Functional differences between rabbit and human Na(+)-dicarboxylate cotransporters, NaDC-1 and hNaDC-1

Mutations in the Na(+)/citrate cotransporter NaCT (SLC13A5) in pediatric patients with epilepsy and developmental delay

Mutations in the SLC13A5 gene that codes for the Na(+)/citrate cotransporter, NaCT, are associated with early onset epilepsy, developmental delay and tooth dysplasia in children. In the present study we identify additional SLC13A5 mutations in nine epilepsy patients from six families. To better characterize the syndrome, families with affected children answered questions about the scope of illness and treatment strategies. There are currently no effective treatments, but some anti-epileptic drugs targeting the GABA system reduce seizure frequency. Acetazolamide, a carbonic anhydrase inhibitor and atypical anti-seizure medication decreases seizures in 4 patients. In contrast to previous reports, the ketogenic diet and fasting produce worsening of symptoms. The effects of the mutations on NaCT transport function and protein expression were examined by transient transfections of COS-7 cells. There was no transport activity from any of the mutant transporters, although some of the mutant transporter proteins were present on the plasma membrane. The structural model of NaCT suggests that these mutations can affect helix packing or substrate binding. We tested various treatments, including chemical chaperones and low temperatures, but none improve transport function in the NaCT mutants. Interestingly, coexpression of NaCT and the mutants results in decreased protein expression and activity of the wild-type transporter, indicating functional interaction. In conclusion, our study has identified additional SLC13A5 mutations in patients with chronic epilepsy starting in the neonatal period, with the mutations producing inactive Na(+)/citrate transporters.

Effects of chronic lithium administration on renal acid excretion in humans and rats

Lithium therapy's most common side effects affecting the kidney are nephrogenic diabetes insipidus (NDI) and chronic kidney disease. Lithium may also induce a distal renal tubular acidosis. This study investigated the effect of chronic lithium exposure on renal acid–base homeostasis, with emphasis on ammonia and citrate excretion. We compared 11 individuals on long‐term lithium therapy with six healthy individuals. Under basal conditions, lithium‐treated individuals excreted significantly more urinary ammonia than did control subjects. Following an acute acid load, urinary ammonia excretion increased approximately twofold above basal rates in both lithium‐treated and control humans. There were no significant differences between lithium‐treated and control subjects in urinary pH or urinary citrate excretion. To elucidate possible mechanisms, rats were randomized to diets containing lithium or regular diet for 6 months. Similar to humans, basal ammonia excretion was significantly higher in lithium‐treated rats; in addition, urinary citrate excretion was also significantly greater. There were no differences in urinary pH. Expression of the critical ammonia transporter, Rhesus C Glycoprotein (Rhcg), was substantially greater in lithium‐treated rats than in control rats. We conclude that chronic lithium exposure increases renal ammonia excretion through mechanisms independent of urinary pH and likely to involve increased collecting duct ammonia secretion via the ammonia transporter, Rhcg.

This study investigated the effect of chronic lithium exposure on renal acid–base homeostasis, with emphasis on ammonia and citrate excretion. Chronic lithium exposure increases renal ammonia excretion through mechanisms independent of urinary pH and likely to involve increased collecting duct ammonia secretion via the ammonia transporter, Rhcg.

Functional characterization of a Na+-dependent dicarboxylate transporter from Vibrio cholerae

VcINDY, a bacterial homolog of transporters implicated in lifespan in fruit flies and insulin resistance in mammals, is a high affinity, electrogenic, Na⁺-dependent dicarboxylate transporter.

The SLC13 transporter family, whose members play key physiological roles in the regulation of fatty acid synthesis, adiposity, insulin resistance, and other processes, catalyzes the transport of Krebs cycle intermediates and sulfate across the plasma membrane of mammalian cells. SLC13 transporters are part of the divalent anion:Na⁺ symporter (DASS) family that includes several well-characterized bacterial members. Despite sharing significant sequence similarity, the functional characteristics of DASS family members differ with regard to their substrate and coupling ion dependence. The publication of a high resolution structure of dimer VcINDY, a bacterial DASS family member, provides crucial structural insight into this transporter family. However, marrying this structural insight to the current functional understanding of this family also demands a comprehensive analysis of the transporter’s functional properties. To this end, we purified VcINDY, reconstituted it into liposomes, and determined its basic functional characteristics. Our data demonstrate that VcINDY is a high affinity, Na⁺-dependent transporter with a preference for C₄- and C₅-dicarboxylates. Transport of the model substrate, succinate, is highly pH dependent, consistent with VcINDY strongly preferring the substrate’s dianionic form. VcINDY transport is electrogenic with succinate coupled to the transport of three or more Na⁺ ions. In contrast to succinate, citrate, bound in the VcINDY crystal structure (in an inward-facing conformation), seems to interact only weakly with the transporter in vitro. These transport properties together provide a functional framework for future experimental and computational examinations of the VcINDY transport mechanism.

Localization of the calcium-regulated citrate transport process in proximal tubule cells

Urinary citrate is an important inhibitor of calcium-stone formation. Most of the citrate reabsorption in the proximal tubule is thought to occur via a dicarboxylate transporter NaDC1 located in the apical membrane. OK cells, an established opossum kidney proximal tubule cell line, transport citrate but the characteristics change with extracellular calcium such that low calcium solutions stimulate total citrate transport as well as increase the apparent affinity for transport. The present studies address several fundamental properties of this novel process: the polarity of the transport process, the location of the calcium-sensitivity and whether NaDC1 is present in OK cells. OK cells grown on permeable supports exhibited apical >basolateral citrate transport. Apical transport of both citrate and succinate was sensitive to extracellular calcium whereas basolateral transport was not. Apical calcium, rather than basolateral, was the predominant determinant of changes in transport. Also 2,3-dimethylsuccinate, previously identified as an inhibitor of basolateral dicarboxylate transport, inhibited apical citrate uptake. Although the calcium-sensitive transport process in OK cells is functionally not typical NaDC1, NaDC1 is present in OK cells by Western blot and PCR. By immunolocalization studies, NaDC1 was predominantly located in discrete apical membrane or subapical areas. However, by biotinylation, apical NaDC1 decreases in the apical membrane with lowering calcium. In sum, OK cells express a calcium-sensitive/regulated dicarboxylate process at the apical membrane which responds to variations in apical calcium. Despite the functional differences of this process compared to NaDC1, NaDC1 is present in these cells, but predominantly in subapical vesicles.

Sodium-coupled dicarboxylate and citrate transporters from the SLC13 family

The SLC13 family in humans and other mammals consists of sodium-coupled transporters for anionic substrates: three transporters for dicarboxylates/citrate and two transporters for sulfate. This review will focus on the di- and tricarboxylate transporters: NaDC1 (SLC13A2), NaDC3 (SLC13A3), and NaCT (SLC13A5). The substrates of these transporters are metabolic intermediates of the citric acid cycle, including citrate, succinate, and α-ketoglutarate, which can exert signaling effects through specific receptors or can affect metabolic enzymes directly. The SLC13 transporters are important for regulating plasma, urinary and tissue levels of these metabolites. NaDC1, primarily found on the apical membranes of renal proximal tubule and small intestinal cells, is involved in regulating urinary levels of citrate and plays a role in kidney stone development. NaDC3 has a wider tissue distribution and high substrate affinity compared with NaDC1. NaDC3 participates in drug and xenobiotic excretion through interactions with organic anion transporters. NaCT is primarily a citrate transporter located in the liver and brain, and its activity may regulate metabolic processes. The recent crystal structure of the Vibrio cholerae homolog, VcINDY, provides a new framework for understanding the mechanism of transport in this family. This review summarizes current knowledge of the structure, function, and regulation of the di- and tricarboxylate transporters of the SLC13 family.

Role of isoleucine-554 in lithium binding by the Na+/dicarboxylate cotransporter NaDC1

Sodium-coupled transport of citric acid cycle intermediates, such as succinate and citrate, is mediated by the NaDC1 transporter located on the apical membrane of kidney proximal tubule and small intestine cells. Our previous study showed that transmembrane helix (TM) 11 of NaDC1 is important for sodium and lithium binding, as well as for determining citrate affinity [Kahn and Pajor (1999) Biochemistry 38, 6151]. In the present study, 21 amino acids in TM11 and the extracellular loop of NaDC1 were mutated one at a time to cysteine. All of the mutants were well expressed on the plasma membrane, but many of them had decreased transport activity. The G550C, W561C, and L568C mutants were inactive, suggesting that these residues may be critical for function. None of the cysteine mutants was sensitive to inhibition by the membrane-impermeant cysteine reagents, MTSET or MTSES, suggesting that the helix is inaccessible to the extracellular solvent. Although NaDC1 is inhibited by low concentrations of lithium in the presence of sodium, the I554C mutant was stimulated by lithium with a K(0.5) of 4.8 mM. The I554C mutant also had decreased affinity for sodium. We conclude that TM11 is likely to be an outer helix in NaDC1 that contains several residues critical for transport. Ile-554 in the middle of the helix may be an important determinant of cation affinity and selectivity, in particular the high affinity cation binding site that recognizes lithium.

Single nucleotide polymorphisms in the human Na+-dicarboxylate cotransporter affect transport activity and protein expression

The sodium-coupled transport of citric acid cycle intermediates in the intestine and kidney is mediated by the Na(+)-dicarboxylate cotransporter, NaDC1. In the kidney, NaDC1 plays an important role in regulating succinate and citrate concentrations in the urine, which may have physiological consequences including the development of kidney stones. In the present study, the impact of nonsynonymous single nucleotide polymorphisms (SNPs) on NaDC1 expression and function was characterized using the COS-7 cell heterologous expression system. The I550V variant had an increased sensitivity to lithium inhibition although there were no significant effects on protein abundance. The L44F variant had no significant effects on expression or function. The membrane protein abundance of the M45L, V117I, and F254L variants was decreased, with corresponding decreases in transport activity. The A310P variant had decreased protein abundance as well as a change in substrate selectivity. The P385S variant had a large decrease in succinate transport V(max), as well as altered substrate selectivity, and a change in the protein glycosylation pattern. The most damaging variant was V477M, which had decreased affinity for both succinate and sodium. The V477M variant also exhibited stimulation by lithium, indicating a change in the high-affinity cation binding site. We conclude that most of the naturally occurring nonsynonymous SNPs affect protein processing of NaDC1, and several also affect functional properties. All of these mutations are predicted to decrease transport activity in vivo, which would result in decreased intestinal and renal absorption of citric acid cycle intermediates.

Threonine-509 is a determinant of apparent affinity for both substrate and cations in the human Na+/dicarboxylate cotransporter

The Na+/dicarboxylate cotransporter (NaDC1) is involved in the absorption of citric acid cycle intermediates from the lumen of the renal proximal tubule and small intestine. The NaDC1 orthologues from human (h) and rabbit (rb) exhibit differences in citrate and cation transport properties. The citrate Km and sodium KNa values are much larger in human than rabbit NaDC1. Our previous study showed that transmembrane helices (TM) 7, 10, and 11 and associated loop regions contain the amino acids that are important in determining the differences in apparent citrate affinity, whereas TM10 and 11 determine differences in apparent sodium affinity. Chimera R10 (hNaDC1 with a substitution of TM10 and associated loop from rbNaDC1) contains only four amino acid differences between rb and hNaDC1. This chimera has similar apparent affinity for succinate and sodium as the wild-type rbNaDC1, and an intermediate Km for citrate. To identify individual residues in the TM10 region that determine functional differences between rb and hNaDC1, four mutants were made in which the rabbit sequence was substituted for that of the hNaDC1. Mutants with a serine or threonine at position 509 (or 512 in rbNaDC1) in TM10 have partial changes in Km for citrate and succinate but larger changes in apparent affinity for cations and substrate specificity for four-carbon dicarboxylates. The results show that the serine or threonine at position 509 (h) or 512 (rb) is the most important determinant of functional differences in apparent affinity for substrate and cations. Furthermore, the results suggest that the cation and substrate binding sites are located in close proximity to one another in NaDC1.

Ala-504 is a determinant of substrate binding affinity in the mouse Na(+)/dicarboxylate cotransporter

The Na(+)/dicarboxylate cotransporters from mouse (mNaDC1) and rabbit (rbNaDC1) differ in their ability to handle adipate, a six-carbon terminal dicarboxylic acid. The mNaDC1 and rbNaDC1 amino acid sequences are 75% identical. The rbNaDC1 does not transport adipate and only succinate produced inward currents under two-electrode voltage clamp. In contrast, oocytes expressing mNaDC1 had adipate-dependent inward currents that were about 60% of those induced by succinate. In order to identify domains involved in adipate transport, we examined the functional properties of a series of chimeric transporters made between mouse and rabbit NaDC1. We find that multiple transmembrane helices (TM), particularly TM 8, 9, and 10, are involved in adipate transport. In TM 10 there is only one amino acid difference between the two proteins, corresponding to Ala-504 in mouse and Ser-512 in rabbit NaDC1. The mNaDC1-A504S mutant had decreased adipate-dependent currents relative to succinate-dependent currents and an increase in the K(0.5) for both succinate and glutarate. We conclude that multiple amino acids from TM 8, 9 and 10 contribute to the transport of adipate in NaDC1. Furthermore, Ala-504 in TM 10 is an important determinant of K(0.5) for both adipate and succinate.

Molecular properties of the SLC13 family of dicarboxylate and sulfate transporters

The SLC13 gene family consists of five members in humans, with corresponding orthologs from different vertebrate species. All five genes code for sodium-coupled transporters that are found on the plasma membrane. Two of the transporters, NaS1 and NaS2, carry substrates such as sulfate, selenate and thiosulfate. The other members of the family (NaDC1, NaDC3, and NaCT) are transporters for di- and tri-carboxylates including succinate, citrate and alpha-ketoglutarate. The SLC13 transporters from vertebrates are electrogenic and they produce inward currents in the presence of sodium and substrate. Substrate-independent leak currents have also been described. Structure-function studies have identified the carboxy terminal half of these proteins as the most important for determining function. Transmembrane helices 9 and 10 may form part of the substrate permeation pathway and participate in conformational changes during the transport cycle. This review also discusses new members of the SLC13 superfamily that exhibit both sodium-dependent and sodium-independent transport mechanisms. The Indy protein from Drosophila, involved in determining lifespan, and the plant vacuolar malate transporter are both sodium-independent dicarboxylate transporters, possibly acting as exchangers. The purpose of this review is to provide an update on new advances in this gene family, particularly on structure-function studies and new members of the family.

Functional characterization of high-affinity Na(+)/dicarboxylate cotransporter found in Xenopus laevis kidney and heart

The SLC13 gene family includes sodium-coupled transporters for citric acid cycle intermediates and sulfate. The present study describes the sequence and functional characterization of a SLC13 family member from Xenopus laevis, the high-affinity Na(+)/dicarboxylate cotransporter xNaDC-3. The cDNA sequence of xNaDC-3 codes for a protein of 602 amino acids that is approximately 70% identical to the sequences of mammalian NaDC-3 orthologs. The message for xNaDC-3 is found in the kidney, liver, intestine, and heart. The xNaDC-3 has a high affinity for substrate, including a K(m) for succinate of 4 muM, and it is inhibited by the NaDC-3 test substrates 2,3-dimethylsuccinate and adipate. The transport of succinate by xNaDC-3 is dependent on sodium, with sigmoidal activation kinetics, and lithium can partially substitute for sodium. As with other members of the family, xNaDC-3 is electrogenic and exhibits inward substrate-dependent currents in the presence of sodium. However, other electrophysiological properties of xNaDC-3 are unique and involve large leak currents, possibly mediated by anions, that are activated by binding of sodium or lithium to a single site.

Substrate specificity of the human renal sodium dicarboxylate cotransporter, hNaDC-3, under voltage-clamp conditions

Proximal tubule cells extract dicarboxylates from filtrate and blood, using cotransporters located in the brush border [sodium dicarboxylate cotransporter (NaDC-1)] and basolateral cell membrane (NaDC-3). We expressed the human NaDC-3 (hNaDC-3) in Xenopus laevis oocytes and characterized it by the two-electrode voltage-clamp technique. At −60 mV, succinate (4 carbons) and glutarate (5 carbons) generated inward currents due to translocation of three sodium ions and one divalent dicarboxylate, whereas oxalate (2 carbons) and malonate (3 carbons) did not. The cis-dicarboxylate maleate produced currents smaller in magnitude, whereas the trans-dicarboxylate fumarate generated currents similar to succinate. The substituted succinate derivatives, malate, 2,2- and 2,3-dimethylsuccinate, and 2,3-dimercaptosuccinate elicited inward currents, whereas aspartate and guanidinosuccinate showed hardly detectable currents. The C-5 dicarboxylates glutarate and α-ketoglutarate produced larger currents than succinate; glutamate and folate failed to cause inward currents. Kinetic analysis revealed, at −60 mV, K0.5 values of 25 ± 12 μM for succinate and 45 ± 13 μM for α-ketoglutarate, values close to the plasma concentration of these compounds. For both compounds, the K0.5 was independent of voltage, whereas the maximal current increased with hyperpolarization. As opposed to the rat and flounder orthologs, hNaDC-3 was hardly inhibited by lithium concentrations up to 5 mM. In the absence of sodium, however, lithium can mediate succinate-dependent currents. The narrow substrate specificity prevents interaction of drugs with dicarboxylate-like structure with hNaDC-3 and ensures sufficient support of the proximal tubule cells with α-ketoglutarate for anion secretion via organic anion transporter 1 or 3.

Relevance of NAC-2, an Na+-coupled citrate transporter, to life span, body size and fat content in Caenorhabditis elegans

We have cloned and functionally characterized an Na+-coupled citrate transporter from Caenorhabditis elegans (ceNAC-2). This transporter shows significant sequence homology to Drosophila Indy and the mammalian Na+-coupled citrate transporter NaCT (now known as NaC2). When heterologously expressed in a mammalian cell line or in Xenopus oocytes, the cloned ceNAC-2 mediates the Na+-coupled transport of various intermediates of the citric acid cycle. However, it transports the tricarboxylate citrate more efficiently than dicarboxylates such as succinate, a feature different from that of ceNAC-1 (formerly known as ceNaDC1) and ceNAC-3 (formerly known as ceNaDC2). The transport process is electrogenic, as evidenced from the substrate-induced inward currents in oocytes expressing the transporter under voltage-clamp conditions. Expression studies using a reporter-gene fusion method in transgenic C. elegans show that the gene is expressed in the intestinal tract, the organ responsible for not only the digestion and absorption of nutrients but also for the storage of energy in this organism. Functional knockdown of the transporter by RNAi (RNA interference) not only leads to a significant increase in life span, but also causes a significant decrease in body size and fat content. The substrates of ceNAC-2 play a critical role in metabolic energy production and in the biosynthesis of cholesterol and fatty acids. The present studies suggest that the knockdown of these metabolic functions by RNAi is linked to an extension of life span and a decrease in fat content and body size.

Conformationally sensitive residues in transmembrane domain 9 of the Na+/dicarboxylate co-transporter

The Na(+)/dicarboxylate co-transporter, NaDC-1, couples the transport of sodium and Krebs cycle intermediates, such as succinate and citrate. Previous studies identified two functionally important amino acids, Glu-475 and Cys-476, located in transmembrane domain (TMD) 9 of NaDC-1. In the present study, each amino acid in TMD-9 was mutated to cysteine, one at a time, and the accessibility of the membrane-impermeant reagent [2-(trimethylammonium)ethyl]methanethiosulfonate (MTSET) to the replacement cysteines was determined. Cysteine substitution was tolerated at all but five of the sites: the A461C mutant was not present at the plasma membrane, whereas the F473C, T474C, E475C, and N479C mutants were inactive proteins located on the plasma membrane. Cysteine substitution of four residues found near the extracellular surface of TMD-9 (Ser-478, Ala-480, Ala-481, and Thr-482) resulted in proteins that were sensitive to inhibition by MTSET. The accessibility of MTSET to the four substituted cysteines was highest in the presence of the transported cations, sodium or lithium, and low in choline. The four mutants also exhibited substrate protection of MTSET accessibility. The MTSET accessibility to S478C, A481C, and A480C was independent of voltage. In contrast, T482C was more accessible to MTSET in choline buffer at negative holding potentials, but there was no effect of voltage in sodium buffer. In conclusion, TMD-9 may be involved in transducing conformational changes between the cation-binding sites and the substrate-binding site in NaDC-1, and it may also form part of the translocation pathway through the transporter.

Cloning and functional characterization of a high-affinity Na(+)/dicarboxylate cotransporter from mouse brain

Neurons contain a high-affinity Na(+)/dicarboxylate cotransporter for absorption of neurotransmitter precursor substrates, such as alpha-ketoglutarate and malate, which are subsequently metabolized to replenish pools of neurotransmitters, including glutamate. We have isolated the cDNA coding for a high-affinity Na(+)/dicarboxylate cotransporter from mouse brain, called mNaDC-3. The mRNA coding for mNaDC-3 is found in brain and choroid plexus as well as in kidney and liver. The mNaDC-3 transporter has a broad substrate specificity for dicarboxylates, including succinate, alpha-ketoglutarate, fumarate, malate, and dimethylsuccinate. The transport of citrate is relatively insensitive to pH, but the transport of succinate is inhibited by acidic pH. The Michaelis-Menten constant for succinate in mNaDC-3 is 140 microM in transport assays and 16 microM at -50 mV in two-electrode voltage clamp assays. Transport is dependent on sodium, although lithium can partially substitute for sodium. In conclusion, mNaDC-3 likely codes for the high-affinity Na(+)/dicarboxylate cotransporter in brain, and it has some unusual electrical properties compared with the other members of the family.

Molecular cloning, chromosomal organization, and functional characterization of a sodium-dicarboxylate cotransporter from mouse kidney

The sodium-dicarboxylate cotransporter of the renal proximal tubule, NaDC-1, reabsorbs filtered Krebs cycle intermediates and plays an important role in the regulation of urinary citrate concentrations. (1) Low urinary citrate is a risk factor for the development of kidney stones. As an initial step in the characterization of NaDC-1 regulation, the genomic structure and functional properties of the mouse Na(+)-dicarboxylate cotransporter (mNaDC-1) were determined. The gene coding for mNaDC-1, Slc13a2, is found on chromosome 11. The gene is approximately 24.9 kb in length and contains 12 exons. The mRNA coding for mNaDC-1 is found in kidney and small intestine. Expression of mNaDC-1 in Xenopus laevis oocytes results in increased transport of di- and tricarboxylates. The Michaelis-Menten constant (K(m)) for succinate was 0.35 mM, and the K(m) for citrate was 0.6 mM. The transport of citrate was stimulated by acidic pH, whereas the transport of succinate was insensitive to pH changes. Transport by mNaDC-1 is electrogenic, and substrates produced inward currents in the presence of sodium. The sodium affinity was relatively high in mNaDC-1, with half-saturation constants for sodium of 10 mM (radiotracer experiments) and 28 mM at -50 mV (2-electrode voltage clamp experiments). Lithium acts as a potent inhibitor of transport, but it can also partially substitute for sodium. In conclusion, the mNaDC-1 is related in sequence and function to the other NaDC-1 orthologs. However, its function more closely resembles the rabbit and human orthologs rather than the rat NaDC-1, with which it shares higher sequence similarity.

The transport properties of the human renal Na(+)- dicarboxylate cotransporter under voltage-clamp conditions

The transport properties of the human Na(+)-dicarboxylate cotransporter, (hNaDC-1), expressed in Xenopus laevis oocytes were characterized using the two-electrode voltage clamp technique. Steady-state succinate-evoked inward currents in hNaDC-1 were dependent on the concentrations of succinate and sodium, and on the membrane potential. At -50 mV, the half-saturation constant for succinate (K(0.5)(succinate)) was 1.1 mM and the half-saturation constant for sodium (K(0.5)(sodium)) was 65 mM. The Hill coefficient was 2.3, which is consistent with a transport stoichiometry of 3 Na(+):1 divalent anion substrate. The hNaDC-1 exhibits a high-cation selectivity. Sodium is the preferred cation and other cations, such as lithium, were not able to support transport of succinate. The preferred substrates of hNaDC-1 are fumarate (K(0.5) 1.8 mM) and succinate, followed by methylsuccinate (K(0.5) 2.8 mM), citrate (K(0. 5) 6.8 mM) and alpha-ketoglutarate (K(0.5) 16 mM). The hNaDC-1 may also transport sodium ions through an uncoupled leak pathway, which is sensitive to phloretin inhibition. We propose a transport model for hNaDC-1 in which the binding of three sodium ions is followed by substrate binding.

Citrate and succinate transport in proximal tubule cells

Urinary citrate, which inhibits calcium nephrolithiasis, is determined by proximal reabsorption via an apical dicarboxylate transporter. Citrate is predominantly trivalent at physiological pH, but citrate(-2) is transported at the apical membrane. We now demonstrate that low-Ca solutions induce transport of citrate(-2) and succinate in opossum kidney cells. With 1.2 mM extracellular Ca, citrate uptake was pH insensitive and not competed by succinate(-2). In contrast, with low extracellular Ca, citrate uptake increased twofold, was inhibited by succinate (and other dicarboxylates), was stimulated by lowering extracellular pH (consistent with citrate(-2) transport), and increased further by lowering extracellular Mg. The effect of Ca was incrementally concentration dependent, between 0 and 1.2 mM. The effect of Ca was not simply complexation with citrate because succinate (which is complexed significantly less) was affected by Ca similarly. Incubation of cells for 48 h in a low-pH media increased citrate transport (studied at control pH) more than twofold, suggesting induction of transporters.

Molecular and functional analysis of SDCT2, a novel rat sodium-dependent dicarboxylate transporter

Kidney proximal tubule cells take up Krebs cycle intermediates for metabolic purposes and for secretion of organic anions through dicarboxylate/organic anion exchange. Alteration in reabsorption of citrate is closely related to renal stone formation. The presence of distinct types of sodium-coupled dicarboxylate transporters has been postulated on either side of the polarized epithelial membrane in the kidney proximal tubule. Using a PCR-based approach, we isolated a novel member of the sodium-dependent dicarboxylate/sulfate transporter called SDCT2. SDCT2 is a 600-amino acid residue protein that has 47-48% amino acid identity to SDCT1 and NaDC-1, previously identified in kidney and intestine. Northern analysis gave a single band of 3.3 kb for SDCT2 in kidney, liver, and brain. In situ hybridization revealed that SDCT2 is prominently expressed in kidney proximal tubule S3 segments and in perivenous hepatocytes, consistent with the sites of high-affinity dicarboxylate transport identified based on vesicle studies. A signal was also detected in the meningeal layers of the brain. SDCT2 expressed in Xenopus oocytes mediated sodium-dependent transport of di- and tricarboxylates with substrate preference for succinate rather than citrate, but excluding monocarboxylates. SDCT2, unlike SDCT1, displayed a unique pH dependence for succinate transport (optimal pH 7.5-8.5) and showed a high affinity for dimethylsuccinate, two features characteristic of basolateral transport. These data help to interpret the mechanisms of renal citrate transport, their alteration in pathophysiological conditions, and their role in the elimination of organic anions and therapeutic drugs.

Mutational analysis of histidine residues in the rabbit Na+/dicarboxylate co-transporter NaDC-1

Succinate transport by the rabbit Na+/dicarboxylate co-transporter, NaDC-1, expressed in Xenopus oocytes was inhibited by the histidyl-selective reagent diethyl pyrocarbonate (DEPC). Therefore the role of histidine residues in the function of NaDC-1 was examined by site-directed mutagenesis. All 11 histidine residues in NaDC-1 were converted to alanine, but only mutant H106A exhibited a decrease in succinate transport. Additional mutations of NaDC-1 at position 106 showed that aspartic acid and asparagine, but not arginine, can substitute for histidine. Examination of succinate and citrate kinetics of H106A revealed a decrease in Vmax with no change in Km. Cell surface biotinylation experiments showed that the transport activity of all four mutants at position 106 was correlated with the amount of cell surface expression, suggesting a role of His-106 in membrane expression rather than function. Two of the histidine mutants, H153A and H569A, exhibited insensitivity to inhibition by DEPC, indicating that these residues are involved in binding DEPC. Neither of these residues is required for transport activity; thus DEPC probably inhibits NaDC-1 function by hindrance of the mobility of the carrier. We conclude that histidine residues are not critical for transport function in NaDC-1, although His-106 might be involved in determining protein expression or stability in the membrane.