Determinants of substrate affinity for the oligopeptide/H+ symporter in the renal brush border membrane

Lacticaseibacillus chiayiensis mediate intestinal microbiome and microbiota-derived metabolites regulating the growth and immunity of chicks

Emerging evidence confirms beneficial properties of probiotics in promoting growth and immunity of farmed chicken. However, the molecular mechanisms underlying the host-microbiome interactions mediated by probiotics are not fully understood. In this study, the internal mechanisms of Lacticaseibacillus chiayiensis-mediated host-microbiome interactions and to elucidate how it promotes host growth were investigated by additional supplementation with L. chiayiensis. We conducted experiments, including intestinal cytokines, digestive enzymes test, intestinal microbiome, metabolome and transcriptome analysis. The results showed that chickens fed L. chiayiensis exhibited higher body weight gain and digestive enzyme activity, and lower pro-inflammatory cytokines, compared to controls. Microbiota sequencing analysis showed that the gut microbiota structure was reshaped with L. chiayiensis supplementation. Specifically, Lactobacillus and Escherichia increased in abundance and Enterococcus, Lactococcus, Corynebacterium, Weissella and Gallicola decreased. In addition, the bacterial community diversity was significantly increased compared to controls. Metabolomic and transcriptomic analyses revealed that higher bile acids and N-acyl amides concentrations and lower carbohydrates concentrations in L. chiayiensis-fed chickens. Meanwhile, the expression of genes related to nutrient transport and absorption in the intestine was upregulated, which reflected the enhanced digestion and absorption of nutrients in chickens supplemented with L. chiayiensis. Moreover, supplementation of L. chiayiensis down-regulated genes involved in inflammation-related, mainly involved in NF-κB signaling pathway and MHC-II mediated antigen presentation process. Cumulatively, these findings highlight that host-microbiota crosstalk enhances the host growth phenotype in two ways: by enhancing bile acid metabolism and digestive enzyme activity, and reducing the occurrence of intestinal inflammation to promote nutrient absorption and maintain intestinal health. This provides a basis for the application of LAB as an alternative to antibiotics in animal husbandry.

Functional Characterization of the Putative POT from Clostridium perfringens

Proton-coupled oligopeptide transporters (POTs) are a fundamental part of the cellular transport machinery that provides plants, bacteria, and mammals with nutrition in the form of short peptides. However, POTs are not restricted to peptide transport; mammalian POTs have especially been in focus due to their ability to transport several peptidomimetics in the small intestine. Herein, we studied a POT from Clostridium perfringens (CPEPOT), which unexpectedly exhibited atypical characteristics. First, very little uptake of a fluorescently labelled peptide β-Ala-Lys-AMCA, an otherwise good substrate of several other bacterial POTs, was observed. Secondly, in the presence of a competitor peptide, enhanced uptake of β-Ala-Lys-AMCA was observed due to trans-stimulation. This effect was also observed even in the absence of a proton electrochemical gradient, suggesting that β-Ala-Lys-AMCA uptake mediated by CPEPOT is likely through the substrate-concentration-driving exchange mechanism, unlike any other functionally characterized bacterial POTs.

Anticancer Gold(III) Peptidomimetics: From Synthesis to in vitro and ex vivo Biological Evaluations

Five new Au^III -peptidodithiocarbamato complexes of the type [Au^III Br₂ (dtc-AA₁ -AA₂ -OR] (in which AA₁ =N-methylglycine (Sar), l/d-Pro; AA₂ =l/d-Ala, α-aminoisobutyric acid (Aib); R=OtBu, triethylene glycol methyl ether), differing with regard to the amino acid sequence and/or the chiral amino acid configuration, were designed to enhance tumor selectivity and bioavailability. The gold(III)-based moiety was functionalized to exploit the targeting properties of the peptidomimetic ligand toward two peptide transporters (namely PEPT1 and PEPT2), which are upregulated in several tumor cells. The compounds were synthesized and fully characterized, mainly by means of elemental analysis, one- and two-dimensional NMR spectroscopy, FT-IR, and UV/Vis spectrophotometry. The crystal structures of three compounds were also solved by X-ray diffraction. In vitro cytotoxicity studies using a panel of human tumor cell lines (A549 [non-small-cell lung carcinoma], MCF-7 [breast cancer], A2780 [ovarian carcinoma], H1975 [non-small-cell lung carcinoma], H460 [large-cell lung carcinoma], and A431 [human epidermoid carcinoma]) showed the dtc-Pro-Aib-OtBu derivative to be very effective, with GI₅₀ values much lower than those of cisplatin. This complex was thus selected for evaluating stability under physiological conditions and possible interactions with serum albumin, as well in PARP-1 enzyme inhibition assays and preliminary ex vivo toxicity experiments on healthy rat tissues.

From Peptides to Peptidomimetics: A Strategy Based on the Structural Features of Cyclopropane

Peptidomimetics, non-natural mimicries of bioactive peptides, comprise an important class of drug molecules. The essence of the peptidomimetic design is to mimic the key conformation assumed by the bioactive peptides upon binding to their targets. Regulation of the conformation of peptidomimetics is important not only to enhance target binding affinity and selectivity, but also to confer cell-membrane permeability for targeting protein-protein interactions in cells. The rational design of peptidomimetics with suitable three-dimensional structures is challenging, however, due to the inherent flexibility of peptides and their dynamic conformational changes upon binding to the target biomolecules. In this Minireview, a three-dimensional structural diversity-oriented strategy based on the characteristic structural features of cyclopropane to address this challenging issue in peptidomimetic chemistry is described.

Are intact peptides absorbed from the healthy gut in the adult human?

For over 100 years it was believed that dietary protein must be completely hydrolysed before its constituent amino acids could be absorbed via specific amino acid transport systems. It is now known that the uptake of di- and tripeptides into the enterocyte is considerable, being transported across the intestinal endothelium by the PepT1 H+/peptide co-transporter. There is also evidence that some di- and tripeptides may survive cytosolic hydrolysis and be transported intact across the basolateral membrane. However, other than antigen sampling, the transport of larger intact macromolecules across the intestinal endothelium of the healthy adult human remains a controversial issue as there is little unequivocal in vivo evidence to support this postulation. The aim of the present review was to critically evaluate the scientific evidence that peptides/proteins are absorbed by healthy intestinal epithelia and pass intact into the hepatic portal system. The question of the absorption of oliogopeptides is paramount to the emerging science of food-derived bioactive peptides, their mode of action and physiological effects. Overall, we conclude that there is little unequivocal evidence that dietary bioactive peptides, other than di- and tripeptides, can cross the gut wall intact and enter the hepatic portal system in physiologically relevant concentrations.

Combination of yeast hydrolysates to improve CHO cell growth and IgG production

Many studies underlined the great benefits of hydrolysates used as additives in animal free media on cell culture performances. However, to precisely define hydrolysate supplementation strategies, a deeper understanding of their effect on cell growth and protein production is required. In the present study, the effect of addition of one yeast extract (YE) and two yeast peptones (named YP.A and YP.B) in a chemically defined medium was first assessed on cell culture performances. Interestingly, specific effects were found depending on the degree of degradation of yeast hydrolysates. The YE at 1 g L(-1) increased the maximal cell density by 70 %, while a mixture of YE (1 g L(-1)) and YP.A (4 g L(-1)) increased IgG production by 180 %. These conditions were then evaluated on the CHO cell kinetics all over cultures. Hydrolysates extended the cell growth phase in Erlenmeyer flask and increased the maximal growth rate in bioreactor up to 20 %. Cell growth stimulation induced by hydrolysates addition was linked with energetic metabolism improvement suggesting that they promote oxidative pathway. Furthermore, hydrolysates provided an additional source of substrate that supported cell growth despite glutamine limitation.

Random mutagenesis of the prokaryotic peptide transporter YdgR identifies potential periplasmic gating residues

The peptide transporter (PTR) family represents a group of proton-coupled secondary transporters responsible for bulk uptake of amino acids in the form of di- and tripeptides, an essential process employed across species ranging from bacteria to humans. To identify amino acids critical for peptide transport in a prokaryotic PTR member, we have screened a library of mutants of the Escherichia coli peptide transporter YdgR using a high-throughput substrate uptake assay. We have identified 35 single point mutations that result in a full or partial loss of transport activity. Additional analysis, including homology modeling based on the crystal structure of the Shewanella oneidensis peptide transporter PepT(so), identifies Glu(56) and Arg(305) as potential periplasmic gating residues. In addition to providing new insights into transport by members of the PTR family, these mutants provide valuable tools for further study of the mechanism of peptide transport.

Functional properties of the Arabidopsis peptide transporters AtPTR1 and AtPTR5

The Arabidopsis di- and tripeptide transporters AtPTR1 and AtPTR5 were expressed in Xenopus laevis oocytes, and their selectivity and kinetic properties were determined by voltage clamping and by radioactive uptake. Dipeptide transport by AtPTR1 and AtPTR5 was found to be electrogenic and dependent on protons but not sodium. In the absence of dipeptides, both transporters showed proton-dependent leak currents that were inhibited by Phe-Ala (AtPTR5) and Phe-Ala, Trp-Ala, and Phe-Phe (AtPTR1). Phe-Ala was shown to reduce leak currents by binding to the substrate-binding site with a high apparent affinity. Inhibition of leak currents was only observed when the aromatic amino acids were present at the N-terminal position. AtPTR1 and AtPTR5 transport activity was voltage-dependent, and currents increased supralinearly with more negative membrane potentials and did not saturate. The voltage dependence of the apparent affinities differed between Ala-Ala, Ala-Lys, and Ala-Asp and was not conserved between the two transporters. The apparent affinity of AtPTR1 for these dipeptides was pH-dependent and decreased with decreasing proton concentration. In contrast to most proton-coupled transporters characterized so far, -I(max) increased at high pH, indicating that regulation of the transporter by pH overrides the importance of protons as co-substrate.

Pharmaceutical and pharmacological importance of peptide transporters

Peptide transport is currently a prominent topic in membrane research. The transport proteins involved are under intense investigation because of their physiological importance in protein absorption and also because peptide transporters are possible vehicles for drug delivery. Moreover, in many tissues peptide carriers transduce peptidic signals across membranes that are relevant in information processing. The focus of this review is on the pharmaceutical relevance of the human peptide transporters PEPT1 and PEPT2. In addition to their physiological substrates, both carriers transport many β-lactam antibiotics, valaciclovir and other drugs and prodrugs because of their sterical resemblance to di- and tripeptides. The primary structure, tissue distribution and substrate specificity of PEPT1 and PEPT2 have been well characterized. However, there is a dearth of knowledge on the substrate binding sites and the three-dimensional structure of these proteins. Until this pivotal information becomes available by X-ray crystallography, the development of new drug substrates relies on classical transport studies combined with molecular modelling. In more than thirty years of research, data on the interaction of well over 700 di- and tripeptides, amino acid and peptide derivatives, drugs and prodrugs with peptide transporters have been gathered. The aim of this review is to put the reports on peptide transporter-mediated drug uptake into perspective. We also review the current knowledge on pharmacogenomics and clinical relevance of human peptide transporters. Finally, the reader's attention is drawn to other known or proposed human peptide-transporting proteins.

Escherichia coli, an Intestinal Microorganism, as a Biosensor for Quantification of Amino Acid Bioavailability

In animal diets optimal amino acid quantities and balance among amino acids is of great nutritional importance. Essential amino acid deficiencies have negative impacts on animal physiology, most often expressed in sub-optimal body weight gains. Over supplementation of diets with amino acids is costly and can increase the nitrogen emissions from animals. Although in vivo animal assays for quantification of amino acid bioavailability are well established, Escherichia coli-based bioassays are viable potential alternatives in terms of accuracy, cost, and time input. E. coli inhabits the gastrointestinal tract and although more abundant in colon, a relatively high titer of E. coli can also be isolated from the small intestine, where primary absorption of amino acids and peptides occur. After feed proteins are digested, liberated amino acids and small peptides are assimilated by both the small intestine and E. coli. The similar pattern of uptake is a necessary prerequisite to establish E. coli cells as accurate amino acid biosensors. In fact, amino acid transporters in both intestinal and E. coli cells are stereospecific, delivering only the respective biological l-forms. The presence of free amino- and carboxyl groups is critical for amino acid and dipeptide transport in both biological subjects. Di-, tri- and tetrapeptides can enter enterocytes; likewise only di-, tri- and tetrapeptides support E. coli growth. These similarities in addition to the well known bacterial genetics make E. coli an optimal bioassay microorganism for the assessment of nutritionally available amino acids in feeds.

Peptide transporters and their roles in physiological processes and drug disposition

1. The peptide transporters belong to the peptide transporter (PTR) family and serve as integral membrane proteins for the cellular uptake of di- and tripeptides in the organism. By their ability also to transport peptidomimetics and other substrates with therapeutic activities or precursors of pharmacologically active agents, they are of considerable importance in pharmacology. 2. PEPT1 is the low-affinity, high-capacity transporter and is mainly expressed in the small intestine, whereas PEPT2 is the high-affinity, low-capacity transporter and has a broader distribution in the organism. 3. Targeted mouse models have revealed PEPT2 to be the dominant transporter for the reabsorption of di- and tripeptides and its pharmacological substrates in the organism, and for the removal of these substrates from the cerebrospinal fluid. Moreover, the peptide transporters undergo physiological and pharmacological regulation and, of great interest, are present in disease states where PEPT1 exhibits ectopic expression in colonic inflammation. 4. The paper reviews the structural characteristics of the peptide transporters, the structural requirements for substrates, the distribution of the peptide transporters in the organism, and finally their regulation in the organism in healthy and pathological situations.

Enhanced absorption and growth inhibition with amino acid monoester prodrugs of floxuridine by targeting hPEPT1 transporters

A series of amino acid monoester prodrugs of floxuridine was synthesized and evaluated for the improvement of oral bioavailability and the feasibility of target drug delivery via oligopeptide transporters. All floxuridine 5'-amino acid monoester prodrugs exhibited PEPT1 affinity, with inhibition coefficients of Gly-Sar uptake (IC50) ranging from 0.7 - 2.3 mM in Caco-2 and 2.0 - 4.8 mM in AsPC-1 cells, while that of floxuridine was 7.3 mM and 6.3 mM, respectively. Caco-2 membrane permeabilities of floxuridine prodrugs (1.01 - 5.31 x 10(-6 )cm/sec) and floxuridine (0.48 x 10(-6 )cm/sec) were much higher than that of 5-FU (0.038 x 10(-6) cm/sec). MDCK cells stably transfected with the human oligopeptide transporter PEPT1 (MDCK/hPEPT1) exhibited enhanced cell growth inhibition in the presence of the prodrugs. This prodrug strategy offers great potential, not only for increased drug absorption but also for improved tumor selectivity and drug efficacy.

DtpB (YhiP) and DtpA (TppB, YdgR) are prototypical proton-dependent peptide transporters of Escherichia coli

The genome of Escherichia coli contains four genes assigned to the peptide transporter (PTR) family. Of these, only tppB (ydgR) has been characterized, and named tripeptide permease, whereas protein functions encoded by the yhiP, ybgH and yjdL genes have remained unknown. Here we describe the overexpression of yhiP as a His-tagged fusion protein in E. coli and show saturable transport of glycyl-sarcosine (Gly-Sar) with an apparent affinity constant of 6.5 mm. Overexpression of the gene also increased the susceptibility of cells to the toxic dipeptide alafosfalin. Transport was strongly decreased in the presence of a protonophore but unaffected by sodium depletion, suggesting H(+)-dependence. This was confirmed by purification of YhiP and TppB by nickel affinity chromatography and reconstitution into liposomes. Both transporters showed Gly-Sar influx in the presence of an artificial proton gradient and generated transport currents on a chip-based sensor. Competition experiments established that YhiP transported dipeptides and tripeptides. Western blot analysis revealed an apparent mass of YhiP of 40 kDa. Taken together, these findings show that yhiP encodes a protein that mediates proton-dependent electrogenic transport of dipeptides and tripeptides with similarities to mammalian PEPT1. On the basis of our results, we propose to rename YhiP as DtpB (dipeptide and tripeptide permease B), by analogy with the nomenclature in other bacteria. We also propose to rename TppB as DtpA, to better describe its function as the first protein of the PTR family characterized in E. coli.

Recognition of 2-aminothiazole-4-acetic acid derivatives by the peptide transporters PEPT1 and PEPT2

The H(+)/peptide cotransporters PEPT1 and PEPT2 have gained considerable interest in pharmaceutical sciences as routes for drug delivery. It is, therefore, of interest to develop uncommon artificial substrates for the two carriers. This study was initiated to investigate the binding affinity of 2-aminothiazole-4-acetic acid (ATAA) conjugates with amino acids to PEPT1 and PEPT2. The 2-aminothiazole-4-acetic acid derivatives have been synthesised and tested for their affinity to PEPT1 and PEPT2. The K(i) values were compared with in silico predicted values from CoMSIA models. C-terminal ATAA-Xaa conjugates proved to be low to medium inhibitors of the [(14)C]Gly-Sar uptake at both carrier systems whereas N-terminal Xaa-ATAA conjugates exhibited medium to high affinity. A promising candidate for further functionalisation is Val-ATAA which shows extraordinary high affinity to PEPT1.

Structural requirements for the substrates of the H+/peptide cotransporter PEPT2 determined by three-dimensional quantitative structure-activity relationship analysis

The renal type H(+)/peptide cotransporter PEPT2 has a substantial influence on the in vivo disposition of dipeptides and tripeptides as well as peptide-like drugs within the body, particularly in kidney, lung, and the brain. The comparative molecular similarity indices analysis (CoMSIA) method was applied to identify those regions in the substrate structures that are responsible for recognition and for differences in affinity. We have developed a comprehensive 3D quantitative structure-activity relationship (3D-QSAR) model based on 83 compounds that is able to explain and predict the binding affinities of new PEPT2 substrates. This 3D-QSAR model possesses a high predictive power (q(2) = 0.755; r(2) = 0.893). An additional 3D-QSAR model based on the same compounds was generated and correlated with affinity data of the intestinal H(+)/peptide cotransporter PEPT1. By comparing the CoMSIA contour plots, differences in selectivity between the intestinal and the renal type peptide carrier become evident.

The renal type H+/peptide symporter PEPT2: structure-affinity relationships

The H(+)/peptide cotransporter PEPT2 is expressed in a variety of organs including kidney, lung, brain, mammary gland, and eye. PEPT2 substrates are di- and tripeptides as well as peptidomimetics, such as beta-lactam antibiotics. Due to the presence of PEPT2 at the bronchial epithelium, the aerosolic administration of peptide-like drugs might play a major role in future treatment of various pulmonary and systemic diseases. Moreover, PEPT2 has a significant influence on the in vivo disposition and half-life time of peptide-like drugs within the body, particularly in kidney and brain. PEPT2 is known to have similar but not identical structural requirements for substrate recognition and transport compared to PEPT1, its intestinal counterpart. In this review we compiled available affinity constants of 352 compounds, measured at different mammalian tissues and expression systems and compare the data whenever possible with those of PEPT1.

Transport of L-proline, L-proline-containing peptides and related drugs at mammalian epithelial cell membranes

Membrane transport of L-proline has received considerable attention in basic and pharmaceutical research recently. Of the most recently cloned members of the solute carrier family, two are "proline transporters". The amino acid transporter PAT1, expressed in intestine, kidney, brain and other organs, mediates the uptake of proline and derivatives in a pH gradient-dependent manner. The Na(+)-dependent proline transporter SIT1, cloned in 2005, exhibits the properties of the long-sought classical IMINO system. Proline-containing peptides are of interest for several reasons. Many biologically important peptide sequences contain highly conserved proline residues. Xaa-Pro peptides are very often resistant to enzymatic hydrolysis and display, in contrast to Pro-Xaa peptides, a high affinity to the H(+)/peptide cotransporter PEPT1 which is expressed in intestinal, renal, lung and biliary duct epithelial cells. Furthermore, several orally available drugs are recognized by PEPT1 as Xaa-Pro analogues due to their sterical resemblance to small peptides.

In vitro evaluation of N-methyl amide tripeptidomimetics as substrates for the human intestinal di-/tri-peptide transporter hPEPT1

Oral absorption of tripeptides is generally mediated by the human intestinal di-/tri-peptide transporter, hPEPT1. However, the bioavailability of tripeptides is often limited due to degradation in the GI-tract by various peptidases. The aim of the present study was to evaluate the general application of N-methyl amide bioisosteres as peptide bond replacements in tripeptides in order to decrease degradation by peptidases and yet retain affinity for and transport via hPEPT1. Seven structurally diverse N-methyl amide tripeptidomimetics were selected based on a principal component analysis of structural properties of 6859 N-methyl amide tripeptidomimetics. In vitro extracellular degradation of the selected tripeptidomimetics as well as affinity for and transepithelial transport via hPEPT1 were investigated in Caco-2 cells. Decreased apparent degradation was observed for all tripeptidomimetics compared to the corresponding natural tripeptides. However, affinity for and transepithelial transport via hPEPT1 were only seen for Gly-Sar-Sar, AsnPsi[CONCH(3)]PhePsi[CONCH(3)]Trp, and Gly-Sar-Leu. This implies that tripeptidomimetics originating from tripeptides with neutral side chains are more likely to be substrates for hPEPT1 than tripeptidomimetics with charged side chains. The results of the present study indicate that the N-methyl amide peptide bond replacement approach for increasing bioavailability of tripeptidomimetic drug candidates is not generally applicable to all tripeptides. Nevertheless, retained affinity for and transport via hPEPT1 were shown for three of the evaluated N-methyl amide tripeptidomimetics.

Peptide uptake in the ectomycorrhizal fungus Hebeloma cylindrosporum: characterization of two di- and tripeptide transporters (HcPTR2A and B)

Constraints on plant growth imposed by low availability of nitrogen are a characteristic feature of ecosystems dominated by ectomycorrhizal plants. Ectomycorrhizal fungi play a key role in the N nutrition of plants, allowing their host plants to access decomposition products of dead plant and animal materials. Ectomycorrhizal plants are thus able to compensate for the low availability of inorganic N in forest ecosystems. The capacity to take up peptides, as well as the transport mechanisms involved, were analysed in the ectomycorrhizal fungus Hebeloma cylindrosporum. The present study demonstrated that H. cylindrosporum mycelium was able to take up di- and tripeptides and use them as sole N source. Two peptide transporters (HcPTR2A and B) were isolated by yeast functional complementation using an H. cylindrosporum cDNA library, and were shown to mediate dipeptide uptake. Uptake capacities and expression regulation of both genes were analysed, indicating that HcPTR2A was involved in the high-efficiency peptide uptake under conditions of limited N availability, whereas HcPTR2B was expressed constitutively.

Affinity prediction for substrates of the peptide transporter PepT1

A quantitative method has been developed for determining the affinity of substrates for the peptide transporter PepT1, allowing oral availability of drugs via PepT1 to be estimated.

Conformational restrictions in ligand binding to the human intestinal di-/tripeptide transporter: implications for design of hPEPT1 targeted prodrugs

The aim of the present study was to develop a computational method aiding the design of dipeptidomimetic pro-moieties targeting the human intestinal di-/tripeptide transporter hPEPT1. First, the conformation in which substrates bind to hPEPT1 (the bioactive conformation) was identified by conformational analysis and 2D dihedral driving analysis of 15 hPEPT1 substrates, which suggested that psi(1) approximately 165 degrees , omega(1) approximately 180 degrees , and phi(2) approximately 280 degrees were descriptive of the bioactive conformation. Subsequently, the conformational energy required to change the peptide backbone conformation (DeltaE(bbone)) from the global energy minimum conformation to the identified bioactive conformation was calculated for 20 hPEPT1 targeted model prodrugs with known K(i) values. Quantitatively, an inverse linear relationship (r(2)=0.81, q(2)=0.80) was obtained between DeltaE(bbone) and log1/K(i), showing that DeltaE(bbone) contributes significantly to the experimentally observed affinity for hPEPT1 ligands. Qualitatively, the results revealed that compounds classified as high affinity ligands (K(i)<0.5 mM) all have a calculated DeltaE(bbone)<1 kcal/mol, whereas medium and low-affinity compounds (0.5 mM<K(i)<15 mM) have DeltaE(bbone) values in the range 1-3 kcal/mol. The findings also shed new light on the basis for the experimentally observed stereoselectivity of hPEPT1.

[11C]Glycylsarcosine: synthesis and in vivo evaluation as a PET tracer of PepT2 transporter function in kidney of PepT2 null and wild-type mice

[11C]Glycylsarcosine (Gly-Sar) was synthesized as a potential radiotracer to investigate the localization and in vivo function of the peptide transporter PepT2 in mouse kidney. Its C-11 labeled diketopiperazine derivative, [11C]cyclo(Gly-Sar) [1-methylpiperazine-2,5-dione], was also evaluated as a potential tracer. [11C]Gly-Sar exhibited rapid initial uptake into kidneys with slow clearance from the medulla, consistent with uptake and retention of the radiotracer through the actions of PepT2. In contrast, the corresponding cyclized dipeptide [11C]cyclo(Gly-Sar) showed rapid clearance and accumulation only in the renal pelvis region. Involvement of PepT2 in reabsorption and delayed clearance of [11C]Gly-Sar was confirmed using the PepT2 knockout mouse, where rapid renal elimination of [11C]Gly-Sar and the absence of radioactivity in medulla were observed. This study demonstrates using in vivo imaging technique that PepT2 is primarily responsible for renal tubular active reabsorption of Gly-Sar, and provides a new tool for studying tubular peptide reabsorption and clearance.

Mechanisms and functional properties of two peptide transporters, AtPTR2 and fPTR2

The Arabidopsis AtPTR2 and fungal fPTR2 genes, which encode H+/dipeptide cotransporters, belong to two different subgroups of the peptide transporter (PTR) (NRT1) family. In this study, the kinetics, substrate specificity, stoichiometry, and voltage dependence of these two transporters expressed in Xenopus oocytes were investigated using the two-microelectrode voltage-clamp method. The results showed that: 1) although AtPTR2 belongs to the same PTR family subgroup as certain H+/nitrate cotransporters, neither AtPTR2 nor fPTR2 exhibited any nitrate transporting activity; 2) AtPTR2 and fPTR2 transported a wide spectrum of dipeptides with apparent affinity constants in the range of 30 microM to 3 mM, the affinity being dependent on the side chain structure of both the N- and C-terminal amino acids; 3) larger maximal currents (Imax) were evoked by positively charged dipeptides in AtPTR2- or fPTR2-injected oocytes; 4) a major difference between AtPTR2 and fPTR2 was that, whereas fPTR2 exhibited low Ala-Asp- transporting activity, AtPTR2 transported Ala-Asp- as efficiently as some of the positively charged dipeptides; 5) kinetic analysis suggested that both fPTR2 and AtPTR2 transported by a random binding, simultaneous transport mechanism. The results also showed that AtPTR2 and fPTR2 were quite distinct from PepT1 and PepT2, two well characterized animal PTR transporters in terms of order of binding of substrate and proton(s), pH sensitivity, and voltage dependence.

Enantiospecific disposition of pranoprofen in beagle dogs and rats

The pharmacokinetic characteristics of pranoprofen enantiomer were examined and compared with the disposition of the corresponding isomer after the administration of racemic pranoprofen to beagle dogs and rats. The plasma levels of (+)-(S)-isomer were significantly higher than those of (-)-(R)-isomer in dogs and rats by either intravenous or oral administration. Although the oral bioavailability and absorption rate constant between the (-)-(R)- and (+)-(S)-form was the same, the elimination rate constant of the (+)-(S)-form was significantly lower than that of the (-)-(R)-form in both dogs and rats. This discrepancy can be explained on the basis of differences in protein binding and the metabolism of the two enantiomers. The (-)-(R)-isomer was predominantly conjugated depending on its higher free plasma level and its faster metabolic rate than the (+)-(S)-form, and thus was excreted more rapidly in the urine and bile in the form of pranoprofen glucuronide. Furthermore, a (-)-(R)- to (+)-(S)-inversion occurred to the extent of 14% in beagle dogs, but not in rats. This chiral inversion might be an important factor in the slow elimination of the (+)-(S)-form in dogs. The most efficient organ for chiral inversion was the liver, followed by kidney and intestine.

An update on renal peptide transporters

The brush-border membrane of renal epithelial cells contains PEPT1 and PEPT2 proteins that are rheogenic carriers for short-chain peptides. The carrier proteins display a distinct surface expression pattern along the proximal tubule, suggesting that initially di- and tripeptides, either filtered or released by surface-bound hydrolases from larger oligopeptides, are taken up by the low-affinity but high-capacity PEPT1 transporter and then by PEPT2, which possesses a higher affinity but lower transport capacity. Both carriers transport essentially all possible di- and tripeptides and numerous structurally related drugs. A unique feature of the mammalian peptide transporters is the capability of proton-dependent electrogenic cotransport of all substrates, regardless of their charge, that is achieved by variable coupling in proton movement along with the substrate down the transmembrane potential difference. This review focuses on the postcloning research efforts to understand the molecular physiology of peptide transport processes in renal tubules and summarizes available data on the underlying genes, protein structures, and transporter function as derived from studies in heterologous expression systems.

Synthesis and characterization of high affinity inhibitors of the H+/peptide transporter PEPT2

In this study, we describe the rational synthesis and functional analysis of novel high affinity inhibitors for the mammalian peptide transporter PEPT2. Moreover, we demonstrate which structural properties convert a transported compound into a non-translocated inhibitor. Starting from Lys[Z(NO(2))]-Pro (where Z is benzyloxycarbonyl), which we recently identified as the first competitive high affinity inhibitor of the intestinal peptide transporter PEPT1, a series of different lysine-containing dipeptide derivatives was synthesized and studied for interaction with PEPT2 based on transport competition assays in Pichia pastoris yeast cells expressing PEPT2 heterologously and in renal SKPT cells expressing PEPT2. In addition, the two-electrode voltage clamp technique in Xenopus laevis oocytes expressing PEPT2 was used to determine whether the compounds are transported electrogenically or block the uptake of dipeptides. Synthesis and functional analysis of Lys-Lys derivatives containing benzyloxycarbonyl or 4-nitrobenzyloxycarbonyl side chain protections provided a set of inhibitors that reversibly inhibited the uptake of dipeptides by PEPT2 with K(i) values as low as 10 +/- 1 nm. This is the highest affinity of a ligand of PEPT2 ever reported. Moreover, based on the structure-function relationship, we conclude that the spatial location of the side chain amino protecting group in a dipeptide containing a diaminocarbonic acid and its intramolecular distance from the Calpha atom are key factors for the transformation of a substrate into an inhibitor of PEPT2.

Defining minimal structural features in substrates of the H(+)/peptide cotransporter PEPT2 using novel amino acid and dipeptide derivatives

The peptide transporter PEPT2, expressed in a variety of tissues, including kidney, lung, and the central nervous system, mediates the uphill transport of di- and tripeptides, as well as a variety of peptidomimetic drugs. To identify the essential molecular features of substrates that determine affinity and transport by PEPT2, we synthesized a series of amino acid derivatives as well as modified dipeptides. Kinetic constants for the interaction of test compounds with PEPT2 were obtained in a competition assay using Pichia pastoris yeast cells expressing mammalian PEPT2. The two-electrode voltage-clamp technique in Xenopus laevis oocytes was used to assess the substrate's electrogenic transport properties. Whereas omega bar-amino fatty acids showed no affinity for PEPT2, the introduction of a single carbonyl group into the backbone increased both affinity and transport currents more than 30-fold. omega bar-amino fatty acids, at their amino or carboxyl group coupled to an alanine residue, allowed us to determine the importance of the spatial position of functional groups within the molecule. Affinity and transport function declined by elongating the omega bar-amino acid chain when located in the N-terminal position, whereas the elongation in the carboxyl terminal with an N-terminal alanine caused less pronounced effects. The results clearly establish that a free N terminus, a correctly positioned backbone carbonyl group, and a carboxylic group that is in a suitable distance from the intramolecular carbonyl function and the amino terminal head group are the main features for substrate recognition and transport by PEPT2. This information provides the framework for a rational design of peptidomimetic drugs for delivery via PEPT2.

Absorption of a protein gavage in Zucker lean rats. Influence of protein content in the diet

The rate of protein absorption was measured in Zucker lean rats. Rats were fed with a bolus that contained ca. 300 mg of 14C-labelled protein at the beginning of the light cycle. Blood was extracted from the portal vein at intervals up to 9 hours after gavage. Label incorporation into tissue protein was monitored. The digestion and absorption of protein was slow, and 9 hours after the gavage, 20% of the bolus remained in the stomach. Forty percent of the protein was absorbed in the first hour. This was followed first by a linear absorption process, then by the amino acid incorporation into tissue proteins. The appearance of label in the portal vein increased progressively for up to four hours, shifting to a progressive decrease that coincides with the maintenance of this label in the tissues. The skin, the striated muscle and the liver showed the highest amounts of labelled proteins. The application of this model to animals fed low-(LP) or high-protein (HP) content diets showed that the HP group digested the protein faster than the LP group, and that catabolism of the amino acids was higher in the HP group. The LP group digested protein much more slowly than the RD (control) group, but protein accretion was more efficient.

An apparatus to assay opioid activity in the infused lumen of the intact isolated guinea pig ileum

A modified apparatus is described that provides for the simultaneous bathing of the serosa of an intact piece of isolated guinea pig ileum while allowing infusion of the isolated lumen. The comparative compartmental potency of the opioid agonists morphine, casomorphins, and enkephalins to inhibit electrically driven contractions are described in this system. The rank-order potency for serosally applied opioid agonists was (IC(50) values, nM): [D-Ala(2),N-Me-Phe(4),Gly-ol(5)]-enkephalin (DAMGO) (15)>[D-Ala(2),D-Leu(5)]-enkephalin (DADLE) (35)> or =morphine (46)> or =[D-Ala(2)]-met-enkephalinamide (55)>[D-Ala(2)]-beta-casomorphin[1--4] amide (122)>beta-casomorphin[1--4] amide (940)>met- and leu-enkephalin (>6000). This contrasted to the rank-order potency for the luminally applied opioid agonists: DADLE (63)>DAMGO (135)>[D-Ala(2)]-met-enkephalinamide=morphine (4700)>[D-Ala(2)]-beta-casomorphin[1--4] amide (29000). beta-Casomorphin[1--4] amide, leu-enkephalin and met-enkephalin are mostly inactive when applied luminally. Furthermore, the opioid antagonists, casoxin 4 and [D-Ala(2)]-casoxin 4, when infused into the lumen, significantly overcame the inhibitory effect of morphine added to the serosal side. This model provides an assay and screening system to differentiate between the effects of chemical agents applied via the blood stream (serosa) or food side (lumen) on quiescent or electrically driven gut activity of the nervous plexi or receptor systems of the ileum.

Membrane transporters

Carrier-mediated drug transport is relatively unexplored in comparison with passive transcellular and paracellular drug transport. Yet, there is a host of transporter proteins that can be targeted for improving epithelial drug absorption. Generally, these are transport mechanisms for amino acids, dipeptides, monosaccharides, monocarboxylic acids, organic cations, phosphates, nucleosides, and water-soluble vitamins. Among them, the dipeptide transporter mechanism has received the most attention. Dipeptide transporters are H(+)-coupled, energy-dependent transporters that are known to play an essential role in the oral absorption of beta-lactam antibiotics, angiotensin-converting enzyme (ACE) inhibitors, renin inhibitors, and an anti-tumor drug, bestatin. Moreover, several investigators have demonstrated the utility of the dipeptide transporter as a platform for improving the oral bioavailability of drugs such as zidovudine and acyclovir through dipeptide prodrug derivatization. Thus far, at least four proton-coupled peptide transporters have been cloned. The first one cloned was PepT1 from the rabbit small intestine. The focus of this presentation will be structure-function, intracellular trafficking, and regulation of PepT1. Disease, dietary, and possible excipient influences on PepT1 function will also be discussed.

Modified amino acids and peptides as substrates for the intestinal peptide transporter PepT1

The binding affinities of a number of amino-acid and peptide derivatives by the mammalian intestinal peptide transporter PepT1 were investigated, using the Xenopus laevis expression system. A series of blocked amino acids, namely N-acetyl-Phe (Ac-Phe), phe-amide (Phe-NH2), N-acetyl-Phe-amide (Ac-Phe-NH2) and the parent compound Phe, was compared for efficacy in inhibiting the uptake of the peptide [3H]-D-Phe-L-Gln. In an equivalent set of experiments, the blocked peptides Ac-Phe-Tyr, Phe-Tyr-NH2 and Ac-Phe-Tyr-NH2 were compared with the parent compound Phe-Tyr. Comparing amino acids and derivatives, only Ac-Phe was an effective inhibitor of peptide uptake (Ki = 1.81+/- 0.37 mM). Ac-Phe-NH2 had a very weak interaction with PepT1 (Ki = 16.8+/-5.64 mM); neither Phe nor Phe-NH2 interacted with PepT1 with measurable affinity. With the dipeptide and derivatives, unsurprisingly the highest affinity interaction was with Phe-Tyr (Ki = 0.10+/-0.04 mM). The blocked C-terminal peptide Phe-Tyr-NH2 also interacted with PepT1 with a relatively high affinity (Ki = 0.94+/-0.38 mM). Both Ac-Phe-Tyr and Ac-Phe-Tyr-NH2 interacted weakly with PepT1 (Ki = 8.41+/-0.11 and 9.97+/-4.01 mM, respectively). The results suggest that the N-terminus is the primary binding site for both dipeptides and tripeptides. Additional experiments with four stereoisomers of Ala-Ala-Ala support this conclusion, and lead us to propose that a histidine residue is involved in binding the C-terminus of dipeptides. In addition, a substrate binding model for PepT1 is proposed.

Kinetics and substrate specificity of membrane-reconstituted peptide transporter DtpT of Lactococcus lactis

The peptide transport protein DtpT of Lactococcus lactis was purified and reconstituted into detergent-destabilized liposomes. The kinetics and substrate specificity of the transporter in the proteoliposomal system were determined, using Pro-[(14)C]Ala as a reporter peptide in the presence of various peptides or peptide mimetics. The DtpT protein appears to be specific for di- and tripeptides, with the highest affinities for peptides with at least one hydrophobic residue. The effect of the hydrophobicity, size, or charge of the amino acid was different for the amino- and carboxyl-terminal positions of dipeptides. Free amino acids, omega-amino fatty acid compounds, or peptides with more than three amino acid residues do not interact with DtpT. For high-affinity interaction with DtpT, the peptides need to have free amino and carboxyl termini, amino acids in the L configuration, and trans-peptide bonds. Comparison of the specificity of DtpT with that of the eukaryotic homologues PepT(1) and PepT(2) shows that the bacterial transporter is more restrictive in its substrate recognition.

Inhibitory effect of novel oral hypoglycemic agent nateglinide (AY4166) on peptide transporters PEPT1 and PEPT2

The novel oral hypoglycemic agent nateglinide (AY4166) is a nonsulfonylurea insulin secretagogue, and its pharmacokinetic features include rapid absorption and elimination. As nateglinide is a dipeptide-like drug, we investigated the interaction of nateglinide with peptide transporters PEPT1 and PEPT2, which mediate the absorption of various peptide-like drugs. Nateglinide exhibited a potent inhibitory effect on [14C]glycylsarcosine uptake by the human colon adenocarcinoma cell line Caco-2 and rat PEPT-transfectants. Kinetic analysis revealed that these inhibitory effects were noncompetitive. Na(+)-coupled alanine or threonine uptake by Caco-2 cells was not inhibited by nateglinide, suggesting that the inhibitory effect of nateglinide on peptide transporters was not due to nonspecific interaction. There was little uptake of [14C]nateglinide by peptide transporters. Various sulfonylureas, such as glibenclamide, also inhibited [14C]glycylsarcosine uptake by rat PEPT-transfectants. In conclusion, nateglinide as well as sulfonylureas inhibit the transport activity of PEPT1 and PEPT2, although nateglinide itself is not transported by these transporters.

Substrate specificity of the brush border K+-leucine symport of Bombyx mori larval midgut

L-leucine uptake into membrane vesicles from Bombyx mori larval midgut was tested for inhibition by 55 compounds, which included sugars, N-methylated, alpha-, beta-, gamma-, delta-, epsilon-amino acids, primary amines, alpha-amino alcohols, monocarboxylic organic acids and alpha-ketoacids. Based on cis-inhibition experiments performed at the high pH (10.8) characteristic of the midgut luminal content in vivo, we find that the carrier binding site interacts with molecules which possess a well-defined set of structural features. Amino acids are preferentially accepted as anions and the ideal inhibitor must have an hydrophobic region and a polar head constituted by a chiral carbon atom bearing two hydrophilic groups, a deprotonated amino-group and a dissociated carboxylic group. Binding is reduced if one of the two hydrophilic groups is removed. Lowering the pH to less alkaline value (8.8) only affects the affinity of delta- and epsilon-amino acids, which are excluded from binding because of their positively charged side-chain. Modifications of the potassium electrochemical gradient increased the affinity constant values of the molecules, but have little effect on the rank of specificity. Physiological implications of the data reported are discussed.

Decisive structural determinants for the interaction of proline derivatives with the intestinal H+/peptide symporter

To elucidate the decisive structural factors relevant for dipeptide-carrier interaction, the affinity of short amide and imide derivatives for the intestinal H+/peptide symporter (PEPT1) was investigated by measuring their ability to inhibit Gly-Sar transport in Caco-2 cells. Dipeptides with proline or alanine in the C-terminal position displayed affinity constants (Ki) of 0.15-1.2 mM and 0.08-9.5 mM, respectively. There was no clear relationship between hydrophobicity, size or ionization status of the N-terminal amino acid and the affinity of the dipeptides. However, analyzing the individual peptide bond conformations of Xaa-Pro dipeptides, a striking correlation between the cis/trans ratios (trans contents 24-70%) and the affinity constants was observed. After correcting the Ki values for the incompetent cis isomers, the Ki corr values of most dipeptides were in a small range of 0.1-0.16 mM. This result revealed the decisive role of peptide bond conformation even for a transport protein that is quite promiscuous in substrate translocation. When measuring affinity constants of Xaa-Pro and Xaa-Sar dipeptides, the cis/trans ratios cannot be ignored. Lower affinities of Lys-Pro, Arg-Pro and Pro-Pro indicate that additional molecular factors affect their binding at PEPT1. The Ki values obtained for the corresponding Xaa-Ala dipeptides support this conclusion. Potential substrates or inhibitors of peptide transport were found among Xaa-piperidides and Xaa-thiazolidides. Dipeptides with N-terminal proline displayed a very diverse affinity profile. However, in contrast to current knowledge, several Pro-Xaa dipeptides such as Pro-Leu, Pro-Tyr and Pro-Pro are recognized by PEPT1 with appreciable affinities. Binding seems mainly determined by the hydrophobicity of the C-terminal amino acid and the rigidity of the structure.

Endogenous expression of the renal high-affinity H+-peptide cotransporter in LLC-PK1 cells

The reabsorption of filtered di- and tripeptides as well as certain peptide mimetics from the tubular lumen into renal epithelial cells is mediated by an H+-coupled high-affinity transport process. Here we demonstrate for the first time H+-coupled uptake of dipeptides into the renal proximal tubule cell line LLC-PK1. Transport was assessed 1) by uptake studies using the radiolabeled dipeptide D-[3H]Phe-L-Ala, 2) by cellular accumulation of the fluorescent dipeptide D-Ala-Lys-AMCA, and 3) by measurement of intracellular pH (pHi) changes as a consequence of H+-coupled dipeptide transport. Uptake of D-Phe-L-Ala increased linearly over 11 days postconfluency and showed all the characteristics of the kidney cortex high-affinity peptide transporter, e.g., a pH optimum for transport of D-Phe-L-Ala of 6.0, an apparent Km value for influx of 25.8 +/- 3. 6 microM, and affinities of differently charged dipeptides or the beta-lactam antibiotic cefadroxil to the binding site in the range of 20-80 microM. pHi measurements established the peptide transporter to induce pronounced intracellular acidification in LLC-PK1 cells and confirm its postulated role as a cellular acid loader.

4-aminomethylbenzoic acid is a non-translocated competitive inhibitor of the epithelial peptide transporter PepT1

1. 4-Aminomethylbenzoic acid, a molecule which mimics the special configuration of a dipeptide, competitively inhibits peptide influx in both Xenopus Laevis oocytes expressing rabbit PepT1 and through PepT1 in rat renal brush border membrane vesicles. 2. This molecule is not translocated through PepT1 as measured both by direct HPLC analysis in PepT1-exp ressing oocytes and indirectly by its failure to trans-stimulate labelle d peptide efflux through PepT1 in oocytes and in renal membrane vessicle s. 3. However 4-aminiomethylbenzoic acid does reverse trans-stimulation through expressed PepT1 of labelled peptid efflux induced by unlabelled peptide. Quantitatively this reversal is compatible with 4-aminomethyl benzoic acid competitively binding to the external surface of PepT1. 4. 4-Aminomethylbenzoic acid (the first molecule discovered to be a non-translocated competitive inhibitor of proton-coupled oligopeptide transport) and its derivatives may thus be particularly useful as experimental tools.

Structure-affinity relationships of Val-Val and Val-Val-Val stereoisomers with the apical oligopeptide transporter in human intestinal Caco-2 cells

The objective of this study was to elucidate the structural features of the stereoisomers of Val-Val and Val-Val-Val that afford optimal binding affinity for the apical oligopeptide transporter in human intestinal Caco-2 cells. Three-dimensional conformations of cephalexin and Val stereoisomers were optimized using Chem-X molecular modeling software. Molecular features associated with the optimized conformations of the Val stereoisomers were analyzed to identify potential relationships with their binding affinities for the apical oligopeptide transporter. For Val-Val stereoisomers, the distance between the N-terminal amino group and the C-terminal carboxyl group, d(N1-C7), was found to have a linear relationship with their binding affinities at the 95% confidence level. For Val-Val-Val stereoisomers, three molecular features were found to have linear relationships with their binding affinities at the 95% confidence level. These features included: a) the distance between the N-terminal amino group and the C-terminal carboxyl group, d(N1-C11); b) the distance between the N-terminal amino group and the second peptide bond, d(N1-N9); and c) the molecular dipole moment. Principal component analysis on all molecular features of Val-Val-Val stereoisomers identified three components that accounted for 90% of the variance. A linear model built with these three components by multiple linear regression adequately described the binding affinities (r2 = 0.90). Results from the current study suggest that the distance between the N-terminal amino group and the C-terminal carboxyl group is important for interaction with the apical oligopeptide transporter in Caco-2 cells. In addition, the binding affinities of the Val-Val-Val stereoisomers appear to be influenced by additional factors, including the position of the second peptide bond and the molecular dipole moment.

One or more serum factors promote peptide utilization in cultured animal cells

We have shown previously that MACT-T and C2C12 cells utilize methionine-containing di- to octapeptides as methionine sources for protein accretion and cell proliferation in the presence of 60 mL/L desalted fetal bovine serum. In this study, serum factors that may regulate the use of peptides as amino acid sources in C2C12 and MAC-T cells were examined. The basal media contained methionine-free Dulbecco's modified Eagle's medium supplemented with 4.0 mL/L bovine serum lipids, 10 mL/L chemically defined lipid concentrate, bovine insulin (1 mg/L), 30 mL/L low protein serum replacement (LPSR-1) or 60 mL/L desalted animal serum. Treatment media included basal media supplemented with no methionine, L-methionine, or one of the methionine-containing peptides. L-Methionine promoted protein and DNA accretion (P < 0.05) in the presence of desalted animal sera, insulin or LPSR-1. Methionine-containing peptides also promoted protein and DNA accretion (P < 0.05) in the presence of desalted animal sera or LPSR-1, but not with insulin, except methionylleucine. In a cell-free medium, fetal bovine serum hydrolyzed peptides to varying degrees. We conclude that animal sera contain one or more factors that regulate utilization of peptides as amino acid sources for C2C12 and MAC-T cells.