent, and a change in its distribution into the light and heavy chains. Calcium had no effect on the dinitrophenylation of this myosin. these results indicate that the reagent, 1-fluoro-2,4-dinitrobenzene, could detect chemical differences in smooth muscle myosin when compared to the reactivity of other myosins. Thiol groups of the 17 000-light chain (and some heavy chains) are probably located peripheral to the active site region of gizzard myosin and they are involved in maintaining the enzymic activity of this protein.     

A trafficking pathway for anthocyanins overlaps with the endoplasmic reticulum-to-vacuole protein-sorting route in Arabidopsis and contributes to the formation of vacuolar inclusions

Plants produce a very large number of specialized compounds that must be transported from their site of synthesis to the sites of storage or disposal. Anthocyanin accumulation has provided a powerful system to elucidate the molecular and cellular mechanisms associated with the intracellular trafficking of phytochemicals. Benefiting from the unique fluorescent properties of anthocyanins, we show here that in Arabidopsis (Arabidopsis thaliana), one route for anthocyanin transport to the vacuole involves vesicle-like structures shared with components of the secretory pathway. By colocalizing the red fluorescence of the anthocyanins with green fluorescent protein markers of the endomembrane system in Arabidopsis seedlings, we show that anthocyanins are also sequestered to the endoplasmic reticulum and to endoplasmic reticulum-derived vesicle-like structures targeted directly to the protein storage vacuole in a Golgi-independent manner. Moreover, our results indicate that vacuolar accumulation of anthocyanins does not depend solely on glutathione S-transferase activity or ATP-dependent transport mechanisms. Indeed, we observed a dramatic increase of anthocyanin-filled subvacuolar structures, without a significant effect on total anthocyanin levels, when we inhibited glutathione S-transferase activity, or the ATP-dependent transporters with vanadate, a general ATPase inhibitor. Taken together, these results provide evidence for an alternative novel mechanism of vesicular transport and vacuolar sequestration of anthocyanins in Arabidopsis.     

Novel function of human RLIP76: ATP-dependent transport of glutathione conjugates and doxorubicin

Active transport of conjugated and unconjugated electrophiles out of cells is essential for cellular homeostasis. We have previously identified in human tissues a transporter, DNP-SG [S-(2, 4-dinitrophenyl)glutathione] ATPase, capable of carrying out this function [Awasthi et al. (1998) Biochemistry 37, 5231-5238, 5239-5248]. We now report the cloning of DNP-SG ATPase. The sequence of the cDNA clone was identical to that of human RLIP76, a known Ral-binding protein. RLIP76 expressed in E. coli was purified by DNP-SG affinity chromatography. Purified recombinant RLIP76: (1) had ATPase activity stimulated by DNP-SG or doxorubicin (DOX), and the K(m) values of RLIP76 for ATP, DOX, and DNP-SG were similar to those reported for DNP-SG ATPase; (2) upon reconstitution with asolectin as well as with defined lipids, catalyzed ATP-dependent transport of DNP-SG and DOX with kinetic parameters similar to those of DNP-SG ATPase; (3) when transfected into K562 cells, resulted in increased resistance to DOX, and increased ATP-dependent transport of DNP-SG and DOX by inside-out membrane vesicles from transfected cells; (4) direct uptake of purified RLIP76 protein into mammalian cells from donor proteoliposomes confers DOX resistance. These results indicate that RLIP76, in addition to its role in signal transduction, can catalyze transport of glutathione conjugates and xenobiotics, and may contribute to the multidrug resistance phenomenon.     

Dinitrophenylation of rabbit skeletal sarcoplasmic reticulum ATPase protein

The ATPase (ATP phosphohydrolase (EC 3.6.1.3)) protein of rabbit skeletal sarcoplasmic reticulum rapidly incorporated three mol of 1-fluoro-2,4-dinitrobenzene per 10(5) g of protein with little change in the Ca2+-dependent ATPase activity. When 2 additional mol of the reagent were bound the Ca2+-dependent ATPase activity was inhibited. The dinitrophenyl group was located mainly in the ATPase protein and a small amount of the label was found in the proteolipid component of the ATPase preparation as judged by dissociation experiments in sodium dodecyl sulfate. Cysteine and tyrosine residues were dinitrophenylated in the modified ATPase protein. Thiolysis of the dinitrophenylated ATPase protein with 2-mercaptoethanol under various conditions did not restore the Ca2+-dependent ATPase activity. Solubilization of the ATPase protein with sodium deoxycholate increased the reactivity of the reagent and the Ca2+-dependent ATPase activity was inhibited to a greater extent. Dinitrophenylation of the ATPase protein was Ca2+-dependent; in the presence of high Ca2+ the incorporation increased by 50% and a large decrease in the Ca2+-ATPase activity was noted. The half-maximal changes for the incorporation of the reagent and the inhibition of the Ca2+-ATPase activity occurred at 3--4 microgram Ca2+-concentration, consistent with the binding of Ca2+ to high affinity sites on the ATPase protein. These results indicate that the ATPase protein as a Ca2+-free and a Ca2+-bound conformation. The reagent, 1-fluoro-2,4-dinitrobenzene reacts differentially and thus characterizes these two conformations.     

Complete inhibition of the Pdr5p multidrug efflux pump ATPase activity by its transport substrate clotrimazole suggests that GTP as well as ATP may be used as an energy source

The yeast Pdr5p transporter is a 160 kDa protein that effluxes a large variety of xenobiotic compounds. In this study, we characterize its ATPase activity and demonstrate that it has biochemical features reminiscent of those of other ATP-binding cassette multidrug transporters: a relatively high Km for ATP (1.9 mM), inhibition by orthovanadate, and the ability to specifically bind an azidoATP analogue at the nucleotide-binding domains. Pdr5p-specific ATPase activity shows complete, concentration-dependent inhibition by clotrimazole, which is also known to be a potent transport substrate. Our results indicate, however, that this inhibition is noncompetitive and caused by the interaction of clotrimazole with the transporter at a site that is distinct from the ATP-binding domains. Curiously, Pdr5p-mediated transport of clotrimazole continues at intracellular concentrations of substrate that should eliminate all ATPase activity. Significantly, however, we observed that the Pdr5p has GTPase and UTPase activities that are relatively resistant to clotrimazole. Furthermore, the Km(GTPase) roughly matches the intracellular concentrations of the nucleotide reported for yeast. Using purified plasma membrane vesicles, we demonstrate that Pdr5p can use GTP to fuel substrate transport. We propose that Pdr5p increases its multidrug transport substrate specificity by using more than one nucleotide as an energy source.