Reconstitution of the L-lactate carrier from rat and rabbit erythrocyte plasma membranes

A look at the smelly side of physiology: transport of short chain fatty acids

Fermentative organs such as the caecum, the colon, and the rumen have evolved to produce and absorb energy rich short chain fatty acids (SCFA) from otherwise indigestible substrates. Classical models postulate diffusional uptake of the undissociated acid (HSCFA). However, in net terms, a major part of SCFA absorption occurs with uptake of Na⁺ and resembles classical, coupled electroneutral NaCl transport. Considerable evidence suggests that the anion transporting proteins expressed by epithelia of fermentative organs are poorly selective and that their main function may be to transport acetate^-, propionate^-, butyrate^- and HCO₃^- as the physiologically relevant anions. Apical uptake of SCFA thus involves non-saturable diffusion of the undissociated acid (HSCFA), SCFA^-/HCO₃^- exchange via DRA (SLC26A3) and/or SCFA^--H⁺ symport (MCT1, SLC16A1). All mechanisms lead to cytosolic acidification with stimulation of Na⁺/H⁺ exchange via NHE (SLC9A2/3). Basolaterally, Na⁺ leaves via the Na⁺/K⁺-ATPase with recirculation of K⁺. Na⁺ efflux drives the transport of SCFA^- anions through volume-regulated anion channels, such as maxi-anion channels (possibly SLCO2A1), LRRC8, anoctamins, or uncoupled exchangers. When luminal buffering is inadequate, basolateral efflux will increasingly involve SCFA^-/ HCO₃^- exchange (AE1/2, SCL4A1/2), or efflux of SCFA^- with H⁺ (MCT1/4, SLC16A1/3). Furthermore, protons can be basolaterally removed by NHE1 (SCL9A1) or NBCe1 (SLC4A4). The purpose of these transport proteins is to maximize the amount of SCFA transported from the tightly buffered ingesta while minimizing acid transport through the epithelium. As known from the rumen for many decades, a disturbance of these processes is likely to cause severe colonic disease.

The SLC16A family of monocarboxylate transporters (MCTs)--physiology and function in cellular metabolism, pH homeostasis, and fluid transport

The SLC16A family of monocarboxylate transporters (MCTs) is composed of 14 members. MCT1 through MCT4 (MCTs 1-4) are H(+)-coupled monocarboxylate transporters, MCT8 and MCT10 transport thyroid hormone and aromatic amino acids, while the substrate specificity and function of other MCTs have yet to be determined. The focus of this review is on MCTs 1-4 because their role in lactate transport is intrinsically linked to cellular metabolism in various biological systems, including skeletal muscle, brain, retina, and testis. Although MCTs 1-4 all transport lactate, they differ in their transport kinetics and vary in tissue and subcellular distribution, where they facilitate "lactate-shuttling" between glycolytic and oxidative cells within tissues and across blood-tissue barriers. However, the role of MCTs 1-4 is not confined to cellular metabolism. By interacting with bicarbonate transport proteins and carbonic anhydrases, MCTs participate in the regulation of pH homeostasis and fluid transport in renal proximal tubule and corneal endothelium, respectively. Here, we provide a comprehensive review of MCTs 1-4, linking their cellular distribution to their functions in various parts of the human body, so that we can better understand the physiological roles of MCTs at the systemic level.

Studies on the DIDS-binding site of monocarboxylate transporter 1 suggest a homology model of the open conformation and a plausible translocation cycle

Site-directed mutagenesis of MCT1 was performed on exofacial lysines Lys³⁸, Lys⁴⁵, Lys²⁸², and Lys⁴¹³. K38Q-MCT1 and K38R-MCT1 were inactive when expressed at the plasma membrane of Xenopus laevis oocytes, whereas K45R/K282R/K413R-MCT1 and K45Q/K282Q/K413Q-MCT1 were active. The former exhibited normal reversible and irreversible inhibition by DIDS, whereas the latter showed less reversible and no irreversible inhibition. K45Q/K413Q-MCT1 retained some irreversible inhibition, whereas K45Q/K282Q-MCT1 and K282Q/K413Q-MCT1 did not. These data suggest that the two DIDS SO₃⁻ groups interact with positively charged Lys²⁸² together with Lys⁴⁵ and/or Lys⁴¹³. This positions one DIDS isothiocyanate group close to Lys³⁸, leading to its covalent modification and irreversible inhibition. Additional mutagenesis revealed that DIDS cross-links MCT1 to its ancillary protein embigin using either Lys³⁸ or Lys²⁹⁰ of MCT1 and Lys¹⁶⁰ or Lys¹⁶⁴ of embigin. We have modeled a possible structure for the outward facing (open) conformation of MCT1 by employing modest rotations of the C-terminal domain of the inner facing conformation modeled previously. The resulting model structure has a DIDS-binding site consistent with experimental data and locates Lys³⁸ in a hydrophobic environment at the bottom of a substrate-binding channel. Our model suggests a translocation cycle in which Lys³⁸ accepts a proton before binding lactate. Both the lactate and proton are then passed through the channel via Asp³⁰²⁻ and Asp³⁰⁶⁺, an ion pair already identified as important for transport and located adjacent to Phe³⁶⁰, which controls channel selectivity. The cross-linking data have also been used to model a structure of MCT1 bound to embigin that is consistent with published data.

Enhanced detergent extraction for analysis of membrane proteomes by two-dimensional gel electrophoresis

Background

The analysis of hydrophobic membrane proteins by two-dimensional gel electrophoresis has long been hampered by the concept of inherent difficulty due to solubility issues. We have optimized extraction protocols by varying the detergent composition of the solubilization buffer with a variety of commercially available non-ionic and zwitterionic detergents and detergent-like phospholipids.

Results

After initial analyses by one-dimensional SDS-PAGE, quantitative two-dimensional analyses of human erythrocyte membranes, mouse liver membranes, and mouse brain membranes, extracted with buffers that included the zwitterionic detergent MEGA 10 (decanoyl-N-methylglucamide) and the zwitterionic lipid LPC (1-lauroyl lysophosphatidylcholine), showed selective improvement over extraction with the common 2-DE detergent CHAPS (3 [(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate). Mixtures of the three detergents showed additive improvements in spot number, density, and resolution. Substantial improvements in the analysis of a brain membrane proteome were observed.

Conclusion

This study demonstrates that an optimized detergent mix, coupled with rigorous sample handling and electrophoretic protocols, enables simple and effective analysis of membrane proteomes using two-dimensional electrophoresis.

Basigin (CD147) is the target for organomercurial inhibition of monocarboxylate transporter isoforms 1 and 4: the ancillary protein for the insensitive MCT2 is EMBIGIN (gp70)

Translocation of monocarboxylate transporters MCT1 and MCT4 to the plasma membrane requires CD147 (basigin) with which they remain tightly associated. However, the importance of CD147 for MCT activity is unclear. MCT1 and MCT4 are both inhibited by the cell-impermeant organomercurial reagent p-chloromercuribenzene sulfonate (pCMBS). Here we demonstrate by site-directed mutagenesis that removal of all accessible cysteine residues on MCT4 does not prevent this inhibition. pCMBS treatment of cells abolished co-immunoprecipitation of MCT1 and MCT4 with CD147 and enhanced labeling of CD147 with a biotinylated-thiol reagent. This suggested that CD147 might be the target of pCMBS, and further evidence for this was obtained by treatment of cells with the bifunctional organomercurial reagent fluorescein dimercury acetate that caused oligomerization of CD147. Site-directed mutagenesis of CD147 implicated the disulfide bridge in the Ig-like C2 domain of CD147 as the target of pCMBS attack. MCT2, which is pCMBS-insensitive, was found to co-immunoprecipitate with gp70 rather than CD147. The interaction between gp70 and MCT2 was confirmed using fluorescence resonance energy transfer between the cyan fluorescent protein- and yellow fluorescent protein-tagged MCT2 and gp70. pCMBS strongly inhibited lactate transport into rabbit erythrocytes, where MCT1 interacts with CD147, but not into rat erythrocytes where it interacts with gp70. These data imply that inhibition of MCT1 and MCT4 activity by pCMBS is mediated through its binding to CD147, whereas MCT2, which associates with gp70, is insensitive to pCMBS. We conclude that ancillary proteins are required to maintain the catalytic activity of MCTs as well as for their translocation to the plasma membrane.

Lactate transporters (MCT proteins) in heart and skeletal muscles

Lactate traverses the cell membranes of many tissues, including the heart and skeletal muscle via a facilitated monocarboxylate transport system that functions as a proton symport and is stereoselective for L-lactate. In the past few years, seven monocarboxylate transporters have been cloned. Monocarboxylate transporters are ubiquitously distributed among many tissues, and the transcripts of several monocarboxylate transporters are present within many of the same tissues. This complicates the identification of their metabolic function. There is also evidence that that there is some species specificity, with differences in MCT tissue distributions in hamsters, rats, and humans. MCT1 and MCT3-M/MCT4 are present in rat and human muscles, and MCT1 expression is highly correlated with the oxidative capacity of skeletal muscles and with their capacity to take up lactate from the circulation. MCT1 is also present in heart and is located on the plasma membrane (in subdomains), T-tubules, and in caveolae. With training, MCT1 is increased in rat and human muscle, and in rat hearts, resulting in an increased uptake of lactate from the buffers perfused through these tissues and an increase in lactate efflux out of purified vesicles. In humans, the training-induced increases in MCT1 are associated with an increased lactate efflux out of muscle. MCT3-M/MCT4 is not correlated with the muscles' oxidative capacities but is equally abundant in Type IIa and IIb muscles, whereas it is markedly lower in slow-twitch (Type I) muscles. Clearly, we are at the threshold of a new era in understanding the regulation of lactate movement into and out of skeletal muscle and cardiac cells.

The proton-linked monocarboxylate transporter (MCT) family: structure, function and regulation

Monocarboxylates such as lactate and pyruvate play a central role in cellular metabolism and metabolic communication between tissues. Essential to these roles is their rapid transport across the plasma membrane, which is catalysed by a recently identified family of proton-linked monocarboxylate transporters (MCTs). Nine MCT-related sequences have so far been identified in mammals, each having a different tissue distribution, whereas six related proteins can be recognized in Caenorhabditis elegans and 4 in Saccharomyces cerevisiae. Direct demonstration of proton-linked lactate and pyruvate transport has been demonstrated for mammalian MCT1-MCT4, but only for MCT1 and MCT2 have detailed analyses of substrate and inhibitor kinetics been described following heterologous expression in Xenopus oocytes. MCT1 is ubiquitously expressed, but is especially prominent in heart and red muscle, where it is up-regulated in response to increased work, suggesting a special role in lactic acid oxidation. By contrast, MCT4 is most evident in white muscle and other cells with a high glycolytic rate, such as tumour cells and white blood cells, suggesting it is expressed where lactic acid efflux predominates. MCT2 has a ten-fold higher affinity for substrates than MCT1 and MCT4 and is found in cells where rapid uptake at low substrate concentrations may be required, including the proximal kidney tubules, neurons and sperm tails. MCT3 is uniquely expressed in the retinal pigment epithelium. The mechanisms involved in regulating the expression of different MCT isoforms remain to be established. However, there is evidence for alternative splicing of the 5'- and 3'-untranslated regions and the use of alternative promoters for some isoforms. In addition, MCT1 and MCT4 have been shown to interact specifically with OX-47 (CD147), a member of the immunoglobulin superfamily with a single transmembrane helix. This interaction appears to assist MCT expression at the cell surface. There is still much work to be done to characterize the properties of the different isoforms and their regulation, which may have wide-ranging implications for health and disease. In the future it will be interesting to explore the linkage of genetic diseases to particular MCTs through their chromosomal location.

Lactate transport in heart in relation to myocardial ischemia

In this article, the importance of lactic acid transport into and out of heart cells is described and the properties of the monocarboxylate transporters (MCTs) responsible are presented. These are monocarboxylate/proton symporters with a broad substrate specificity that includes L-lactate, pyruvate, and the ketone bodies acetate, acetoacetate, and beta-hydroxybutyrate. Although it is unlikely that lactic acid transport constrains heart metabolism under most conditions, it may do so during severe hypoxia or ischemia. The transporter plays a critical role in maintaining intracellular pH because it removes the protons that are produced stoichiometrically with lactate during glycolysis. The kinetics and substrate and inhibitor specificities of the transport process have been determined in cell suspensions using a radiotracer technique and in single cells using a fluorescent measurement of the decrease in intracellular pH that accompanies transport. The results of these experiments suggest the presence of 2 different transporter isoforms in heart cells, at least one of which is different from the cloned MCT1 and MCT2. Immunofluorescence microscopy shows that MCT1 expression is restricted to the intercalated disk region, yet the rate of lactate transport in this region is slower than in the center of the cell, where there is no MCT1. New cDNA sequences with strong homology to MCT1 have been found in human cDNA libraries and Northern blots show that the corresponding mRNA is expressed in rat heart. Expressions of these new MCT isoforms have yet to be demonstrated and their properties and cellular distribution defined.

Studies of the membrane topology of the rat erythrocyte H+/lactate cotransporter (MCT1)

1. Hydrophobicity analysis of the monocarboxylate/proton cotransporter MCT1 (lactate transporter) suggests a structure with 12 transmembrane (TM) segments, presumed to be alpha-helical. 2. A series of anti-peptide antibodies have been raised against regions of the MCT1 sequence, which each recognize a polypeptide of approx. 40 kDa in rat erythrocytes. The topology of rat MCT1 was investigated by studying the immunoreactive fragments derived from proteolytic digestion of the protein in intact rat erythrocytes and leaky membranes. 3. Reactivity with an anti-(C-terminus) antibody was prevented on treatment of leaky membranes, but not intact cells, with carboxypeptidase Y, indicating that the C-terminus of the protein is cytoplasmically disposed. 4. Treatment of intact cells in saline buffer with trypsin, chymotrypsin, bromelain and protease K (up to 1 mg/ml) resulted in no degradation of MCT1, indicating the absence of any large exposed extracellular loop. In a buffer of low ionic strength (containing sucrose), cleavage was observed with bromelain at an extracellular site, probably TM9/10.5. Treatment of leaky membranes with low (less than 100 micrograms/ml) concentrations of several proteases resulted in fragmentation of MCT1, reflecting cleavage at the cytoplasmic face of the membrane. These treatments generated N-terminal fragments of apparent molecular mass approx. 17-19 kDa that were resistant to further degradation. The epitopes for the TM6/7 and C-terminal antibodies were either lost from the membrane or destroyed under most of these conditions, indicating that these regions of the protein are located in the cytoplasm. 6. More detailed structural prediction analysis of MCT-related sequences was made assuming the constraints placed upon the possible arrangements by the experimental data outlined above. This analysis provided additional strong evidence for the 12-TM-segment model, with cytoplasmic N- and C-terminal ends and a large internal loop between TM6 and TM7. The predicted helices were assigned moments of hydrophobicity and residue substitution; for a number of TM segments this permitted the prediction of the sides of the helix that faced membrane lipid and the interior of the protein.

Role of the lactate transporter (MCT1) in skeletal muscles

We used an antibody, constructed against the monocarboxylate transporter 1 (MCT1) protein (L. Carpenter, R. C. Poole, and A. P. Halestrap. Biochim. Biophys. Acta 1279: 157-165, 1996), to study the expression and role of MCT1 in rat skeletal muscles. MCT1 was higher in red than in white muscles (P < 0.05) and was highly correlated with the oxidative fiber content (%slow-twitch oxidative + %fast-twitch oxidative glycolytic) of skeletal muscles (r = 0.91). MCT1 was highly related to lactate uptake in skeletal muscles (r = 0.90). Total lactate dehydrogenase (LDH) activity, an index of glycolysis, was negatively correlated with MCT1 in rat muscles (r = -0.80). MCT1 was also strongly correlated with the heart-type forms of LDH (LDH-1 vs. MCT1, r = 0.83; LDH-2 vs. MCT1, r = 0.89). There was no relationship between MCT1 and the muscle form of LDH (LDH-5; P > 0.05). MCT1 was highly correlated with citrate synthase activity, a marker of the oxidative capacity of muscle (r = 0.82). Therefore, MCT1 may have kinetics that favor the uptake of L-lactate into the muscle cell for oxidative metabolism, and MCT1 may be coordinately expressed with the heart forms of LDH and enzymes of oxidative metabolism.

Characterization of the enhanced transport of L- and D-lactate into human red blood cells infected with Plasmodium falciparum suggests the presence of a novel saturable lactate proton cotransporter

Human erythrocytes parasitized with the malarial protozoan Plasmodium falciparum showed rates of L-lactate, D-lactate, and pyruvate uptake many fold greater than control cells. Thus it was necessary to work at 0 degrees C to resolve true initial rates of transport. Studies on the dependence of the rate of transport on substrate concentration implied the presence in parasitized cells of both a saturable mechanism blocked by alpha-cyano-4-hydroxycinnamate (CHC) and a nonsaturable mechanism insensitive to CHC. The former was dominant at physiological substrate concentrations with Km values for pyruvate and D-lactate of 2.3 and 5.2 mM, respectively, with no stereoselectivity for L- over D-lactate. CHC was significantly less effective as an inhibitor of lactate transport in parasitized erythrocytes than in uninfected cells, whereas p-chloromercuribenzenesulfonate, a potent inhibitor in control cells, gave little or no inhibition of lactate transport into parasitized erythrocytes. Inhibition of transport into infected cells was also observed with phloretin, furosemide, niflumic acid, stilbenedisulfonate derivatives, and 5-nitro-2-(3-phenylpropylamino)benzoic acid at concentrations similar to those that inhibit the lactate carrier of control erythrocytes. These compounds were more effective inhibitors of the rapid transport of chloride into infected cells than of lactate transport, whereas CHC was more effective against lactate transport. This implies that different pathways are involved in the parasite-induced transport pathways for lactate and chloride. The transport of L-lactate into infected erythrocytes was also inhibited by D-lactate, pyruvate, 2-oxobutyrate, and 2-hydroxybutyrate. The intracellular accumulation of L-lactate at equilibrium was dependent on the transmembrane pH gradient, suggesting a protogenic transport mechanism. Our data are consistent with lactate and pyruvate having direct access to the malarial parasite, perhaps via the proposed parasitophorous duct or some close contact between the host cell and parasite plasma membranes, with transport across the latter by both a proton-linked carrier (CHC-sensitive, saturable, and the major route) and free diffusion of the undissociated acid (CHC-insensitive, unsaturable, and a minor route).

L(+)-lactate binding to a protein in rat skeletal muscle plasma membranes

Biochemical studies were conducted to determine the location of a putative lactate transport protein in rat skeletal muscle plasma membranes (PM). PM (50-100 micrograms protein) were incubated with [U-14C] L(+)-lactate, in the presence or absence of unlabeled monocarboxylates or potential inhibitors, after which proteins were separated by SDS-PAGE. Gel slices (2 mm) were cut and analyzed for 14C. [U-14C] L(+)-lactate was bound to plasma membranes in the 30 to 40 kDa molecular mass range. Binding of [U-14C] L(+)-lactate was inhibited by N-ethylmaleimide, unlabeled L-lactate and pyruvate, and in a dose dependent manner by alpha-cyano-4-hydroxycinnamate (r = 0.995), but not by cytochalasin-B. The inhibition of [U-14C] L(+)-lactate binding was similar to the inhibition of lactate transport. Therefore the transport of L(+)-lactate across skeletal muscle plasma membranes involves a polypeptide of 30 to 40 kDa.

The kinetics, substrate and inhibitor specificity of the lactate transporter of Ehrlich-Lettre tumour cells studied with the intracellular pH indicator BCECF

1. Suspensions of cultured Ehrlich-Lettre tumour cells were loaded with the pH-sensitive fluorescent indicator 2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF), and changes in intracellular pH upon addition of L-lactate and other monocarboxylates were continuously monitored by fluorimetry using dual-wavelength excitation (450/500 nm) and single-wavelength emission (> 520 nm). 2. The rapid fluorescence changes were analysed by first-order regression analysis, and with suitable calibration procedures this enabled calculation of initial rates of proton uptake associated with monocarboxylate transport. 3. The stoichiometry was shown to be one proton per lactate molecule transported. 4. The kinetics of carrier-mediated transport of a wide range of monocarboxylates were determined at 25 degrees C. The Km values for L-lactate, pyruvate and D-lactate were found to be 4.54, 0.72 and 27.5 mM respectively, similar to values found previously for rat erythrocytes. This similarity was shared with a wide range of variously substituted C2, C3 and C4 monocarboxylates, all of which were transported with similar Vmax. No stereoselectivity was found in the Km values for D- and L-2-chloropropionate (0.75 mM) or D- and L-3-hydroxybutyrate (11 mM), but in the latter case the Vmax. of the D-isomer was twice that of the L-isomer. 5. The temperature-dependence of L-lactate transport demonstrated a transition point, with activation energies of 60 and 109 kJ.mol-1 above and below 19 degrees C respectively The Km for L-lactate below the transition temperature was about half that above it. 6. Inhibition of lactate transport into tumour cells by a wide range of compounds known to inhibit the erythrocyte monocarboxylate carrier was analysed. Patterns of inhibition were similar to those seen in the erythrocyte, but the Ki values were 2-4-fold higher in the tumour cells. 7. It is concluded that tumour cells contain an isoform of the monocarboxylate carrier with functional properties almost identical with that found in erythrocytes. This is probably identical with MCT1, which was recently cloned and sequenced from Chinese Hamster Ovary cells [Kim Garcia, Goldstein, Pathak, Anderson and Brown (1994) Cell 76, 865-873].

N-terminal protein sequence analysis of the rabbit erythrocyte lactate transporter suggests identity with the cloned monocarboxylate transport protein MCT1

An improved purification for the rabbit erythrocyte lactate transporter, using aminoethyl-Sepharose chromatography, is described. The process of purification of the 40-50 kDa transporter, labelled with 4,4'-diisothiocyanostilbene-2,2'-disulphonate (DIDS), was followed by Western blotting with anti-DIDS antibodies [Poole, R. C. and Halestrap, A. P. (1992) Biochem. J. 283, 855-862]. Fractions highly-enriched in transporter were further purified by SDS/PAGE and the 40-50 kDa DIDS-labelled polypeptide was subjected to N-terminal protein sequencing. This analysis identified the first 16 amino acids of the protein. With the exception of one conservative substitution, this protein sequence is identical to the N-terminal protein sequence predicted from a cDNA isolated from Chinese hamster ovary cells that encode a monocarboxylate transporter, MCT1 [Kim Garcia, C., Goldstein, J. L., Pathak, R. K., Anderson, R. G. W. and Brown, M. S. (1994) Cell 76, 865-873]. This observation, along with similarities in functional properties, leads us to conclude that lactate transport in rabbit erythrocytes is mediated by the MCT1 monocarboxylate transporter isoform.

Reconstitution of the lactate carrier from rat skeletal-muscle sarcolemma

The lactate carrier was solubilized from purified rat skeletal-muscle sarcolemma with the detergent decanoyl-N-methyl-glucamide and the solubilized carrier was reconstituted into phospholipid vesicles. Reconstituted proteoliposomes showed a faster time course of L-lactate uptake than did protein-free liposomes. The rate of L-lactate uptake into the proteoliposomes was inhibited by the lactate-transport inhibitors p-chloromercuribenzenesulphonate, diethyl pyrocarbonate, alpha-cyano-4-hydroxycinnamate and quercetin. In contrast, the anion-exchange inhibitor DIDS (4,4'-di-isothiocyanostilbene-2,2'-disulphonate) had almost no effect on the uptake. The extent of L-lactate uptake at equilibrium was not affected by the presence of the transport inhibitors, but was sensitive to osmotic strength. L-Lactate and pyruvate, but not D-lactate, inhibited L-lactate uptake when present at 10-fold excess. The properties of L-lactate transport in reconstituted proteoliposomes were similar to those observed in native sarcolemmal vesicles, i.e. the lactate carrier seems to retain its transport characteristics during the solubilization and reconstitution steps.

Molecular characterization of a membrane transporter for lactate, pyruvate, and other monocarboxylates: implications for the Cori cycle

Lactate and pyruvate cross cell membranes via a monocarboxylate transporter (MCT) with well-defined properties but undefined molecular structure. We report the cloning of a cDNA encoding MCT1, a monocarboxylate transporter whose properties resemble those of the erythrocyte MCT, including proton symport, trans acceleration, and sensitivity to alpha-cyanocinnammates. A Phe to Cys substitution in MCT1 converts it to Mev, a mevalonate transporter. MCT1 is abundant in erythrocytes, cardiac muscle, and basolateral intestinal epithelium. In skeletal muscle it is restricted to mitochondria-rich myocytes. As sperm traverse the epididymis, MCT1 switches from sperm to epithelial cells. MCT1 is present at low levels in liver, suggesting another MCT in this tissue. By exporting lactate from intestine and erythrocytes, MCT1 participates in the Cori cycle. It also participates in novel pathways of monocarboxylate metabolism in muscle and sperm.

Reconstitution studies of amino acid transport system L in rat erythrocytes

In many cell types, including human erythrocytes, membrane transport of hydrophobic amino acids such as leucine and phenylalanine is mediated primarily by Na(+)-independent system L. In this paper we demonstrate that erythrocytes from the rat have a 400-fold higher system L transport capacity than human erythrocytes. We have exploited this high transport activity to achieve the first successful reconstitution of an erythrocyte amino acid transporter into phospholipid vesicles. Rat erythrocyte membranes were depleted of extrinsic membrane proteins, solubilized in 50 mM n-octyl glucoside and reconstituted into egg-yolk phospholipid vesicles by a gel filtration freeze-thaw protocol. Optimal reconstitution of transport activity occurred at lipid/protein ratios of 25-35:1. At a lipid/protein ratio of 25:1, one-half of the total uptake of L-[14C]leucine (0.2 mM, 25 degrees C) was inhibited by 2 mM phloretin and thus judged to be carrier-mediated. This component of L-leucine uptake was inhibited by non-radioactive L-phenylalanine and L-leucine, and only to a very much weaker extent by glycine and L-alanine. Two other inhibitors of system L in intact cells, MK196 and PCMBS (p-chloromercuriphenylsulphonate), were also effective inhibitors of phloretin-sensitive L-leucine transport in reconstituted proteoliposomes. Phloretin-insensitive uptake of L-leucine in proteoliposomes occurred by simple diffusion across the lipid bilayer.

A basolateral lactate/H+ co-transporter in Madin-Darby Canine Kidney (MDCK) cells

Monolayers of Madin-Darby Canine Kidney (MDCK) cells grown on permeable filters generated lactate aerobically and accumulated it preferentially in the basolateral compartment, suggesting the presence of a lactate carrier. The mechanism of lactate transport across apical and basolateral membranes was examined by determining intracellular pH (pHi) microspectrofluorimetrically after addition of lactate to the extracellular solutions and by measuring uptake of [14C]lactate. Addition of 20 mM lactate to the apical compartment produced no change in pHi, whereas lactate added to the basolateral compartment rapidly and reversibly lowered pHi. Pyruvate produced similar results. Inhibitors of lactate/H+ co-transporters, alpha-cyano-4-hydroxycinnamate (CnCN) and quercetin, partially inhibited the fall in pHi produced by basolateral lactate. In contrast, the disulphonic stilbene. DIDS (4,4'-di-isothiocyanostilbene-2,2'-disulphonic acid) produced no inhibition at 0.5 mM. Kinetic analysis was performed by applying basolateral lactate at various concentrations and measuring the rate of entry (delta pHi/min) in the presence and absence of CnCN. Lactate flux was shown to occur by both non-ionic diffusion and a alpha-cyano-4-hydroxycinnamate-sensitive component (carrier). The latter has a Km of approximately 7 mM for the lactate anion. Propionate, but not formate, lowered pHi to the same degree as did equimolar lactate, but the propionate effect was not inhibited by CnCN. Influx of [14C]lactate was substantially greater across the basolateral membrane than across the apical membrane and occurred in the absence of Na+. We conclude that MDCK cells grown on permeable filters generate lactate aerobically and transport it across the basolateral membrane by way of a lactate/H+ cotransporter.

Identification and partial purification of the erythrocyte L-lactate transporter

1. Intact erythrocytes were incubated with 100 microM-4,4'-di-isothiocyanostilbene-2,2'-disulphonate (DIDS), a concentration sufficient to inhibit lactate transport irreversibly by 65%. DIDS-labelled proteins were detected by immunoblotting of erythrocyte membrane proteins with an anti-DIDS antibody. Labelled polypeptides of 35-45 kDa in rat erythrocytes, and of 40-50 kDa in rabbit and guinea pig erythrocytes, were detected by this technique. In human erythrocytes, which have 10-fold less transport activity, no labelled polypeptide in this molecular mass range was detected. 2. Labelling of these 35-50 kDa polypeptides was decreased markedly in the presence of the specific inhibitors of lactate transport alpha-cyano-4-hydroxycinnamate and 4,4'-dibenzamidostilbene-2,2'-disulphonate (DBDS), which compete with DIDS for binding to the transporter. However, the weakly bound inhibitor 4,4'-dinitrostilbene-2,2'-disulphonate (DNDS) afforded little protection against labelling by DIDS. 3. The lactate transporter from rat erythrocytes was solubilized with decanoyl-N-methyl glucamide (MEGA-10) and partially purified by Mono-Q anion-exchange chromatography, with transport activity eluting at 0.1-0.15 M-NaCl. The 35-45 kDa DIDS-labelled polypeptide from rat erythrocytes was eluted in the same peak of protein as lactate transporter activity during Mono-Q chromatography. 4. These observations provide strong evidence that the lactate transporter is a polypeptide of 35-45 kDa in rat erythrocytes and of 40-50 kDa in rabbit and guinea pig erythrocytes.

Reconstitution and identification of the major Na(+)-dependent neutral amino acid-transport protein from bovine renal brush-border membrane vesicles

Amino acid transport activity from bovine renal brush-border membrane vesicles (BBMV) was reconstituted into phospholipid vesicles composed of phosphatidylcholine/5% stearylamine. Reconstitutable transport activity was enhanced in protein fractions binding to various lectins. When solubilized BBMV were fractionated on peanut lectin, a single protein band of average molecular mass 132 kDa was obtained. When this protein fraction was reconstituted into phospholipid membrane vesicles, amino acid transport activity was obtained with properties similar to those in native BBMV with regard to amino acid specificity, although the cation specificity was different. A monoclonal antibody which reacted with the same protein removed reconstitutable amino acid transport activity from solubilized BBMV. These findings may provide the first identification of a renal amino acid-transporting protein, although confirmation of this identification by other approaches will be required.

Inhibition of L-lactate transport and band 3-mediated anion transport in erythrocytes by the novel stilbenedisulphonate N,N,N',N'-tetrabenzyl-4,4'-diaminostilbene-2,2'-disulpho nat e (TBenzDS)

(1) The synthesis of the novel stilbenedisulphonate N,N,N',N'-tetrabenzyl- 4,4'-diaminostilbene-2,2'-disulphonate (TBenzDS) is described, and its interaction with the lactate transporter and band 3 protein of erythrocytes investigated. At 10% haematocrit the IC50 (concn. required for 50% inhibition) for inhibition of transport of 0.5 mM L-lactate into rat erythrocytes at 7 degrees C was approx. 1.6 microM, as low as any other inhibitor of the transporter. In human erythrocytes at 10% haematocrit the IC50 value was increased from approx. 3 microM to 9 microM upon raising the temperature from 7 degrees C to 25 degrees C. (2) TBenzDS inhibited transport of L-lactate into rat erythrocytes in a manner that was competitive with the substrate, as is the case for some other stilbene disulphonate derivatives (Poole, R.C. and Halestrap, A.P. (1991) Biochem. J. 275, 307-312). (3) Increasing the haematocrit from 5 to 20% caused a 3-fold increase in the IC50 value for inhibition of L-lactate transport in rat erythrocytes. (4) TBenzDS was found to bind to erythrocyte membranes, with a partition coefficient (Pm) of 6000-7000 under all conditions tested. (5) TBenzDS also inhibited band 3-mediated sulphate transport in rat erythrocytes; 50% inhibition required approx. 2.5 microM TBenzDS for cells at 10% haematocrit. (6) TBenzDS is fluorescent, and an enhancement of this fluorescence occurs upon addition of BSA or erythrocyte membranes. The fluorescence enhancement caused by erythrocyte membranes is due to binding of the inhibitor to the band 3 protein at the same site as the stilbenedisulphonate 4,4'-diisothiocyanodihydrostilbene-2,2'-disulphonate (H2DIDS).

Reversible and irreversible inhibition, by stilbenedisulphonates, of lactate transport into rat erythrocytes. Identification of some new high-affinity inhibitors

1. Inhibition of L-lactate transport into rat erythrocytes by stilbenedisulphonates was studied under conditions which allowed the contribution of reversible and irreversible inhibition to be assessed. 2. At low temperatures (7 degrees C), 4,4'-di-isothiocyanostilbene-2,2'-disulphonate (DIDS) and other stilbenedisulphonates were found to inhibit lactate transport instantaneously, in a manner which was fully reversible. The most potent reversible inhibitors were 4,4'-dibenzamidostilbene-2,2'-disulphonate (DBDS), DIDS and 4-acetamido-4'isothiocyanostilbene-2,2'-disulphonate (SITS), for which apparent Ki values at 0.5 mM-L-lactate were approx. 36, 53 and 130 microM respectively. 3. DIDS and DBDS were competitive inhibitors with respect to L-lactate, with Ki values of approx 40 microM and 22 microM respectively. 4. After incubation for 1 h at 37 degrees C with DIDS or its dihydro derivative (H2DIDS), which contain the amino-reactive isothiocyanate group, most of the inhibition observed was irreversible. Under these conditions the IC50 value (concn. causing 50% inhibition) for irreversible inhibition by both compounds was approx 100 microM. SITS was much less potent as an irreversible inhibitor of L-lactate transport, approx. 20% inhibition being obtained at 100 microM. 5. The reversible inhibitor DBDS (1 mM) afforded protection against irreversible inhibition by DIDS and H2DIDS (100 microM); protection was 60 and 65% respectively after a 60 min incubation. This indicates that specific binding of the irreversible inhibitors is required before covalent modification can take place. 6. These compounds may be useful high-affinity probes for lactate transport in other tissues and might act as affinity labels for the transport protein(s).

High channel-mediated water permeability in rabbit erythrocytes: characterization in native cells and expression in Xenopus oocytes

Erythrocytes from several mammalian species contain mercurial-sensitive water transporters. By a stopped-flow light scattering technique, osmotic water permeability (Pf) was exceptionally high in rabbit erythrocytes (0.053 +/- 0.002 cm/s) and reversibly inhibited by 98% by p-(chloromercuri)benzenesulfonate (pCMBS). The activation energy (Ea) was 4.6 kcal/mol (15-37 degrees C). pCMBS inhibition was half-maximal at 0.1 mM (60-min incubation); at 1 mM pCMBS, half-maximal inhibition occurred in 8 min. Pf was also inhibited by HgCl2 and pCMB with greater than 90% inhibition in 5 min. There was no inhibition by high concentrations of phloretin, DNDS, cytochalasin B, amiloride, ouabain, furosemide, and several proteases. In defolliculated Xenopus oocytes microinjected with 50 nL of water or unfractionated mRNA (1 mg/mL) from rabbit reticulocytes, oocyte Pf assayed at 10 degrees C after 72-h incubation increased from (4 +/- 1) X 10(-4) cm/s (water injected) to (18 +/- 2) X 10(-4) cm/s (mRNA injected). Pf increased linearly with [mRNA] (0-75 ng/oocyte) and was inhibited slowly and reversibly by pCMBS and immediately by HgCl2 but not by cytochalasin B, phloretin, or DNDS. Ea was 9.6 kcal/mol (water injected) and 2.6 kcal/mol (mRNA injected). These results demonstrate that rabbit erythrocytes have the highest Pf and the greatest percentage inhibition of Pf by mercurials of any mammalian erythrocyte studied. The characteristics of the expressed and native water channels were similar, suggesting that the erythrocyte water channel is a membrane protein suitable for expression cloning.

Substrate and inhibitor specificity of monocarboxylate transport into heart cells and erythrocytes. Further evidence for the existence of two distinct carriers

A range of short-chain aliphatic monocarboxylates, both unsubstituted and substituted with hydroxy, chloro and keto groups, were shown to inhibit transport of L-lactate and pyruvate into both guinea-pig cardiac myocytes and rat erythrocytes. The carrier of heart cells exhibited a higher affinity (approx. 10-fold) for most of the monocarboxylates than did the erythrocyte carrier. A notable exception was L-lactate, whose Km for both carriers was similar. The K1 values of the two carriers for inhibitors such as phenylpyruvate and alpha-cyanocinnamate derivatives were also different. The high affinity of the heart cell carrier for ketone bodies and acetate may be physiologically important, since these substrates are used as fuels by the heart.

The kinetics of transport of lactate and pyruvate into isolated cardiac myocytes from guinea pig. Kinetic evidence for the presence of a carrier distinct from that in erythrocytes and hepatocytes

1. Time courses for the uptake of L-lactate, D-lactate and pyruvate into isolated cardiac ventricular myocytes from guinea pig were determined at 11 degrees C or 0 degrees C (for pyruvate) in a citrate-based buffer by using a silicone-oil-filtration technique. These conditions enabled initial rates of transport to be measured without interference from metabolism of the substrates. 2. At a concentration of 0.5 mM, transport of all these substrates was inhibited by approx. 90% by 5 mM-alpha-cyano-4-hydroxycinnamate; at 10 mM-L-lactate a considerable portion of transport could not be inhibited. 3. Initial rates of L-lactate and pyruvate uptake in the presence of 5 mM-alpha-cyano-4-hydroxycinnamate were linearly related to the concentration of the monocarboxylate and probably represented diffusion of the free acid. The inhibitor-sensitive component of uptake obeyed Michaelis-Menten kinetics, with Km values for L-lactate and pyruvate of 2.3 and 0.066 mM respectively. 4. Pyruvate and D-lactate inhibited the transport of L-lactate, with Ki values (competitive) of 0.077 and 6.6 mM respectively; the Ki for pyruvate was very similar to its Km for transport. The Ki for alpha-cyano-4-hydroxycinnamate as a non-competitive inhibitor was 0.042 mM. 5. These results indicate that L-lactate, D-lactate and pyruvate share a common carrier in guinea-pig cardiac myocytes; the low stereoselectivity for L-lactate over D-lactate and the high affinity for pyruvate distinguish it from the carrier in erythrocytes and hepatocytes. The metabolic roles for this novel carrier in heart are discussed.

A rapid method for the functional reconstitution of amino acid transport systems from rat liver plasma membranes. Partial purification of System A

A rapid method for the functional reconstruction of amino acid transport from liver plasma-membrane vesicles using the neutral detergent decanoyl-N-glucamide ('MEGA-10') is described. The method is a modification of that previously employed in this laboratory for reconstitution of amino acid transport systems from kidney brush-border membranes [Lynch & McGivan (1987) Biochem. J. 244, 503-508]. The transport activities termed 'System A', 'System N', and 'System L' are all reconstituted. The reconstitution procedure is rapid and efficient and is suitable as an assay for transport activity in studies involving membrane fractionation. By using this reconstitution procedure, System A transport activity was partially purified by lectin-affinity chromatography.