The chloride dependence of the human organic anion transporter 1 (hOAT1) is blunted by mutation of a single amino acid

Organic anion transporter 1 (OAT1) is key for the secretion of organic anions in renal proximal tubules. These organic anions comprise endogenous as well as exogenous compounds including frequently used drugs of various chemical structures. The molecular basis for the polyspecificity of OAT1 is not known. Here we mutated a conserved positively charged arginine residue (Arg(466)) in the 11(th) transmembrane helix of human OAT1. The replacement by the positively charged lysine (R466K) did not impair expression of hOAT1 at the plasma membrane of Xenopus laevis oocytes but decreased the transport of p-aminohippurate (PAH) considerably. Extracellular glutarate inhibited and intracellular glutarate trans-stimulated wild type and mutated OAT1, suggesting for the mutant R466K an unimpaired interaction with dicarboxylates. However, when Arg(466) was replaced by the negatively charged aspartate (R466D), glutarate no longer interacted with the mutant. PAH uptake by wild type hOAT1 was stimulated in the presence of chloride, whereas the R466K mutant was chloride-insensitive. Likewise, the uptake of labeled glutarate or ochratoxin A was chloride-dependent in the wild type but not in R466K. Kinetic experiments revealed that chloride did not alter the apparent K(m) for PAH but influenced V(max) in wild type OAT1-expressing oocytes. In R466K mutants the apparent K(m) for PAH was similar to that of the wild type, but V(max) was not changed by chloride removal. We conclude that Arg(466) influences the binding of glutarate, but not interaction with PAH, and interacts with chloride, which is a major determinant in substrate translocation.     

Identification of a new urate and high affinity nicotinate transporter, hOAT10 (SLC22A13)

The orphan transporter hORCTL3 (human organic cation transporter like 3; SLC22A13) is highly expressed in kidneys and to a weaker extent in brain, heart, and intestine. hORCTL3-expressing Xenopus laevis oocytes showed uptake of [(3)H]nicotinate, [(3)H]p-aminohippurate, and [(14)C]urate. Hence, hORCTL3 is an organic anion transporter, and we renamed it hOAT10. [(3)H]Nicotinate transport by hOAT10 into X. laevis oocytes and into Caco-2 cells was saturable with Michaelis constants (K(m)) of 22 and 44 microm, respectively, suggesting that hOAT10 may be the molecular equivalent of the postulated high affinity nicotinate transporter in kidneys and intestine. The pH dependence of hOAT10 suggests p-aminohippurate(-)/OH(-), urate(-)/OH(-), and nicotinate(-)/OH(-) exchange as possible transport modes. Urate inhibited [(3)H]nicotinate transport by hOAT10 with an IC(50) value of 759 microm, assuming that hOAT10 represents a low affinity urate transporter. hOAT10-mediated [(14)C]urate uptake was elevated by an exchange with l -lactate, pyrazinoate, and nicotinate. Surprisingly, we have detected urate(-)/glutathione exchange by hOAT10, consistent with an involvement of hOAT10 in the renal glutathione cycle. Uricosurics, diuretics, and cyclosporine A showed substantial interactions with hOAT10, of which cyclosporine A enhanced [(14)C]urate uptake, providing the first molecular evidence for cyclosporine A-induced hyperuricemia.     

Characterization of ochratoxin A transport by human organic anion transporters.

The purpose of this study was to investigate the characteristics of ochratoxin A (OTA) transport by multispecific human organic anion transporters (hOAT1 and hOAT3, respectively) using the second segment of proximal tubule (S2) cells from mice stably expressing hOAT1 and hOAT3 (S2 hOAT1 and S2 hOAT3). S2 hOAT1 and S2 hOAT3 exhibited a time- and dose-dependent, and a saturable increase in uptake of [3H]-OTA, with apparent Km values of 0.42 microM (hOAT1) and 0.75 microM (hOAT3). These OTA uptakes were inhibited by several substrates for the OATs. Para-aminohippuric acid (PAH), probenecid, piroxicam, octanoate and citrinin inhibited [3H]-OTA uptake by hOAT1 and hOAT3 in a competitive manner (Ki = 4.29-3080 microM), with the following order of potency: probenecid > octanoate > PAH > piroxicam > citrinin for hOAT1; probenecid > piroxicam > octanoate> citrinin > PAH for hOAT3. These results indicate that hOAT1, as well as hOAT3, mediates a high-affinity transport of OTA on the basolateral side of the proximal tubule, but hOAT1- and hOAT3-mediated OTA transport are differently influenced by the substrates for the OATs. These pharmacological characteristics of hOAT1 and hOAT3 may be significantly related with the events in the development of OTA-induced nephrotoxicity in the human kidney.     

A three-dimensional model of human organic anion transporter 1: aromatic amino acids required for substrate transport

Organic anion transporters (OATs) play a critical role in the handling of endogenous and exogenous organic anions by excretory and barrier tissues. Little is known about the OAT three-dimensional structure or substrate/protein interactions involved in transport. In this investigation, a theoretical three-dimensional model was generated for human OAT1 (hOAT1) based on fold recognition to the crystal structure of the glycerol 3-phosphate transporter (GlpT) from Escherichia coli. GlpT and hOAT1 share several sequence motifs as major facilitator superfamily members. The structural hOAT1 model shows that helices 5, 7, 8, 10, and 11 surround an electronegative putative active site ( approximately 830A(3)). The site opens to the cytoplasm and is surrounded by three residues not previously examined for function (Tyr(230) (domain 5) and Lys(431) and Phe(438) (domain 10)). Effects of these residues on p-aminohippurate (PAH) and cidofovir transport were assessed by point mutations in a Xenopus oocyte expression system. Membrane protein expression was severely limited for the Y230A mutant. For the K431A and F438A mutants, [(3)H]PAH uptake was less than 30% of wild-type hOAT1 uptake after protein expression correction. Reduced V(max) values for the F438A mutant confirmed lower protein expression. In addition, the F438A mutant exhibited an increased affinity for cidofovir but was not significantly different for PAH. Differences in handling of PAH and cidofovir were also observed for the Y230F mutant. Little uptake was determined for cidofovir, whereas PAH uptake was similar to wild-type hOAT1. Therefore, the hOAT1 structural model has identified two new residues, Tyr(230) and Phe(438), which are important for substrate/protein interactions.     

Organic anion transporters involved in the excretion of bestatin in the kidney

Bestatin, a dipeptide, a low molecular weight aminopeptidase inhibitor, has been demonstrated to be an immunomodulator with an antitumor activity. However, the transporter-mediated renal excretion of bestatin is not fully understood. The purpose of this study was to elucidate the transporter-mediated renal excretion mechanism for bestatin. The plasma concentration of bestatin was increased markedly and both the accumulative renal excretion and renal clearance of bestatin were decreased significantly after intravenous administration of bestatin in combination with probenecid. p-Aminohippuric acid (PAH), a substrate of organic anion transporter (OAT) 1, benzylpenicillin (PCG), a substrate of OAT3 and JBP485, a substrate of OAT1 and OAT3, reduced the uptake of bestatin in rat kidney slices and in hOAT1- or hOAT3-HEK 293 cells. The accumulation of bestatin in hOAT1-HEK and hOAT3-HEK 293 cells was significantly greater than that in vector-HEK, and the K(m) and V(max) were 0.679 ± 0.007 mM and 0.807 ± 0.006 nmol/mg protein/30s for OAT1, 0.632 ± 0.014 mM and 1.303 ± 0.015 nmol/mg protein/30s for OAT3 respectively. PAH and JBP485 inhibited significantly the uptake of bestatin in hOAT1-HEK with the K(i) values of 92 ± 9 μM and 197 ± 21 μM; and PCG, JBP485 inhibited significantly the uptake of bestatin in hOAT3-HEK 293 cells with the K(i) values of 88 ± 12 μM and 160 ± 16 μM. Our results are novel in demonstrating for the first time that OAT1 and OAT3 are involved in the renal excretion of bestatin.     

Development and characterization of immobilized human organic anion transporter-based liquid chromatographic stationary phase: hOAT1 and hOAT2

This paper reports the development of liquid chromatographic columns containing immobilized organic anion transporters (hOAT1 and hOAT2). Cellular membrane fragments from MDCK cells expressing hOAT1 and S2 cells expressing hOAT2 were immobilized on the surface of the immobilized artificial membrane (IAM) liquid chromatographic stationary phase. The resulting stationary phases were characterized by frontal affinity chromatography, using the marker ligand [3H]-adefovir for the hOAT1 and [14C]-p-aminohippurate for the hOAT2 in the presence of multiple displacers. The determined binding affinities (Kd) for eight OAT1 ligands and eight OAT2 ligands were correlated with literature values and a statistically significant correlation was obtained for both the hOAT1 and hOAT2 columns: r2=0.688 (p<0.05) and r2=0.9967 (p<0.0001), respectively. The results indicate that the OAT1 and OAT2 have been successfully immobilized with retention of their binding activity. The use of these columns to identify ligands to the respective transporters will be presented.     

Liver X receptor activation downregulates organic anion transporter 1 (OAT1) in the renal proximal tubule

Liver X receptors (LXRs) play an important role in the regulation of cholesterol by regulating several transporters. In this study, we investigated the role of LXRs in the regulation of human organic anion transporter 1 (hOAT1), a major transporter localized in the basolateral membrane of the renal proximal tubule. Exposure of renal S2 cells expressing hOAT1 to LXR agonists (TO901317 and GW3965) and their endogenous ligand [22(R)-hydroxycholesterol] led to the inhibition of hOAT1-mediated [(14)C]PAH uptake. This inhibition was abolished by coincubation of the above agonists with 22(S)-hydroxycholesterol, an LXR antagonist. Moreover, it was found that the effect of LXR agonists was not mediated by changes in intracellular cholesterol levels. Interestingly, the inhibitory effect of LXRs was enhanced in the presence of 9-cis retinoic acid, a retinoic X receptor agonist. Kinetic analysis revealed that LXR activation decreased the maximum rate of PAH transport (J(max)) but had no effect on the affinity of the transporter (K(t)). This result correlated well with data from Western blot analysis, which showed the decrease in hOAT1 expression following LXR activation. Similarly, TO901317 inhibited [(14)C]PAH uptake by the renal cortical slices as well as decreasing mOAT1 protein expression in mouse kidney. Our findings indicated for the first time that hOAT1 was downregulated by LXR activation in the renal proximal tubule.     

Human organic anion transporter hOAT1 forms homooligomers

Human organic anion transporter hOAT1 belongs to a superfamily of organic anion transporters, which play critical roles in the body disposition of clinically important drugs, including anti-human immunodeficiency virus therapeutics, anti-tumor drugs, antibiotics, anti-hypertensives, and anti-inflammatories. To gain insight into the regulation of hOAT1, detailed information on its structural assembly is essential. In the present study, we investigate the quaternary structure of hOAT1 using combined approaches of chemical cross-linking, gel filtration chromatography, co-immunoprecipitation, cell surface biotinylation, and metabolic labeling. Chemical cross-linking of intact membrane proteins from LLC-PK1 cells stably expressing hOAT1 converted quantitatively hOAT1 monomer to putative trimer and higher order of oligomer, indicating that hOAT1 is present in the membrane as multimeric complexes. When co-expressed in LLC-PK1 cells, FLAG-tagged hOAT1 co-immunoprecipitated with myc-tagged hOAT1. The hOAT1 oligomer was also detected in gel filtration chromatography of total membranes from hOAT1-expressing LLC-PK1 cells. Cell surface biotinylation with membrane-impermeable reagents and metabolic labeling with [(35)S]methionine followed by immunoprecipitation showed that the oligomeric hOAT1 did not contain any other proteins. Taken together, this is the first study demonstrating that hOAT1 exists in the plasma membrane as a homooligomer, possibly trimer, and higher order of oligomer.     

Identification and characterization of single nucleotide polymorphisms of SLC22A11 (hOAT4) in Korean women osteoporosis patients

Single nucleotide polymorphisms (SNPs) are the most common form of human genetic variation. Non-synonymous SNPs (nsSNPs) change an amino acid. Organic anion transporters (OATs) play an important role in eliminating or reabsorbing endogenous and exogenous organic anionic compounds. Among OATs, hOAT4 mediates high affinity transport of estrone sulfate and dehydroepiandrosterone sulfate. The rapid bone loss that occurs in post-menopausal women is mainly due to a net decrease of estrogen. In the present study we searched for SNPs within the exon regions of hOAT4 in Korean women osteoporosis patients. Fifty healthy subjects and 50 subjects with osteoporosis were screened for genetic polymorphism in the coding region of SLC22A11 (hOAT4) using GC-clamp PCR and denaturing gradient gel electrophoresis (DGGE). We found three SNPs in the hOAT4 gene. Two were in the osteoporosis group (C483A and G832A) and one in the normal group (C847T). One of the SNPs, G832A, is an nsSNP that changes the 278th amino acid from glutamic acid to lysine (E278K). Uptake of [3H] estrone sulfate by oocytes injected with the hOAT4 E278K mutant was reduced compared with wild-type hOAT4. Km values for wild type and E278K were 0.7 microM and 1.2 microM, and Vmax values were 1.8 and 0.47 pmol/oocyte/h, respectively. The present study demonstrates that hOAT4 variants can causing inter-individual variation in anionic drug uptake and, therefore, could be used as markers for certain diseases including osteoporosis.     

Urate/alpha-ketoglutarate exchange in avian basolateral membrane vesicles

Membrane transport pathways for transcellular secretion of urate across the proximal tubule were investigated in avian kidney. The presence of coupled urate/alpha-ketoglutarate exchange was investigated in basolateral membrane vesicles (BLMV) by [(14)C]urate and [alpha-(3)H]ketoglutarate flux measurements. An inward Na gradient induced accumulation of alpha-ketoglutarate of sufficient magnitude to suggest a Na-dicarboxylate cotransporter. An inward Na gradient also induced concentrative accumulation of urate in the presence of alpha-ketoglutarate but not in its absence, suggesting urate/alpha-ketoglutarate exchange. alpha-Ketoglutarate-dependent stimulation of urate uptake was not observed in brush-border membrane vesicles. An outward urate gradient induced concentrative accumulation of alpha-ketoglutarate. alpha-Ketoglutarate-coupled urate uptake was specific for alpha-ketoglutarate, Cl dependent, and insensitive to membrane potential. alpha-Ketoglutarate-coupled urate uptake was inhibited by increasing p-aminohippurate (PAH) concentrations, and alpha-ketoglutarate-coupled PAH uptake was observed. alpha-Ketoglutarate-coupled PAH uptake was inhibited by increasing urate concentrations, and an outward urate gradient induced concentrative accumulation of PAH. These results suggest a Cl-dependent, alpha-ketoglutarate-coupled anion exchange mechanism as a pathway for active urate uptake across the basolateral membrane of urate-secreting proximal tubule cells.     

Scintillation proximity assay for measuring uptake by the human drug transporters hOCT1, hOAT3, and hOATP1B1

Increasing evidence suggests a key role of transport proteins in the pharmacokinetics of drugs. Within the solute carrier (SLC) family, various organic cation transporters (OCTs), organic anion transporters (OATs), and organic anion transporting polypeptides (OATPs) that interact with drug molecules have been identified. Traditionally, cellular uptake assays require multiple steps and provide low experimental throughput. We here demonstrate the use of a scintillation proximity approach to detect substrate uptake by human drug transporters in real time. HEK293 cells stably transfected with hOCT1, hOATP1B1, or hOAT3 were grown directly in Cytostar-T scintillating microplates. Confluent cell monolayers were incubated with 14C- or 3H-labeled transporter substrates. Cellular uptake brings the radioisotopes into proximity with the scintillation plate base. The resulting light emission signals were recorded on-line in a microplate scintillation counter. Results show time- and concentration-dependent uptake of 14C-tetraethylammonium, 3H-methylphenylpyridinium (HEK-hOCT1), 3H-estradiol-17beta-D-glucuronide (HEK-hOATP1B1), and 3H-estrone-3-sulfate (HEK-hOAT3), while no respective uptake was detected in empty vector-transfected cells. Km of 14C-tetraethylammonium and 3H-estrone-3-sulfate uptake and hOAT3 inhibition by ibuprofen and furosemide were similar to conventional dish uptake studies. The scintillation proximity approach is high throughput, amenable to automation and allows for identification of SLC transporter substrates and inhibitors in a convenient and reliable fashion, suggesting its broad applicability in drug discovery.     

Critical amino acid residues in transmembrane domain 1 of the human organic anion transporter hOAT1

Human organic anion transporter 1 (hOAT1) belongs to a superfamily of organic anion transporters, which play critical roles in the body disposition of clinically important drugs, including anti-human immunodeficiency virus therapeutics, anti-tumor drugs, antibiotics, anti-hypertensives, and anti-inflammatories. Previously we suggested that the predicted transmembrane domain 1 (TM1) of hOAT1 might be important for its function. In the present study, we examined the role of each residue within TM1 of hOAT1 in substrate recognition and transport. Alanine scanning was used to construct mutants of hOAT1, and the uptake of model substrate para-aminohippurate was studied in COS-7 cells expressing the mutant transporters. This approach led to the discovery of two critical amino acid residues, Leu-30 and Thr-36. A substitution of Leu-30 or Thr-36 with alanine resulted in a complete loss of transport activities. We then further characterized Leu-30 and Thr-36 by mutagenizing these residues to amino acids with different physicochemical properties. Leu-30 was replaced with amino acids with varying sizes of side chains, including glycine, valine, and isoleucine. We showed that progressively smaller side chains at position 30 increasingly impaired hOAT1 function mainly because of the impaired surface expression of the transporter. Thr-36, another critical amino acid in TM1, was replaced by serine and cysteine. Similar to the substitution of Thr-36 by alanine, substitution by serine and cysteine at this position abolished transport activity without affecting the surface expression of the transporter. The fact that Thr-36 cannot be substituted with serine and that the side chains of alanine, serine, and cysteine are smaller than that of threonine by a methyl group indicate that both the methyl group and the hydroxyl group of Thr-36 could be critical for hOAT1 activity. Together we conclude that Leu-30 and Thr-36 play distinct roles in hOAT1 function. Leu-30 is important in targeting the transporter to the plasma membrane. In contrast, Thr-36 is critical for substrate recognition. The present study provided the first molecular evidence that transmembrane domain 1 is a critical determinant of hOAT1 function and may provide important insights into the structure-function relationships of the organic anion transporter family.     

Functional expression of pig renal organic anion transporter 3 (pOAT3)

With the cloning of pig renal organic anion transporter 1 (pOAT1) (Biochimie 84 (2002) 1219) we set up a model system for comparative studies of cloned and natively isolated membrane located transport proteins. Meanwhile, another transport protein involved in p-aminohippurate (PAH) uptake on the basolateral side of the proximal tubule cells was identified, designated organic anion transporter 3 (OAT3). To explore the contribution of pOAT1 to the PAH clearance in comparison to OAT3, it was the aim of this study to extend our model by cloning of the pig ortholog of OAT3. Sequence comparisons of human organic anion transporter 3 (hOAT3) with the expressed sequence tag (EST) database revealed a clone and partial sequence of the pig renal organic anion transporter 3 (pOAT3) ortholog. Sequencing of the entire open reading frame resulted in a protein of 543 amino acid residues encoded by 1632 base pairs (EMBL Acc. No. AJ587003). It showed high homologies of 81%, 80%, 76%, and 77% to the human, rabbit, rat, and mouse OAT3, respectively. A functional characterization of pOAT3 in Xenopus laevis oocytes yielded an apparent Km (Kt) for [3H]estrone sulfate of 7.8 +/- 1.3 microM. Moreover, pOAT3 mediated [3H]estrone sulfate uptake was almost abolished by 0.5 mM of glutarate, dehydroepiandosterone sulfate, or probenecid consistent with the hallmarks of OAT3 function.     

Regulation of human organic anion transporter 4 by parathyroid hormone-related protein and protein kinase A

Human organic anion transporter 4 (hOAT4) belongs to a family of organic anion transporters that play critical roles in the body disposition of clinically important drugs, including anti-human immunodeficiency virus therapeutics, anti-tumor drugs, antibiotics, antihypertensives, and anti-inflammatories. hOAT4 is abundantly expressed in the kidney and placenta. In the current study, we examined the regulation of hOAT4 by parathyroid hormone-related protein (PTHrP) and protein kinase A (PKA) in kidney COS-7 cells. PTHrP induced a time- and concentration-dependent stimulation of hOAT4 transport activity. The stimulation of hOAT4 activity by PTHrP mainly resulted from an increased cell surface expression without a change in total cell expression of the transporter. Activation of PKA by Bt2-cAMP also resulted in a stimulation of hOAT4 activity through an increased cell surface expression of the transporter. However, PTHrP-induced stimulation of hOAT4 activity could not be prevented by treating hOAT4-expressing cells with the PKA inhibitor H89. We concluded that both PTHrP and activation of PKA stimulate hOAT4 activity through redistribution of the transporter from intracellular compartments to the cell surface. However, PTHrP regulates hOAT4 activity by mechanisms independent of PKA pathway.     

Cloning of the pig renal organic anion transporter 1 (pOAT1)

A pig kidney cDNA library was screened for the porcine ortholog of the multispecific organic anion transporter 1 (pOAT1). Several positive clones were isolated resulting in two alternatively spliced cDNA clones of pOAT1 (pOAT1 and pOAT1A). pOAT1-cDNAs consist of 2126 or 1895 base pairs (EMBL Acc. No. AJ308234 and AJ308235) encoding 547 or 533 amino acid residue proteins with 89, 87, 83 and 81% homology to the human, rabbit, rat, and mouse OAT1, respectively. Heterologous expression of pOAT1 in Xenopus laevis oocytes revealed an apparent K(m) for [3H]PAH of 3.75 +/- 1.6 microM. [3H]PAH uptake mediated by pOAT1 was abolished by 0.5 mM glutarate or 1 mM probenecid. Functional characterization of pOAT1A did not show any affinity for [3H]PAH. In summary, we cloned two alternative splice variants of the pig ortholog of organic anion transporter 1. One splice form (pOAT1) showed typical functional characteristics of organic anion transporter 1, whereas the second form appears not to transport PAH.     

Human renal organic anion transporter 1 (hOAT1) and its role in the nephrotoxicity of antiviral nucleotide analogs

hOAT1 is a renal membrane protein able to efficiently transport acyclic nucleoside phosphonates (ANPs). When expressed in CHO cells, hOAT1 mediates the uptake and cytotoxicity of ANPs suggesting that it plays an active role in the nephrotoxicity associated with cidofovir CMV therapy and high-dose adefovir HIV therapy. Although efficiently transported by hOAT1, tenofovir did not show any significant cytotoxicity in isolated human proximal tubular cells, which correlates with the lack of nephrotoxicity observed in HIV-infected patients on prolonged tenofovir therapy.     

Inhibition of the human organic anion transporter 1 by the caffeine metabolite 1-methylxanthine

Caffeine (1,3,7-trimethylxanthine) is daily and widely consumed in beverages and food and is mainly metabolized to 1,7-dimethylxanthine and 1-methylxanthine. Indirect clinical evidence suggests that 1-methylxanthine interacts with the organic anion transport system in the human kidney. In this study the effect of caffeine and its main metabolites on the human organic anion transporter 1 (hOAT1) was investigated using CHO cells overexpressing hOAT1. The uptake of 6-carboxyfluorescein into CHO(hOAT) cells was significantly inhibited by > or = 100 microM of 1-methylxanthine. Five hundred micromolar 1-methylxanthine was equieffective to 100 microM probenecid. In contrast, caffeine and 1,7-dimethylxanthine did not inhibit the transport of 6-carboxyfluorescein at concentrations up to 500 microM. In conclusion, the caffeine metabolite 1-methylxanthine inhibits the transport activity of hOAT1 in vitro. The central involvement of hOAT1 in the renal excretion of numerous drugs suggests that this inhibition may alter the pharmacokinetics of a series of clinically important drugs in humans.     

Cell free expression and functional reconstitution of eukaryotic drug transporters

Polyspecific organic cation and anion transporters of the SLC22 protein family are critically involved in absorption and excretion of drugs. To elucidate transport mechanisms, functional and biophysical characterization of purified transporters is required and tertiary structures must be determined. Here, we synthesized rat organic cation transporters OCT1 and OCT2 and rat organic anion transporter OAT1 in a cell free system in the absence of detergent. We solubilized the precipitates with 2% 1-myristoyl-2-hydroxy- sn-glycero-3-[phospho- rac-(1-glycerol)] (LMPG), purified the transporters in the presence of 1% 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS) or octyl glucoside, and reconstituted them into proteoliposomes. From 1 mL reaction vessels 0.13-0.36 mg of transporter proteins was purified. Thus, from five to ten 1 mL reaction vessels sufficient protein for crystallization was obtained. In the presence of 1% LMPG and 0.5% CHAPS, OCT1 and OAT1 formed homo-oligomers but no hetero-oligomers. After reconstitution of OCT1, OCT2, and OAT1 into proteoliposomes, similar Michaelis-Menten K m values were measured for uptake of 1-methyl-4-phenylpyridinium and p-aminohippurate (PAH (-)) by the organic cation and anion transporters, respectively, as after expression of the transporters in cells. Using the reconstituted system, evidence was obtained that OAT1 operates as obligatory and electroneutral PAH (-)/dicarboxylate antiporter and contains a low-affinity chloride binding site that stimulates turnover. PAH (-) uptake was observed only with alpha-ketoglutarate (KG (2-)) on the trans side, and trans-KG (2-) increased the PAH (-) concentration in voltage-clamped proteoliposomes transiently above equilibrium. The V max of PAH (-)/KG (2-) antiport was increased by Cl (-) in a manner independent of gradients, and PAH (-)/KG (2-) antiport was independent of membrane potential in the absence or presence of Cl (-).