A combined TDDA-PVC pH and reference electrode for use in the upper small intestine

The evolving biology of the proton‐coupled folate transporter: New insights into regulation, structure, and mechanism

The human proton‐coupled folate transporter (PCFT; SLC46A1) or hPCFT was identified in 2006 as the principal folate transporter involved in the intestinal absorption of dietary folates. A rare autosomal recessive hereditary folate malabsorption syndrome is attributable to human SLC46A1 variants. The recognition that hPCFT was highly expressed in many tumors stimulated substantial interest in its potential for cytotoxic drug targeting, taking advantage of its high‐level transport activity under acidic pH conditions that characterize many tumors and its modest expression in most normal tissues. To better understand the basis for variations in hPCFT levels between tissues including human tumors, studies have examined the transcriptional regulation of hPCFT including the roles of CpG hypermethylation and critical transcription factors and cis elements. Additional focus involved identifying key structural and functional determinants of hPCFT transport that, combined with homology models based on structural homologies to the bacterial transporters GlpT and LacY, have enabled new structural and mechanistic insights. Recently, cryo‐electron microscopy structures of chicken PCFT in a substrate‐free state and in complex with the antifolate pemetrexed were reported, providing further structural insights into determinants of (anti)folate recognition and the mechanism of pH‐regulated (anti)folate transport by PCFT. Like many major facilitator proteins, hPCFT exists as a homo‐oligomer, and evidence suggests that homo‐oligomerization of hPCFT monomeric proteins may be important for its intracellular trafficking and/or transport function. Better understanding of the structure, function and regulation of hPCFT should facilitate the rational development of new therapeutic strategies for conditions associated with folate deficiency, as well as cancer.

The proton-coupled folate transporter (PCFT-SLC46A1) and the syndrome of systemic and cerebral folate deficiency of infancy: Hereditary folate malabsorption

The proton-coupled folate transporter (PCFT-SLC46A1) is the mechanism by which folates are absorbed across the brush-border membrane of the small intestine. The transporter is also expressed in the choroid plexus and is required for transport of folates into the cerebrospinal fluid. Loss of PCFT function, as occurs in the autosomal recessive disorder "hereditary folate malabsorption" (HFM), results in a syndrome characterized by severe systemic and cerebral folate deficiency. Folate-receptor alpha (FRα) is expressed in the choroid plexus, and loss of function of this protein, as also occurs in an autosomal recessive disorder, results solely in "cerebral folate deficiency" (CFD), the designation for this disorder. This paper reviews the current understanding of the functional and structural properties and regulation of PCFT, an electrogenic proton symporter, and contrasts PCFT properties with those of the reduced folate carrier (RFC), an organic anion antiporter, that is the major route of folate transport to systemic tissues. The clinical characteristics of HFM and its treatment, based upon the thirty-seven known cases with the clinical syndrome, of which thirty have been verified by genotype, are presented. The ways in which PCFT and FRα might interact at the level of the choroid plexus such that each is required for folate transport from blood to cerebrospinal fluid are considered along with the different clinical presentations of HFM and CFD.

Intestinal brush-border Na+/H+ exchanger-3 drives H+-coupled iron absorption in the mouse

Divalent metal-ion transporter-1 (DMT1), the principal mechanism by which nonheme iron is taken up at the intestinal brush border, is energized by the H(+)-electrochemical potential gradient. The provenance of the H(+) gradient in vivo is unknown, so we have explored a role for brush-border Na(+)/H(+) exchanger (NHE) isoforms by examining iron homeostasis and intestinal iron handling in mice lacking NHE2 or NHE3. We observed modestly depleted liver iron stores in NHE2-null (NHE2(-/-)) mice stressed on a low-iron diet but no change in hematological or blood iron variables or the expression of genes associated with iron metabolism compared with wild-type mice. Ablation of NHE3 strongly depleted liver iron stores, regardless of diet. We observed decreases in blood iron variables but no overt anemia in NHE3-null (NHE3(-/-)) mice on a low-iron diet. Intestinal expression of DMT1, the apical surface ferrireductase cytochrome b reductase-1, and the basolateral iron exporter ferroportin was upregulated in NHE3(-/-) mice, and expression of liver Hamp1 (hepcidin) was suppressed compared with wild-type mice. Absorption of (59)Fe from an oral dose was substantially impaired in NHE3(-/-) compared with wild-type mice. Our data point to an important role for NHE3 in generating the H(+) gradient that drives DMT1-mediated iron uptake at the intestinal brush border.

The intestinal absorption of folates

The properties of intestinal folate absorption were documented decades ago. However, it was only recently that the proton-coupled folate transporter (PCFT) was identified and its critical role in folate transport across the apical brush-border membrane of the proximal small intestine established by the loss-of-function mutations identified in the PCFT gene in subjects with hereditary folate malabsorption and, more recently, by the Pcft-null mouse. This article reviews the current understanding of the properties of PCFT-mediated transport and how they differ from those of the reduced folate carrier. Other processes that contribute to the transport of folates across the enterocyte, along with the contribution of the enterohepatic circulation, are considered. Important unresolved issues are addressed, including the mechanism of intestinal folate absorption in the absence of PCFT and regulation of PCFT gene expression. The impact of a variety of ions, organic molecules, and drugs on PCFT-mediated folate transport is described.

Mechanisms of membrane transport of folates into cells and across epithelia

Until recently, the transport of folates into cells and across epithelia has been interpreted primarily within the context of two transporters with high affinity and specificity for folates, the reduced folate carrier and the folate receptors. However, there were discrepancies between the properties of these transporters and characteristics of folate transport in many tissues, most notably the intestinal absorption of folates, in terms of pH dependency and substrate specificity. With the recent cloning of the proton-coupled folate transporter (PCFT) and the demonstration that this transporter is mutated in hereditary folate malabsorption, an autosomal recessive disorder, the molecular basis for this low-pH transport activity is now understood. This review focuses on the properties of PCFT and briefly addresses the two other folate-specific transporters along with other facilitative and ATP-binding cassette (ABC) transporters with folate transport activities. The role of these transporters in the vectorial transport of folates across epithelia is considered.

Zip14 is a complex broad-scope metal-ion transporter whose functional properties support roles in the cellular uptake of zinc and nontransferrin-bound iron

Recent studies have shown that overexpression of the transmembrane protein Zrt- and Irt-like protein 14 (Zip14) stimulates the cellular uptake of zinc and nontransferrin-bound iron (NTBI). Here, we directly tested the hypothesis that Zip14 transports free zinc, iron, and other metal ions by using the Xenopus laevis oocyte heterologous expression system, and use of this approach also allowed us to characterize the functional properties of Zip14. Expression of mouse Zip14 in RNA-injected oocytes stimulated the uptake of (55)Fe in the presence of l-ascorbate but not nitrilotriacetic acid, indicating that Zip14 is an iron transporter specific for ferrous ion (Fe(2+)) over ferric ion (Fe(3+)). Zip14-mediated (55)Fe(2+) uptake was saturable (K(0.5) ≈ 2 μM), temperature-dependent (apparent activation energy, E(a) = 15 kcal/mol), pH-sensitive, Ca(2+)-dependent, and inhibited by Co(2+), Mn(2+), and Zn(2+). HCO(3)(-) stimulated (55)Fe(2+) transport. These properties are in close agreement with those of NTBI uptake in the perfused rat liver and in isolated hepatocytes reported in the literature. Zip14 also mediated the uptake of (109)Cd(2+), (54)Mn(2+), and (65)Zn(2+) but not (64)Cu (I or II). (65)Zn(2+) uptake also was saturable (K(0.5) ≈ 2 μM) but, notably, the metal-ion inhibition profile and Ca(2+) dependence of Zn(2+) transport differed from those of Fe(2+) transport, and we propose a model to account for these observations. Our data reveal that Zip14 is a complex, broad-scope metal-ion transporter. Whereas zinc appears to be a preferred substrate under normal conditions, we found that Zip14 is capable of mediating cellular uptake of NTBI characteristic of iron-overload conditions.

Stereoselective transport of amethopterin enantiomers by the proton-coupled folate transporter

Stereoselective transport of methotrexate (L-amethopterin, L-MTX) and its antipode (D-amethopterin, D-MTX) by the proton-coupled folate transporter (PCFT) was examined using PCFT-expressing HEK293 cells (PCFT-HEK293 cells). Uptake of both L-MTX and D-MTX was pH-dependent and decreased with an increase in the extracellular pH from 5.0 to 7.4. The initial uptake rate of L-MTX into PCFT-HEK293 cells followed Michaelis-Menten kinetics with a K(m) value of approximately 5.0 microM. Dixon plots revealed that L-MTX uptake was inhibited competitively by unlabeled L-MTX, D-MTX, and folic acid (FA), with K(i) values of approximately 3.6, 180, and 2.1 microM, respectively. The initial uptake rate of D-MTX into PCFT-HEK293 cells also followed Michaelis-Menten kinetics with a K(m) value of 211 microM. The V(max) value of D-MTX was similar to that of L-MTX. The present study revealed that the transport of MTX enantiomers by PCFT is highly stereoselective with the uptake clearance of L-MTX being approximately 40-fold greater than that of D-MTX. It was also revealed that this high stereoselectivity results from the difference in K(m) values, and not V(max) values, between the enantiomers. The observed stereoselectivity was consistent with the differences in the intestinal absorption of MTX enantiomers in humans.

Properties of the Arg376 residue of the proton-coupled folate transporter (PCFT-SLC46A1) and a glutamine mutant causing hereditary folate malabsorption

The proton-coupled folate transporter (PCFT-SLC46A1) is required for intestinal folate absorption and is mutated in the autosomal recessive disorder, hereditary folate malabsorption (HFM). This report characterizes properties and requirements of the R376 residue in PCFT function, including a R376Q mutant associated with HFM. Gln, Cys, and Ala substitutions resulted in markedly impaired transport of 5-formyltetrahydrofolate (5-FTHF) and 5-methyltetrahydrofolate (5-MTHF) due to an increase in K(m) and decrease in V(max) in HeLa R1-11 transfectants lacking endogenous folate transport function. In contrast, although the influx K(m) for pemetrexed was increased, transport was fully preserved at saturating concentrations and enhanced for the like-charged R376K- and R376H-PCFT. Pemetrexed and 5-FTHF influx mediated by R376Q-PCFT was markedly decreased at pH 5.5 compared with wild-type PCFT. However, while pemetrexed transport was substantially preserved at low pH (4.5-5.0), 5-FTHF transport remained very low. Electrophysiological studies in Xenopus oocytes demonstrated that 1) the R376Q mutant, like wild-type PCFT, transports protons in the absence of folate substrate, and in this respect has channel-like properties; and 2) the influx K(m) mediated by R376Q-PCFT is increased for 5-MTHF, 5-FTHF, and pemetrexed. The data suggest that mutation of the R376 residue to Gln impairs proton binding which, in turn, modulates the folate-binding pocket and depresses the rate of conformational alteration of the carrier, a change that appears to be, in part, substrate dependent.

Hypermethylation of the human proton-coupled folate transporter (SLC46A1) minimal transcriptional regulatory region in an antifolate-resistant HeLa cell line

This laboratory recently identified a novel proton-coupled folate transporter (PCFT) that mediates intestinal folate absorption and transport of folates into the central nervous system. The present study focuses on the definition of the minimum transcriptional regulatory region of this gene in HeLa cells and the mechanism(s) underlying the loss of PCFT expression in the methotrexate-resistant HeLa R1-11 cell line. The PCFT transcriptional regulatory controls were localized between -42 and +96 bases from the transcriptional start site using a luciferase-reporter gene system. The promoter is a G + C rich region of 139 nucleotides contained in a CpG island. HeLa R1-11 cells have no mutations in the PCFT open reading frame and its promoter; the transcription/translation machinery is intact because transient transfections in HeLa R1-11 and wild-type HeLa cells produced similar luciferase activities. Hypermethylation at CpG sites within the minimal transcriptional regulatory region was shown in HeLa R1-11 cells as compared with the parental PCFT-competent HeLa cells, using bisulfite conversion and sequence analysis. Treatment with 5-aza-2'-deoxycytidine resulted in a substantial restoration of transport and PCFT mRNA expression and small but significant decreases in methylation in the promoter region. In vitro methylation of the transfected reporter plasmid inhibited luciferase gene expression. Cytogenetics/fluorescence in situ hybridization indicated a loss of half the PCFT gene copies in HeLa R1-11 as compared with PCFT-competent HeLa cells. Taken together, promoter silencing through methylation and gene copy loss accounted for the loss of PCFT activity in antifolate-resistant HeLa R1-11 cells.

Membrane transporters and folate homeostasis: intestinal absorption and transport into systemic compartments and tissues

Members of the family of B9 vitamins are commonly known as folates. They are derived entirely from dietary sources and are key one-carbon donors required for de novo nucleotide and methionine synthesis. These highly hydrophilic molecules use several genetically distinct and functionally diverse transport systems to enter cells: the reduced folate carrier, the proton-coupled folate transporter and the folate receptors. Each plays a unique role in mediating folate transport across epithelia and into systemic tissues. The mechanism of intestinal folate absorption was recently uncovered, revealing the genetic basis for the autosomal recessive disorder hereditary folate malabsorption, which results from loss-of-function mutations in the proton-coupled folate transporter gene. It is therefore now possible to piece together how these folate transporters contribute, both individually and collectively, to folate homeostasis in humans. This review focuses on the physiological roles of the major folate transporters, with a brief consideration of their impact on the pharmacological activities of antifolates.

A role for the proton-coupled folate transporter (PCFT-SLC46A1) in folate receptor-mediated endocytosis

Recently, this laboratory identified a proton-coupled folate transporter (PCFT), with optimal activity at low pH. PCFT is critical to intestinal folate absorption and transport into the central nervous system because there are loss-of-function mutations in this gene in the autosomal recessive disorder, hereditary folate malabsorption. The current study addresses the role PCFT might play in another transport pathway, folate receptor (FR)-mediated endocytosis. FRalpha cDNA was transfected into novel PCFT(+) and PCFT(-) HeLa sublines. FRalpha was shown to bind and trap folates in vesicles but with minimal export into the cytosol in PCFT(-) cells. Cotransfection of FRalpha and PCFT resulted in enhanced folate transport into cytosol as compared with transfection of FRalpha alone. Probenecid did not inhibit folate binding to FR, but inhibited PCFT-mediated transport at endosomal pH, and blocked FRalpha-mediated transport into the cytosol. FRalpha and PCFT co-localized to the endosomal compartment. These observations (i) indicate that PCFT plays a role in FRalpha-mediated endocytosis by serving as a route of export of folates from acidified endosomes and (ii) provide a functional role for PCFT in tissues in which it is expressed, such as the choroid plexus, where the extracellular milieu is at neutral pH.

Transport model of the human Na+-coupled L-ascorbic acid (vitamin C) transporter SVCT1

Vitamin C (L-ascorbic acid) is an essential micronutrient that serves as an antioxidant and as a cofactor in many enzymatic reactions. Intestinal absorption and renal reabsorption of the vitamin is mediated by the epithelial apical L-ascorbic acid cotransporter SVCT1 (SLC23A1). We explored the molecular mechanisms of SVCT1-mediated L-ascorbic acid transport using radiotracer and voltage-clamp techniques in RNA-injected Xenopus oocytes. L-ascorbic acid transport was saturable (K(0.5) approximately 70 microM), temperature dependent (Q(10) approximately 5), and energized by the Na(+) electrochemical potential gradient. We obtained a Na(+)-L-ascorbic acid coupling ratio of 2:1 from simultaneous measurement of currents and fluxes. L-ascorbic acid and Na(+) saturation kinetics as a function of cosubstrate concentrations revealed a simultaneous transport mechanism in which binding is ordered Na(+), L-ascorbic acid, Na(+). In the absence of L-ascorbic acid, SVCT1 mediated pre-steady-state currents that decayed with time constants 3-15 ms. Transients were described by single Boltzmann distributions. At 100 mM Na(+), maximal charge translocation (Q(max)) was approximately 25 nC, around a midpoint (V(0.5)) at -9 mV, and with apparent valence approximately -1. Q(max) was conserved upon progressive removal of Na(+), whereas V(0.5) shifted to more hyperpolarized potentials. Model simulation predicted that the pre-steady-state current predominantly results from an ion-well effect on binding of the first Na(+) partway within the membrane electric field. We present a transport model for SVCT1 that will provide a framework for investigating the impact of specific mutations and polymorphisms in SLC23A1 and help us better understand the contribution of SVCT1 to vitamin C metabolism in health and disease.

Molecular mechanisms involved in intestinal iron absorption

Iron is an essential trace metal in the human diet due to its obligate role in a number of metabolic processes. In the diet, iron is present in a number of different forms, generally described as haem (from haemoglobin and myoglobin in animal tissue) and non-haem iron (including ferric oxides and salts, ferritin and lactoferrin). This review describes the molecular mechanisms that co-ordinate the absorption of iron from the diet and its release into the circulation. While many components of the iron transport pathway have been elucidated, a number of key issues still remain to be resolved. Future work in this area will provide a clearer picture regarding the transcellular flux of iron and its regulation by dietary and humoral factors.

The molecular identity and characterization of a Proton-coupled Folate Transporter--PCFT; biological ramifications and impact on the activity of pemetrexed

Membrane transport of folates is essential for the survival of all mammalian cells and transport of antifolates is an important determinant of antifolate activity. While a major focus of attention has been on transport mediated by the reduced folate carrier and folate receptors, a very prominent carrier-mediated folate transport activity has been recognized for decades with a low-pH optimum and substrate specificity distinct from that of the reduced folate carrier which operates most efficiently at neutral pH. This low-pH transporter represents the mechanism by which folates are absorbed in the small intestine and it is also widely expressed in other human tissues and solid tumors. Recently, this laboratory discovered the molecular identity of this transporter which is genetically unrelated to the reduced folate carrier. This transporter is proton-coupled, electrogenic, and manifests a substrate specificity that is similar to that of the low-pH transport activity previously described in mammalian cells. The key role this transporter plays in intestinal folate absorption has been confirmed by the demonstration of a mutation in this gene in the rare autosomal recessive disorder, hereditary folate malabsorption. This article reviews (1) the characteristics and prevalence of the low-pH folate transport activity, (2) its relationship to, and properties of, the recently identified Proton-Coupled Folate Transporter (PCFT), (3) the physiological and pharmacological roles of this transporter, particularly with respect to pemetrexed, and (4) the historical controversy, now resolved, on the mechanism of intestinal folate absorption.

Functional properties of multiple isoforms of human divalent metal-ion transporter 1 (DMT1)

DMT1 (divalent metal-ion transporter 1) is a widely expressed metal-ion transporter that is vital for intestinal iron absorption and iron utilization by most cell types throughout the body, including erythroid precursors. Mutations in DMT1 cause severe microcytic anaemia in animal models. Four DMT1 isoforms that differ in their N- and C-termini arise from mRNA transcripts that vary both at their 5'-ends (starting in exon 1A or exon 1B) and at their 3'-ends giving rise to mRNAs containing (+) or lacking (-) the 3'-IRE (iron-responsive element) and resulting in altered C-terminal coding sequences. To determine whether these variations result in functional differences between isoforms, we explored the functional properties of each isoform using the voltage clamp and radiotracer assays in cRNA-injected Xenopus oocytes. 1A/IRE+-DMT1 mediated Fe2+-evoked currents that were saturable (K(0.5)(Fe) approximately 1-2 microM), temperature-dependent (Q10 approximately 2), H+-dependent (K(0.5)(H) approximately 1 muM) and voltage-dependent. 1A/IRE+-DMT1 exhibited the provisional substrate profile (ranked on currents) Cd2+, Co2+, Fe2+, Mn2+>Ni2+, V3+>>Pb2+. Zn2+ also evoked large currents; however, the zinc-evoked current was accounted for by H+ and Cl- conductances and was not associated with significant Zn2+ transport. 1B/IRE+-DMT1 exhibited the same substrate profile, Fe2+ affinity and dependence on the H+ electrochemical gradient. Each isoform mediated 55Fe2+ uptake and Fe2+-evoked currents at low extracellular pH. Whereas iron transport activity varied markedly between the four isoforms, the activity for each correlated with the density of anti-DMT1 immunostaining in the plasma membrane, and the turnover rate of the Fe2+ transport cycle did not differ between isoforms. Therefore all four isoforms of human DMT1 function as metal-ion transporters of equivalent efficiency. Our results reveal that the N- and C-terminal sequence variations among the DMT1 isoforms do not alter DMT1 functional properties. We therefore propose that these variations serve as tissue-specific signals or cues to direct DMT1 to the appropriate subcellular compartments (e.g. in erythroid cells) or the plasma membrane (e.g. in intestine).

The spectrum of mutations in the PCFT gene, coding for an intestinal folate transporter, that are the basis for hereditary folate malabsorption

Hereditary folate malabsorption (HFM) is a rare autosomal recessive disorder caused by impaired intestinal folate absorption and impaired folate transport into the central nervous system. Recent studies in 1 family revealed that the molecular basis for this disorder is a loss-of-function mutation in the PCFT gene encoding a proton-coupled folate transporter. The current study broadens the understanding of the spectrum of alterations in the PCFT gene associated with HFM in 5 additional patients. There was no racial, ethnic, or sex pattern. A total of 4 different homozygous mutations were detected in 4 patients; 2 heterozygous mutations were identified in the fifth patient. Mutations involved 4 of the 5 exons, all at highly conserved amino acid residues. A total of 4 of the mutated transporters resulted in a complete loss of transport function, primarily due to decreased protein stability and/or defects in membrane trafficking, while 2 of the mutated carriers manifested residual function. Folate transport at low pH was markedly impaired in transformed lymphocytes from 2 patients. These findings further substantiate the role that mutations in PCFT play in the pathogenesis of HFM and will make possible rapid diagnosis and treatment of this disorder in infants, and prenatal diagnosis in families that carry a mutated gene.

Identification of an intestinal folate transporter and the molecular basis for hereditary folate malabsorption

Folates are essential nutrients that are required for one-carbon biosynthetic and epigenetic processes. While folates are absorbed in the acidic milieu of the upper small intestine, the underlying absorption mechanism has not been defined. We now report the identification of a human proton-coupled, high-affinity folate transporter that recapitulates properties of folate transport and absorption in intestine and in various cell types at low pH. We demonstrate that a loss-of-function mutation in this gene is the molecular basis for hereditary folate malabsorption in a family with this disease. This transporter was previously reported to be a lower-affinity, pH-independent heme carrier protein, HCP1. However, the current study establishes that a major function of this gene product is proton-coupled folate transport required for folate homeostasis in man, and we have thus amended the name to PCFT/HCP1.

Iron Imports. II. Iron uptake at the apical membrane in the intestine

How does iron enter enterocytes? Ablating SLC11A2, the gene for the divalent metal ion transporter DMT1, supports evidence from the Belgrade rat and mk mouse models establishing DMT1 as the primary mechanism serving apical uptake of nonheme iron. DMT1 harnesses the energy from the proton electrochemical potential gradient to drive active transport of Fe(2+) (and perhaps Mn(2+) and other metal ions) into enterocytes. Fe(III) must first be reduced by ascorbic acid and surface ferrireductases. Among these is duodenal cytochrome B (DcytB), but lack of an obvious phenotype in DcytB (Cybrd1) knockout mice suggests ferrireductase redundancy. Our understanding of heme absorption has lagged, but the time is ripe for gains.

Divalent metal-ion transporter DMT1 mediates both H+ -coupled Fe2+ transport and uncoupled fluxes

The H(+) -coupled divalent metal-ion transporter DMT1 serves as both the primary entry point for iron into the body (intestinal brush-border uptake) and the route by which transferrin-associated iron is mobilized from endosomes to cytosol in erythroid precursors and other cells. Elucidating the molecular mechanisms of DMT1 will therefore increase our understanding of iron metabolism and the etiology of iron overload disorders. We expressed wild type and mutant DMT1 in Xenopus oocytes and monitored metal-ion uptake, currents and intracellular pH. DMT1 was activated in the presence of an inwardly directed H(+) electrochemical gradient. At low extracellular pH (pH(o)), H(+) binding preceded binding of Fe(2+) and its simultaneous translocation. However, DMT1 did not behave like a typical ion-coupled transporter at higher pH(o), and at pH(o) 7.4 we observed Fe(2+) transport that was not associated with H(+) influx. His(272) --> Ala substitution uncoupled the Fe(2+) and H(+) fluxes. At low pH(o), H272A mediated H(+) uniport that was inhibited by Fe(2+). Meanwhile H272A-mediated Fe(2+) transport was independent of pH(o). Our data indicate (i) that H(+) coupling in DMT1 serves to increase affinity for Fe(2+) and provide a thermodynamic driving force for Fe(2+) transport and (ii) that His-272 is critical in transducing the effects of H(+) coupling. Notably, our data also indicate that DMT1 can mediate facilitative Fe(2+) transport in the absence of a H(+) gradient. Since plasma membrane expression of DMT1 is upregulated in liver of hemochromatosis patients, this H(+) -uncoupled facilitative Fe(2+) transport via DMT1 can account for the uptake of nontransferrin-bound plasma iron characteristic of iron overload disorders.

Symmetry of H+ binding to the intra- and extracellular side of the H+-coupled oligopeptide cotransporter PepT1

Ion-coupled solute transporters exhibit pre-steady-tate currents that resemble those of voltage-dependent ion channels. These currents were assumed to be mostly due to binding and dissociation of the coupling ion near the extracellular transporter surface. Little attention was given to analogous events that may occur at the intracellular surface. To address this issue, we performed voltage clamp studies of Xenopus oocytes expressing the intestinal H+-coupled peptide cotransporter PepT1 and recorded the dependence of transient charge movements in the absence of peptide substrate on changing intra- (pHi) and extracellular pH (pHo). Rapid steps in membrane potential induced transient charge movements that showed a marked dependence on pHi and pHo. At a pHo of 7.0 and a holding potential (Vh) of -50 mV, the charge movements were mostly inwardly directed, whereas reduction of pHo to below 7.0 resulted in outwardly directed charge movements. When pHi was reduced, inwardly directed charge movements were observed. The data on the voltage dependence of the transient charge movements were fitted by the Boltzmann equation, yielding an apparent valence of 0.65 +/- 0.03 (n = 7). The midpoint voltage (V0.5) of the charge distribution shifted linearly as a function of pHi and pHo. Our results indicate that, as a first approximation, the magnitude and polarity of the transient charge movements depend upon the prevailing H+ electrochemical gradient. We propose that PepT1 has a single proton binding site that is symmetrically accessible from both sides of the membrane and that decreasing the H+ chemical potential (DeltamuH) or increasing the membrane potential (Vm) shifts this binding site from an outwardly to an inwardly facing occluded state. This concept constitutes an important extension of previous kinetic models of ion-coupled solute transporters by including a more detailed description of intracellular events.

Mechanisms of the human intestinal H+-coupled oligopeptide transporter hPEPT1

The hPEPT1 cDNA cloned from human intestine (Liang, R., Fei, Y.-J., Prasad, P. D., Ramamoorthy, S., Han, H., Yang-Feng, T. L., Hediger, M. A., Ganapathy, V., and Leibach, F. H. (1995) J. Biol. Chem. 270, 6456-6463) encodes a H+/oligopeptide cotransporter. Using two-microelectrode voltage-clamp in Xenopus oocytes expressing hPEPT1, we have investigated the transport mechanisms of hPEPT1 with regard to voltage dependence, steady-state kinetics, and transient charge movements. The currents evoked by 20 mM glycyl-sarcosine (Gly-Sar) at pH 5.0 were dependent upon membrane potential (Vm) between -150 mV and +50 mV. Gly-Sar-evoked currents increased hyperbolically with increasing extracellular [H+], with Hill coefficient approximately 1, and the apparent affinity constant (K0.5H) for H+ was in the range of 0.05 1 microM. K0.5 for Gly-Sar (K0.5GS) was dependent upon Vm and pH; at -50 mV, K0.5H was minimal (approximately 0.7 mM) at pH 6.0. Following step-changes in Vm, in the absence of Gly-Sar, hPEPT1 exhibited H+-dependent transient currents with characteristics similar to those of Na+-coupled transporters. These charge movements (which relaxed with time constants of 2-10 ms) were fitted to Boltzmann relations with maximal charge (Qmax) of up to 12 nC; the apparent valence was determined to be approximately 1. Qmax is an index of the level of transporter expression which for hPEPT1 was in the order of 1011/oocyte. In general our data are consistent with an ordered, simultaneous transport model for hPEPT1 in which H+ binds first.

Direct measurement of mucosal surface pH of pig jejunum in vivo

The mucosal surface pH of pig jejunum was measured in vivo. When the pH in the bulk phase of the perfusing buffer solution was 7.10, the pH at the mucosa (as measured by a miniaturised glass pH electrode) was 6.19 +/- 0.04 (n = 19). This relatively acidic mucosal surface pH is not an anoxic artefact since anoxia resulted in a significant alkalinisation of the mucosa. This finding, that the pig, like man and the rat, has a jejunal "acid microclimate" has important implications for weak electrolyte absorption from the upper small intestine of this species as well as for any other pH dependent brush border process.