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

Evidence for the absolute conformational specificity of the intestinal H+/peptide symporter, PEPT1

This study was initiated to determine whether the intestinal H+/peptide symporter PEPT1 differentiates between the peptide bond conformers of substrates. We synthesized a modified dipeptide where the peptide bond is replaced by the isosteric thioxo peptide bond. The Ala-Pro derivative Ala-psi[CS-N]-Pro exists as a mixture of cis and trans conformation in aqueous solution and is characterized by a low cis/trans isomerization rate. The compound was recognized by PEPT1 with high affinity. The Ki value of Ala-psi[CS-N]-Pro for the inhibition of the uptake of radiolabeled glycylsarcosine in Caco-2 cells was 0.30 +/- 0.02 mM, determined in solution with 96% trans conformation. In contrast, the Ki value was 0.51 +/- 0.02 mM when uptake media with 62% trans conformer were used. We conclude that only the trans conformer interacts with the transport system. From our data, a significant affinity of the cis conformer at PEPT1 cannot be derived. In a second approach, conformer-specific uptake of Ala-psi[CS-N]-Pro was studied by analyzing the intracellular content of Caco-2 cells following transport as well as the composition of the extracellular medium using capillary electrophoresis. The percentage of trans conformer that was 62% in the uptake medium increased to 92% inside the cells. This is the first direct evidence that an H+/peptide cotransport system selectively binds and transports the trans conformer of a peptide derivative.

Purification and liposomal reconstitution of the oligopeptide transport activity in rat renal cortex using ceftibuten-affinity chromatography

The carrier protein(s) responsible for the transport of ceftibuten, a peptide-like dianionic cefem, in rat renal brush-border membrane were solubilized and purified by a ceftibuten-ligand specific affinity chromatography technique. The proteoliposomes reconstituted from the solubilized brush-border membrane proteins by dialysis had H+-sensitive uptake of ceftibuten and trans-stimulative effect by cephalexin. A specific uptake activity for ceftibuten was found in the 3.5 M-eluted fraction but not the flowthrough and the 0.5 M-eluted fraction of the affinity chromatography. Analyzing this active fraction by SDS/PAGE after reconstituting into liposomes gave two major proteins (approx. molecular masses of 130 and 107 kDa). The purification protocol presented in this study permitted an efficient isolation of the carrier proteins responsible for the transport of ceftibuten and other peptide-like compounds.

Transepithelial transport of oligopeptides in the human intestinal cell, Caco-2

The transepithelial transport of oligopeptides (of more than 4 residues) was studied by using human intestinal Caco-2 cell monolyers. The susceptibility to the brush-border peptidases was observed to be one of the primary factors which decide the transport rate. The apical-to-basolateral transport mechanism was investigated by using bradykinin and GGYR which were resistant to cellular peptidases. The intracellular pathway, probably the adsorptive transcytosis, was suggested to be involved in the transport of bradykinin and its analogues, the transport rate being particularly dependent on the hydrophobic properties of the peptides. On the other hand, the tetrapeptide such as GGYR was suggested to be transported mainly via the paracellular pathway.

Mechanism of clearance and transfer of dipeptides by perfused human placenta

Glycylglutamine (Gly-Gln) is stable source of glutamine for parenteral nutrition. In the present study we have investigated whether this dipeptide is transferred intact across the human placenta. Although after 90 min of placental perfusion there was almost complete disappearance of Gly-Gln (100 microM) from the maternal compartment, only a small concentration of this dipeptide (< 6 microM) appeared in the fetal compartment. To investigate whether this transfer was due to transcellular transport, brush-border membrane vesicles of the human placenta were probed with [3H]Gly-Gln, which showed no uptake. To investigate whether hydrolysis was the mechanism of disappearance of Gly-Gln, the perfusion study was repeated with glycylsarcosine (Gly-Sar), which is resistant to hydrolysis. In sharp contrast to Gly-Gln, after 90 min of perfusion nearly 80% of Gly-Sar remained in the perfusate (half-life of 24 vs. 235 min). The rest of the Gly-Sar was recovered intact in the fetal compartment. The addition of Gly-Gln to the maternal compartment increased the accumulation of glycine, but not glutamine, in both the maternal and fetal compartments. In conclusion, our data suggest that 1) the mechanism of clearance of Gly-Gln by perfused human placenta is largely hydrolysis, whereas that of Gly-Sar is largely passive diffusion, and 2) the placenta has a greater preference for glutamine than for glycine.

Molecular modeling study of structural requirements for the oligopeptide transporter

The intestinal oligopeptide transporter (OPT) mediates the absorption of di-/tripeptides, beta-lactam antibiotics, angiotensin converting enzyme (ACE) inhibitors and renin inhibitors. This suggests that the targeting of molecules to this transporter could result in orally-absorbed drugs. Results from a recent study with renal brush border membrane vesicles (BBMV) suggested that an alpha-NH2 group is required for interaction with the renal OPT. In general, structural requirements for interaction with the renal and intestinal OPT are similar. However, these recent findings do not agree with earlier studies, which showed that an alpha-NH2 group is not essential for interaction with the intestinal OPT. Thus, it appears that the renal and intestinal OPT may differ in their recognition of compounds containing an alpha-NH2 group. In this study, molecular modeling was used to determine the tridimensional structures of various cephalosporins for which Ki values had been determined using renal BBMV. All cephalosporins which interact with the OPT have two, energetically equivalent, conformations. Most compounds which do not interact with the OPT cannot adopt the two conformations. A key factor which influences the conformation seems to be the substituent group at the alpha position; an electron drawing group at that position alters the common conformations. For the OPT substrates, the distances between the -NH2 and -COOH groups are comparable to those of the tripeptide, GlyGlyGly; and the distances between -NH2 and carbonyl group in the beta-lactam ring are close to the distance between N-terminal and C-terminal in the dipeptide, GlyGly. The corresponding distances in cephamycin C (in which a -NH2 group is located in a different position) and the tetrapeptide, GlyGlyGlyGly, are longer than those in alpha-NH2 cephalosporins and GlyGlyGly. Cephamycin C and the tetrapeptide have low affinity for the renal OPT, suggesting that the distances between functional groups are critical for affinity. The alpha-NH2 group had no effect on the conformations of the molecules. We concluded that the alpha-NH2 group may interact directly with the renal oligopeptide transporter. Whether this is unique to the renal transporter or could be applied to the intestinal transporter will require further investigation.

Substrate-charge dependence of stoichiometry shows membrane potential is the driving force for proton-peptide cotransport in rat renal cortex

The proton dependence of the transport of three labelled, hydrolysis-resistant synthetic dipeptides carrying a net charge of -1, 0 or +1 has been investigated in a brush border membrane vesicle preparation obtained from rat renal cortex. Cross-inhibition studies are consistent with the transport of all peptides studied being through a single system. The extent and time course of uptake in response to an inwardly directed electrochemical gradient of protons differed for each peptide. For the cationic peptide D-Phe-L-Lys this gradient did not stimulate the initial rate of uptake, while for the neutral dipeptide D-Phe-L-Ala and the anionic peptide D-Phe-L-Glu stimulation was observed. However, the effect on D-Phe-L-Glu was more marked than that on D-Phe-L-Ala and the proton activation differed for these two peptides. The calculated Hill coefficients for the two proton-dependent peptides were 1.14 +/- 0.16 and 2.15 +/- 0.10 for D-Phe-L-Ala and D-Phe-L-Glu, respectively, providing evidence that the stoichiometry of proton:peptide cotransport is different for each peptide (0:1, 1:1 and 2:1 for D-Phe-L-Lys, D-Phe-L-Ala and D-Phe-L-Glu respectively); studies on energetics are compatible with this conclusion. The physiological and molecular implications of this model are discussed, as are the applicability of the conclusions to secondary active transport systems more generally.

Luminal transport system for H+/organic cations in the rat proximal tubule. Kinetics, dependence on pH; specificity as compared with the contraluminal organic cation-transport system

The efflux of radiolabelled organic cations from the tubular lumen into proximal tubular cells was investigated by using the stop-flow microperfusion method. The efflux rate increased in the sequence: N1-methylnicotinamide (NMeN+) < cimetidine < tetraethylammonium (TEA+) < N-methyl-4-phenylpyridinium (MPP+). Preloading the animals by i.v. infusion or pre perfusion of the peritubular capillaries with NMeN+ increased the efflux rate of MPP+. Luminal efflux was also augmented when the tubular solution was made alkaline with HCO3- or phosphate, whereby HCO3- is more effective than phosphate. Replacement of Na+ by Cs+ showed no effect. With i.v. preloading the animals with NMeN+ and with 25 mM HCO3- in the luminal perfusate the 2-s efflux follows kinetics with a Michaelis constant Km = 0.21 mmol/l and maximal flux Jmax = 0.42 pmol.cm-1.s-1 and a permeability term with P = 37.7 microns2.s-1. Comparing the apparent luminal inhibitory constant values for MPP+ (Kil,MPP+) with the apparent contraluminal Kicl,NMeN+ values of substrates of homologous series, it was found that (1) limitation by molecular size occurs at the contraluminal cell side earlier than at the luminal cell side; (2) affinity increases with hydrophobicity of the substrates at the luminal cell side, with a steeper or equal slope than at the contraluminal cell side; (3) affinity increases with basicity (i.e. pKa values) at the luminal cell side with a steeper slope than at the contraluminal cell side. Taken together, substrates with low hydrophobicity and low basicity interact at the luminal cell side more weakly than at the contraluminal cell side. On the other hand large, hydrophobic substrates have, at the luminal cell side, a higher affinity than at the contraluminal cell side. Many substrates, however, have equal affinity at the luminal and contraluminal cell sides.

Target size analysis of the peptide/H(+)-symporter in kidney brush-border membranes

The apparent functional molecular mass of the kidney peptide/H(+)-symporter was determined by radiation inactivation in brush-border membrane vesicles (BBMV) of rat kidney cortex. Purified BBMV were irradiated at low temperatures with high energy electrons generated by a 10-MeV linear accelerator at doses from 0 to 30 megarads. Uptake studies were performed with [3H]cefadroxil, a beta-lactam antibiotic which serves as a substrate for the kidney peptide/H(+)-symporter. Inhibition of influx of [3H]cefadroxil into BBMV was used to determine the functional molecular mass of the transporter. Additionally, direct photoaffinity labeling of the transport- and/or binding proteins for [3H]cefadroxil in control and irradiated BBMV was performed to determine the molecular mass of the putative transporter by SDS-polyacrylamide gel electrophoresis. Initial rates of pH-gradient dependent uptake of [3H]cefadroxil decreased progressively as a function of radiation dose. The apparent radiation inactivation size (RIS) of the transport function was found to be 414 +/- 16 kDa. Direct photoaffinity labeling yielded labeled membrane proteins with apparent molecular masses of 130 kDa and 105 kDa, respectively. The proteins displayed different labeling characteristics with respect to incubation time, specificity and the response to irradiation. It appears that only a 105 kDa protein is directly involved in transport function since (a) only it showed a specific pH gradient dependent labeling pattern and (b) the covalent incorporation of [3H]cefadroxil into this protein decreased parallel to the loss of transport function in irradiated BBMV. The peptide/H(+)-symporter in kidney brush-border membranes therefore appears to have a monomer mass of 105 kDa and may function in an oligomeric arrangement.

Substrate specificity of the di/tripeptide transporter in human intestinal epithelia (Caco-2): identification of substrates that undergo H(+)-coupled absorption

1. pH-dependent transepithelial transport and intracellular accumulation of the hydrolysis-resistant dipeptide glycylsarcosine (Gly-Sar) have been demonstrated in the model human intestinal epithelial cell line, Caco-2. 2. Experiments with BCECF (2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein)-loaded Caco-2 cells demonstrated that dipeptide (Gly-Sar) transport across the apical membrane is coupled to proton flow into the cell. 3. A range of postulated substrates for the intestinal di/tripeptide carrier were tested for their abilities to: (a) inhibit pH-dependent [14C]Gly-Sar apical-to-basal transport and intracellular accumulation and (b) stimulate H+ flow across the apical surface of BCECF-loaded Caco-2 cell monolayers. 4. A range of compounds (including Gly-Gly, Leu-Leu, Gly-Gly-Gly, cefadroxil and cephalexin) caused marked acidification of intracellular pH when perfused at the apical surface of Caco-2 cell monolayers. In contrast leucine and D-Leu-D-Leu failed to induce proton flow. The ability to induce proton-flow across the apical surface by these compounds, in this intestinal epithelium, was directly correlated to the relative inhibitory effects on [14C]-Gly-Sar transport and accumulation. 5. The determination of substrate-induced intracellular pH change in the Caco-2 cell system may provide a useful rapid screen for candidate substrates for absorption via H(+)-coupled transport mechanisms such as the intestinal di/tripeptide carrier in an appropriate physiological context.

The inhibitory effects of cephalosporin and dipeptide on ceftibuten uptake by human and rat intestinal brush-border membrane vesicles

The types of inhibitory effects caused by compound V (an analogue of ceftibuten) and alanylproline (dipeptide) on the uptake of ceftibuten by brush-border membrane vesicles (BBMV) prepared from human and rat small intestine were analysed. In the presence of an inward H(+)-gradient, the initial uptake rate of ceftibuten by both human and rat intestinal BBMV was concentration-dependent with apparent Km and Vmax values of 0.35 mM and 2.052 nmol (mg protein)-1 min-1 for human BBMV, and 0.50 mM and 3.056 nmol (mg protein)-1 min-1 for rat BBMV, respectively. For both human and rat BBMV, kinetic analysis by Dixon and Lineweaver-Burk plots demonstrated that the uptake of ceftibuten was competitively inhibited by compound V, whereas inhibition by alanylproline was noncompetitive or partially competitive. These results suggest that there is a stereospecific transport system which is common to ceftibuten and compound V, and that this system is not identical to the carrier system for the dipeptide, alanylproline.

Dipeptides in parenteral nutrition: from basic science to clinical applications

The use of intravenous dipeptides shows great promise as an avenue for the provision of amino acids that may otherwise be difficult to deliver via nutrient infusions. The physical/chemical properties and metabolism of numerous dipeptides have now been explored in experimental and human studies. It has been found that these agents have the capacity to spare nitrogen and support serum protein levels in a fashion equivalent to that of intravenous free amino acids. An additional benefit is the ability to deliver certain amino acids that are relatively unstable or poorly soluble in aqueous solutions. These various aspects of intravenous dipeptides are considered in this review.