Substrate specificity of the intestinal brush-border proline/sodium (IMINO) transporter

Summary l-proline uptake via the intestinal brush-borderIMINO carrier was tested for inhibition by 41 compounds which included sugars, N-methylated, -,-, - and -amino and imino acids, and heterocyclic analogs of pyrrolidine, piperidine and pyridine. Based on competitive inhibitor constants (apparentK/'s) we find that theIMINO carrier binding site interacts with molecules which possess a well-defined set of structural prerequisites. The ideal inhibitor must 1) be a heterocyclic nitrogen ring, 2) have a hydrophobic region, 3) be thel-stereoisomer of 4) an electronegative carbonyl group which is 5) separated by a one-carbon atom spacer from 6) an electropositive tetrahedral imino nitrogen with two H atoms. Finally, 7) the inhibitor conformation determined by dynamic ring puckering must position all these features within a critical domain. The two best inhibitors arel-pipecolate (apparentK/0.2mm) andl-proline (apparentK/0.3mm).     

Transport of lactate and acetate through the energized cytoplasmic membrane of Escherichia coli

Escherichia coli produces lactate and acetate in significant amounts during both aerobic and anaerobic glycolysis. A model describing the mechanism of protein mediated lactate transport has previously bee proposed. A simple theoretical analysis here indicates that the proposed model would be drain cellular energy resources by catalytically dissipating the proton-motive force. An experimental analysis of lactate and acetate transport employ nuclear magnetic resonance (NMR) spectroscopy to measure the relative concentration of these end products on the two sides of the cytoplasmic membrane of anaerobically glycolyzing cells. Comparison of measured concentration rations to those expected at equilibrium for various transport modes indicates that acetate is a classical uncoupling agent, permeating the membrane oat comparable rates in the dissociated and undissociated forms. The lactate concentration ratio changes market markedly after an initial period of sustained glycolysis. This change is most readily explained as resulting from a lactate transport system that responds to an indicator of glycolytic activity. The data further indicates that lactate permeates the membrane in both dissociated and undissociated forms. Both acids, then are capable of catalytically dissipating the proton-motives force. (c) 1995 John Wiley & Sons, Inc.     

Substrate specificity of human ceramide kinase

Previous studies in our laboratory have established ceramide kinase (CERK) as a critical mediator of eicosanoid synthesis. To date, CERK has not been well characterized in vitro. In this study, we investigated the substrate specificity of CERK using baculovirus-expressed human CERK (6 x His) and a newly designed assay based on mixed micelles of Triton X-100. The results indicate that the ability of CERK to recognize ceramide as a substrate is stereospecific. A minimum of a 12 carbon acyl chain was required for normal CERK activity, and the 4-5 trans double bond was important for substrate recognition. A significant discrimination by CERK was not observed between ceramides with long saturated and long unsaturated fatty acyl chains. Methylation of the primary hydroxyl group resulted in a loss of activity, confirming that CERK produces ceramide-1-phosphate versus ceramide-3-phosphate. In addition, methylation of the secondary hydroxyl group drastically decreased the phosphorylation by CERK. These results also indicated that the free hydrogen of the secondary amide group is critical for substrate recognition. Lastly, the sphingoid chain was also required for substrate recognition by CERK. Together, these results indicate a very high specificity for substrate recognition by CERK, explaining the use of ceramide and not sphingosine or diacylglycerol as substrates.     

Effects of variation of ion and methylation of carrier on the rate constants of macrotetralide-mediated ion transport in lipid bilayers

Summary The effects of methylation on the rate constants of carrier-mediated ion transport have been studied on monooleindecane bilayers with K⁺, Rb⁺, NH ₄ ⁺ , and TI⁺ ions, using the series of homologue carriers, nonactin, monactin, dinactin, trinactin, and tetranactin, each member of the series differing from the previous one by only one methyl group. Measurements of the amplitude and time constant of the current relaxation after a voltage jump over a large domain of voltage and permeant ion concentration, together with a computer curve-fitting procedure, have allowed us, without the help of steady-state current-voltage data, to deduce and compare the values of the various rate constants for ion transport: formation (k _Ri) and dissociation (k _Di) of the ion-carrier complex at the interface, translocation across the membrane interior of the carrier (k _s) and the complex (k _is). With the additional information from steady-state low-voltage conductance measurements, we have obtained the value of the aqueous phase-membrane and torus-membrane partition coefficient of the carrier ({ie191-1} and {ie191-2}). From nonactin to tetranactin with the NH ₄ ⁺ ion,k _is, and {ie191-3} are found to increase by factors of 5 and 3, respectively,k _Di and {ie191-4} to decrease respectively by factors 8 and 2, whilek _Ri andk _s are practically invariant. Nearly identical results are found for K⁺, Rb⁺, and Tl⁺ ions.k _Ri,k _s andk _is are quite invariant from one ion to the other except for Tl⁺ wherek _Ri is about five times larger. On the other hand,k _Di depends strongly on the ion, indicating that dissociation is the determining step of the ionic selectivity of a given carrier. The systematic variations in the values of the rate constants with increasing methylation are interpreted in terms of modifications of energy barriers induced by the carrier increasing size. Within this framework, we have been able to establish and verify a fundamental relationship between the variations ofk _is andk _Di with methylation.     

Substrate specificity and ion coupling in the Na+/betaine symporter BetP

BetP is an Na(+)-coupled betaine-specific transporter of the betaine-choline-carnitine (BCC) transporter family involved in the response to hyperosmotic stress. The crystal structure of BetP revealed an overall fold of two inverted structurally related repeats (LeuT-fold) that BetP shares with other sequence-unrelated Na(+)-coupled symporters. Numerous structures of LeuT-fold transporters in distinct conformational states have contributed substantially to our understanding of the alternating access mechanism of transport. Nevertheless, coupling of substrate and co-transported ion fluxes has not been structurally corroborated to the same extent. We converted BetP by a single-point mutation--glycine to aspartate--into an H(+)-coupled choline-specific transporter and solved the crystal structure of this mutant in complex with choline. The structure of BetP-G153D demonstrates a new inward-facing open conformation for BetP. Choline binding to a location close to the second, low-affinity sodium-binding site (Na2) of LeuT-fold transporters is facilitated by the introduced aspartate. Our data confirm the importance of a cation-binding site in BetP, playing a key role in a proposed molecular mechanism of Na(+) and H(+) coupling in BCC transporters.     

The substrate specificity of the human ADP/ATP carrier AAC1

Abstract

The mitochondrial ADP/ATP carrier imports ADP from the cytosol into the mitochondrial matrix for its conversion to ATP by ATP synthase and exports ATP out of the mitochondrion to replenish the eukaryotic cell with chemical energy. Here the substrate specificity of the human mitochondrial ADP/ATP carrier AAC1 was determined by two different approaches. In the first the protein was functionally expressed in Escherichia coli membranes as a fusion protein with maltose binding protein and the effect of excess of unlabeled compounds on the uptake of [³²P]-ATP was measured. In the second approach the protein was expressed in the cytoplasmic membrane of Lactococcus lactis. The uptake of [¹⁴C]-ADP in whole cells was measured in the presence of excess of unlabeled compounds and in fused membrane vesicles loaded with unlabeled compounds to demonstrate their transport. A large number of nucleotides were tested, but only ADP and ATP are suitable substrates for human AAC1, demonstrating a very narrow specificity. Next we tried to understand the molecular basis of this specificity by carrying out molecular-dynamics simulations with selected nucleotides, which were placed at the entrance of the central cavity. The binding of the phosphate groups of guanine and adenine nucleotides is similar, yet there is a low probability for the base moiety to be bound, likely to be rooted in the greater polarity of guanine compared to adenine. AMP is unlikely to engage fully with all contact points of the substrate binding site, suggesting that it cannot trigger translocation.     

Amino acid specificity of the Na/alanine cotransporter in pancreatic acinar cells

The method of tight-seal whole0cell recording was used to study the amino-acid specificity of the Na⁺/alanine cotransporter in pancreatic acinar cells. Single cells or small clusters of electrically coupled cells were obtained by enzymatic dissociation of mouse pancreas. Inward currents were measured under ‘zero-trans’ conditions, i.e., at finite concentrations of Na⁺ and amino acid at the extracellular side and vanishing concentrations at the cytoplasmic side. The cotransporter, which corresponds to ‘system A’, as previously defined in the literature, was found to exhibit a wide tolerance to neutral amino acids (l-cysteine, l-serine, l-alanine, glycine, l-phenylalanine). Competition experiments with 2-methylaminoisobutyric acid (MeAIB) indicate that for glycine a second electrogenic transport system exists in pancreatic acinar cells.     

Regulation of leukotriene synthesis by arachidonic acid in human polymorphonuclear leukocyte adhesive interactions is dependent on the presence of albumin

We have previously demonstrated that the pretreatment of polymorphonuclear leukocytes (PMNs) with the chemotherapeutic drug, Suramin, increases both cell attachment and inhibits calcium ionophore A23187-stimulated leukotriene (LT) synthesis. Here, we examined the effects of extracellular arachidonic acid (AA) and albumin on attachment and LT synthesis in the interaction of PMNs with both collagen-coated surfaces and human umbilical vein endothelial cell (HUVEC) monolayers. Suramin decreased the release of radiolabelled AA and 5-lipoxygenase metabolites by [(14)C-AA]-prelabelled PMNs stimulated with A23187, with and without human serum albumin (HSA) in the culture medium. Addition of 1 microM AA together with calcium ionophore stimulated the release of endogenous AA to the same level as control and Suramin-pretreated cells, but attachment was unaffected and LT synthesis was still inhibited with Suramin treatment. Using 24 microM AA, regulation of LT synthesis was dependent on the presence of HSA in the medium. Without HSA, 24 microM AA induced detachment of PMNs and increased LT synthesis in Suramin-treated cells above the control level. In the presence of HSA, 24 microM AA did not influence PMN attachment or abolish Suramin-induced inhibition of LT synthesis. These results suggest that tight attachment of PMNs to a solid surface leads to decreased LT synthesis during subsequent stimulation of the cells by A23187 in the presence or absence of exogenous substrate.     

Calmodulin-dependents stimulation of the NADPH oxidase of human neutrophils

The NADPH oxidase of human neutrophils is highly sensitive to calcium concentration and is inhibited in intact cells and cell-free preparations by various phenothiazine drugs. Addition of calmodulin to preparations of NADPH oxidase stimulates enzymatic rates from 1.4–2.5-fold. Addition of calmodulin and calcium, but not calcium alone, to NADPH oxidase preparations which have been inactivated by EDTA results in the restoration of activity. No activation is observed when membrane preparations containing latent NADPH oxidase are exposed to calcium and calmodulin. These studies suggest a role for calmodulin in the control of NADPH oxidase but that calmodium alone is not sufficient for activation.     

Substrate specificity of the lactate dehydrogenase isoenzyme C4 from human spermatozoa and a possible selective assay.

The activity of lactate dehydrogenase (EC 1.1.1.27) in normal human sperm lysates and in human heart and liver homogenates was determined by using a variety of 2-oxoacids as substrates. Sperm preparations were active with pyruvate, 2-oxobutanoate, 2-oxopentanoate and 2-oxohexanoate, while heart and liver extracts utilized only pyruvate and 2-oxobutanoate. Selective staining after gel electrophoresis indicated that the fraction corresponding to lactate dehydrogenase C4, the sperm-specific isoenzyme, was responsible for the utilization of substrates with a linear chain of 3 to 6 carbon atoms. The use of 5 mM 2-oxohexanoate allowed the selective determination of isoenzyme C4 in preparations containing different lactate dehydrogenase molecular forms.     

Cryo‐EM structure of the human NKCC1 transporter reveals mechanisms of ion coupling and specificity

The sodium–potassium–chloride transporter NKCC1 of the SLC12 family performs Na⁺‐dependent Cl⁻‐ and K⁺‐ion uptake across plasma membranes. NKCC1 is important for regulating cell volume, hearing, blood pressure, and regulation of hyperpolarizing GABAergic and glycinergic signaling in the central nervous system. Here, we present a 2.6 Å resolution cryo‐electron microscopy structure of human NKCC1 in the substrate‐loaded (Na⁺, K⁺, and 2 Cl⁻) and occluded, inward‐facing state that has also been observed for the SLC6‐type transporters MhsT and LeuT. Cl⁻ binding at the Cl1 site together with the nearby K⁺ ion provides a crucial bridge between the LeuT‐fold scaffold and bundle domains. Cl⁻‐ion binding at the Cl2 site seems to undertake a structural role similar to conserved glutamate of SLC6 transporters and may allow for Cl⁻‐sensitive regulation of transport. Supported by functional studies in mammalian cells and computational simulations, we describe a putative Na⁺ release pathway along transmembrane helix 5 coupled to the Cl2 site. The results provide insight into the structure–function relationship of NKCC1 with broader implications for other SLC12 family members.     

Substrate specificity and inhibitor studies of a membrane-bound ganglioside sialidase isolated from human brain tissue

A ganglioside-specific sialidase that controls cellular functions such as growth, differentiation, and adhesion has been observed in a variety of cells, but its characterization proved difficult due to firm membrane attachment and lability of the purified enzyme. Here we report on the specificity toward gangliosides and susceptibility to certain inhibitors of a ganglioside sialidase solubilized and purified 5100-fold from human brain. The sialidase removed terminal sialic acids from gangliosides GM3, GM4, GD3, GD2, GD1 a, GD1 b, GT1 b and GQ1 b, but was inactive toward gangliosides with sialic acid in a branching position (as in GM1 and GM2). Lyso-GM3 and -GD1a were good substrates, too, whereas O-acetylation of the sialic acid as in 9-O-acetyl-GD3 caused strongly reduced cleavage. The new influenza virus drug 4-guanidino-2-deoxy-2,3-dehydro-N-acetylneuraminic acid (Zanamivir) exhibited an IC50 value of about 7 x 10(-5) M that was in the range of the 'classical' sialidase inhibitor 2-deoxy-2,3-dehydro-N-acetylneuraminic acid; the bacterial sialidase inhibitor 4-nitrophenyloxamic acid, however, was ineffective. The glycosaminoglycans heparan sulfate, heparin, chondroitin sulfates A and B, as well as dextran sulfate and suramin, were all strongly inhibitory, suggesting that glycosaminoglycans present on the cell surface or in the extracellular matrix may influence the ability of the sialidase to alter the ganglioside composition of the membrane.     

The cell membrane and the struggle for life of lactic acid bacteria

The major life-threatening event for lactic acid bacteria (LAB) in their natural environment is the depletion of their energy sources and LAB can survive such conditions only for a short period of time. During periods of starvation LAB can exploit optimally the potential energy sources in their environment usually by applying proton motive force generating membrane transport systems. These systems include in addition to the proton translocating F_oF₁-ATPase: a respiratory chain when hemin is present in the medium, electrogenic solute uptake and excretion systems, electrogenic lactate/proton symport and precursor/ product exchange systems. Most of these metabolic energy-generating systems offer as additional bonus the prevention of a lethal decrease of the internal and external pH. LAB have limited biosynthetic capacities and rely heavily on the presence of essential components such as sources of amino acids in their environment. The uptake of amino acids requires a major fraction of the available metabolic energy of LAB. The metabolic energy cost of amino acid uptake can be reduced drastically by accumulating oligopeptides instead of the individual amino acids and by proton motive force-generating efflux of excessively accumulated amino acids. Other life-threatening conditions that LAB encounter in their environment are rapid changes in the osmolality and the exposure to cytotoxic compounds, including antibiotics. LAB respond to osmotic upshock or downshock by accumulating or releasing rapidly osmolytes such as glycine-betaine. The life-threatening presence of cytotoxic compounds, including antibiotics, is effectively counteracted by powerful drug extruding multidrug resistance systems. The number and variety of defense mechanisms in LAB is surprisingly high. Most defense mechanisms operate in the cytoplasmic membrane to control the internal environment and the energetic status of LAB. Annotation of the functions of the genes in the genomes of LAB will undoubtely reveal additional defense mechanisms.     

Negative hydrophobic ions as transport-mediators for positive ions. Evidence for a carrier mechanism

The permeability of hydrophobic cations, such as tetraphenylarsonium across biological membranes and artificial lipid membranes is strongly increased in the presence of trace amounts of hydrophobic anions like tetraphenylborate (Liberman, Y.A. and Topaly, V.P. (1969) Biofizika 14, 452–461). Voltage-jump relaxation experiments performed on thin lipid membranes support the idea that the anions, A^?, act as carriers for the cations, B⁺, by the formation of neutral ion pairs, A^?B⁺. Their permeability is not affected by the electric dipole potential, which hinders the movement of free cations, B⁺.     

Substrate and inhibitor specificity of the cholesterol oxidase in bovine adrenal cortex

1. Cholesteryl 3β-sulphate is oxidized in vitro by preparations of bovine adrenal-cortex mitochondria to pregnenolone sulphate and isocaproic acid (4-methyl-pentanoic acid) without hydrolysis of the ester linkage. 2. Free cholesterol is the preferred substrate for adrenal-cortex cholesterol oxidase; the apparent K_m for cholesteryl sulphate is 500μm and for free cholesterol 50μm under the same conditions. 3. Cholesteryl 3β-acetate is hydrolysed by bovine adrenal-cortex mitochondria in vitro to free cholesterol, which is subsequently oxidized to more polar steroids and isocaproic acid. Evidence was obtained that other cholesterol esters behave similarly. Cholesterol esters may thus act as precursors of steroid hormones. 4. Cholest-4-en-3-one is only poorly oxidized to isocaproic acid and more polar steroids and thus is probably not a significant precursor of steroid hormones. 5. Cholesteryl esters inhibit the oxidation of cholesterol competitively (K_i for cholesteryl phosphate 28μm, for cholesteryl sulphate 110μm, for cholesteryl acetate 65μm) but pregnenolone esters do not inhibit this system. 6. Pregnenolone and 20α-hydroxycholesterol (both metabolites of cholesterol in this system) inhibit the oxidation of cholesterol non-competitively. K_i for pregnenolone is 130μm and K_i for 20α-hydroxycholesterol is 17μm. 7. 25-Oxo-27-norcholesterol inhibits cholesterol oxidation non-competitively (K_i16μm). A number of other Δ⁵-3β-hydroxy steroids inhibit cholesterol oxidation and evidence was obtained that the 3β-hydroxyl group was necessary for inhibitory activity. 8. Pregnenolone, 20α-hydroxycholesterol and 25-oxo-27-norcholesterol inhibit oxidation of cholesteryl sulphate by this system but their sulphates do not. 9. 3β-Hydroxychol-5-enoic acid, 3α-hydroxy-5β-cholanic acid and 3β-hydroxy-22,23-bisnorchol-5-enoic acid stimulated formation of isocaproic acid from cholesterol. 10. No evidence was obtained that phosphorylation or sulphation are obligatory steps in cholesterol oxidation by adrenal-cortex mitochondria. 11. The cholesteryl 3β-sulphate sulphatase of bovine adrenal cortex was found mostly in the microsomal fraction and was inhibited by inorganic phosphate.     

The comparative specificity of the inner and outer substrate transfer sites in the choline carrier of human erythrocytes

Summary The substrate specificities on the inner and outer surfaces of the cell membrane have been compared by determining the relative affinities, inside and outside, of a series of choline analogs. The results of two different methods were in agreement: (1) the carrier distribution was determined in the presence of a saturating concentration of an equilibrated analog, using N-ethylmaleimide as a probe for the inward-facing carrier; (2) the degree of competition was measured between an equilibrated analog and choline in the external solution. The carrier sites are found to have markedly different specificities: the outer site is more closely complementary to the structure of choline than is the inner, and even a slight enlargement of either the trimethylammonium or hydroxyethyl group gives rise to preferential binding inside. It is also found that a nonpolar binding region, which is adjacent to the outer site, is absent from the inner site. As the transport mechanism involves the exposure of only one site at a time, first on one surface and then the other, it follows that an extensive reorganization of the structure of the substrate site may occur during the carrier-reorientation step, or alternatively that two distinct sites may be present, only one of which is exposed at a time.     

Interactions of F1 fractions from different strains of shape Paracoccidioides brasiliensis with human complement and with human neutrophils

The aim of our study was to investigate differences that might exist in the activation of the human complement system by F1 fractions from four different isolates of P. brasiliensis. Isolates HC and 18 (virulent), 265 (low virulence), and 9 (intermediate virulence, attenuated) were used; before the experiments, the virulence of isolates HC and 18 was recovered by in vivo passage in guinea pigs. The four isolates of the fungus were processed for purification of F1 fractions and the activation of the human complement system was studied by a kinetic method of hemolytic activity measurement. The incubation of F1 fractions in normal human serum resulted in different degrees of inhibition of the classical and alternative pathways. The F1 fraction from the low virulence isolate was more efficient than the F1 fraction from the virulent isolates (HC and 18). Previous absorption of sera with F1 fractions completely abolished classical pathway activation. Using zymosan, instead of F1, in the absorption process caused the same phenomenon, suggesting that natural or nonspecific antibodies are responsible for the classical pathway activation. The alternative pathway activation did not depend on these antibodies, but was enhanced by their presence. On the other hand, F1 fractions from virulent isolates were more active in the stimulation of neutrophil chemiluminescence compared with the F1 fraction from the low virulence isolate. Whole P. brasiliensis yeast cells (WYC) from two distinct strains, 18 and 265, showed the same patterns of response of those observed with the F1 fractions in the functions tested. These differences in the behavior of the F1 fractions as well as WYC in relation to human complement activation and consequently to neutrophil stimulation may correlate with the virulence of individual isolates and may contribute to the understanding of the inflammatory response generation and maintenance processes in paracoccidioidomycosis. This revised version was published online in June 2006 with corrections to the Cover Date.     

Substrate specificity of the amino acid transporter PAT1

Summary. The proton coupled amino acid transporter PAT1 expressed in intestine, brain, and other organs accepts L- and D-proline, glycine, and L-alanine but also pharmaceutically active amino acid derivatives such as 3-amino-1-propanesulfonic acid, L-azetidine-2-carboxylic acid, and cis-4-hydroxy-D-proline as substrates. We systematically analyzed the structural requirements for PAT1 substrates by testing 87 amino acids, proline homologs, indoles, and derivatives. Affinity data and effects on membrane potential were determined using Caco-2 cells. For aliphatic amino acids, a blocked carboxyl group, the distance between amino and carboxyl group, and the position of the hydroxyl group are affinity limiting factors. Methylation of the amino group enhances substrate affinity. Hetero atoms in the proline template are well tolerated. Aromatic α-amino acids display low affinity. PAT1 interacts strongly with heterocyclic aromatic acids containing an indole scaffold. The structural requirements of PAT1 substrates elucidated in this study will be useful for the development of prodrugs.     

Anion specificity of the jejunal folate carrier: Effects of reduced folate analogues on folate uptake and efflux

Summary We previously reported that³H-folate uptake by rabbit jejunal brush-border membrane (BBM) vesicles was markedly stimulated by an outwardly directed OH^– gradient (pH_in 7.7, pH_out 5.5), inhibited by anion exchange inhibitors (DIDS, SITS, furosemide), and saturable (folateK _m=0.19 m) suggesting carrier-mediated folate/OH^– exchange (or H⁺/folate cotransport). In the present study, the anion specificity of this transport process was examined. Under conditions of an outwardly directed OH^– gradient, DIDS-sensitive folate uptake wascis inhibited (>90%) by reduced folate analogues: dihydrofolate (IC₅₀=0.40 m), folinic acid (IC₅₀=0.50 m), 5-methyltetrahydrofolate (IC₅₀=0.53 m), and (+)amethopterin (IC₅₀=0.93 M). In contrast, 10 m (–)amethopterin had only a modest effect on folate uptake (18% inhibition) suggesting stereospecificity of the folate/OH^– exchanger. The nonpteridine compounds which are transported by the folate carrier in L1210 leukemic cells (phthalate, thiamine pyrophosphate, and PO ₄ ^–3 ) did not inhibit jejunal folate uptake. Furthermore, folate uptake was not inhibited by SO ₄ ^–2 (4mm) or oxalate (4mm) thereby distinguishing this carrier from the previously described intestinal SO ₄ ^–2 /OH^– and oxalate/Cl^– exchangers. After BBM vesicles were loaded with³H-folate, the initial velocity of³H-folate efflux was stimulated by unlabeled folate in the efflux medium. The transstimulation of³H-folate efflux by unlabeled folate was furosemide (or DIDS) inhibitable and temperature sensitive. Half-maximal stimulation of furosemide-sensitive³H-folate efflux was observed with 0.25±0.05 m unlabeled folate, a concentration similar to theK _m for folate uptake. These data suggest that folate-stimulated³H-folate efflux is mediated by the folate/OH^– exchanger. With the exception of (–) amethopterin, reduced folate analogues also transstimulated furosemide-sensitive³H-folate efflux in a concentration-dependent manner suggesting stereospecific transport of these analogues by the folate/OH^– exchanger. In summary, folate transport by the jejunal folate/OH^– exchanger demonstrates bothcis inhibition and transstimulation by reduced folate analogues, but not by other inorganic or organic anions suggesting bidirectional transport of folate and a high degree of anion specificity.     

Facilitated transport of lactic acid in a stirred transfer cell

The application of liquid membrane extraction to the recovery of lactic acid from model systems and fermentation media was investigated. An experimental study of the facilitated transport of lactic acid using ALIQUAT 336 as a mobile carrier in a stirred transfer cell is reported. The effect of stirring speed, initial lactic acid concentration, carrier concentration, and NaCl as a reagent in the acceptor phase are considered. (c) 1994 John Wiley & Sons, Inc.