1 Department of Pediatrics, Charité Campus Virchow, Humboldt University, 13353 Berlin; 2 Institute for Anatomy and Cell Biology, Faculty of Medicine, Justus Liebig University, 35385 Giessen; 3 Institute of Nutritional Sciences, Technical University of Munich, 85350 Freising-Weihenstephan, Germany; and 4 National Heart and Lung Institute, Imperial College of Science, Technology, and Medicine, London SW3 6LY, United Kingdom
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ABSTRACT |
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The nature of protein breakdown
products and peptidomimetic drugs such as -lactams is crucial for
their transmembrane transport across apical enterocyte membranes, which
is accomplished by the pH-dependent high-capacity oligopeptide
transporter PEPT1. To visualize oligopeptide
transporter-mediated uptake of oligopeptides, an ex vivo assay using
the fluorophore-conjugated dipeptide derivative D-Ala-Lys-N
-7-amino-4-methylcoumarin-3-acetic
acid (D-Ala-Lys-AMCA) was established in the murine small
intestine and compared with immunohistochemistry for PEPT1 in murine
and human small intestine. D-Ala-Lys-AMCA was accumulated
by enterocytes throughout all segments of the murine small intestine,
with decreasing intensity from the top to the base of the villi. Goblet
cells did not show specific uptake. Inhibition studies revealed
competitive inhibition by the
-lactam cefadroxil, the
angiotensin-converting enzyme inhibitor captopril, and the dipeptide
glycyl-glutamine. Controls were performed using either the inhibitor
diethylpyrocarbonate or an incubation temperature of 4°C to exclude
unspecific uptake. Immunohistochemistry for PEPT1 localized
immunoreactivity to the enterocytes, with the highest intensity at the
apical membrane. This is the first study that visualizes dipeptide
transport across the mammalian intestine and indicates that uptake
assays using D-Ala-Lys-AMCA might be useful for
characterizing PEPT1-specific substrates or inhibitors.
immunohistochemistry; oligopeptide; human; mouse
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INTRODUCTION |
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THE
NATURE OF PROTEIN BREAKDOWN products generated at the luminal
membrane of enterocytes determines the route of their epithelial absorption (27, 28, 45). Carrier-mediated transport of di- and tripeptides accounts for the major fraction of dietary amino acids
absorbed after protein digestion (2). Not only short-chain peptides but also a variety of pharmacologically active
peptidomimetics are taken up into the epithelium by the peptide
transport system (8). In this respect, the high oral
availability of peptide-based drugs such as amino -lactam
antibiotics (6, 21, 34, 50) and angiotensin-converting
enzyme (ACE) inhibitors (42, 48) results from their active
transport mediated by the peptide transport system. In addition,
antiviral (17, 19) and antineoplastic (22)
agents as well as the precursor of porphyrine synthesis,
-aminolevulinic acid (11) (
-ALA), have been
identified as peptide transporter substrates. Uptake of these
chemically distinct endogenous and exogenous compounds is accomplished
by the pH-dependent H+/peptide cotransporter PEPT1. The
cDNAs encoding two mammalian proton-coupled peptide transporters have
been cloned from rat (29, 37, 38), rabbit (4, 5,
15), and human tissues (24, 26). Whereas PEPT1
protein and/or mRNA has been found in enterocytes from rat, rabbit, and
human small intestine (16, 30, 31, 49), and to a smaller
extent in renal tubular cells (39, 40), PEPT2 is expressed
in the kidney (39, 40), central nervous system (3,
10, 14), and in a variety of peripheral tissues, including the
lung (4, 18). Both transporter isoforms possess 12 membrane-spanning domains, share an identity of ~47% at the protein
level, and catalyze the electrogenic uphill transport of peptides by
coupling it to the downhill movement of protons along an inwardly
directed electrochemical proton gradient (9).
Although there is a large body of information on function and kinetics of PEPT1-mediated transport in cell culture models and heterologous expression systems, techniques that visualize the transport of PEPT1-carried substrates in the intact intestine do not exist so far. Here we describe a new assay system employing a reporter molecule for rapid detection and visualization of peptide transport in intact tissue in combination with histochemical localization of PEPT1 immunoreactivity in the murine and human intestine.
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METHODS |
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Adult BALB/c mice housed under standard laboratory conditions and fed ad libitum were used in the experiments. For uptake studies, 8 animals were used; 15 animals were used for immunohistochemistry studies. Ten specimens of human small intestine were obtained from remnants in excess of those required for pathological examination. The tissues were obtained after informed consent from patients. All specimens were examined histologically and did not display pathological features. All protocols were performed according to the Declaration of Helsinki and approved by the local ethics committee.
Ex vivo uptake studies. Animals of both sexes were killed by injection of pentobarbital sodium (40 mg/kg body wt ip). After laparotomy, the intestinal loops were mobilized and clipped proximal to the gastroduodenal junction and distal to the sigmoid. Then the loops were separated by morphological means to segments of 20-30 mm resembling duodenum, jejunum, ileum, and colon and isolated from each other by clipping the segments. The deceased animals were transferred to an incubation chamber (37°C), and a cannula was introduced into each segment.
Ex vivo uptake experiments were carried out by instillation of MEM (MEM 21011, GIBCO, Paisley, UK; gassed with 95% O2-5% CO2 at 37°C). MEM solution contained 25 µM of the fluorophore-conjugated dipeptide D-Ala-L-Lys-N
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Tissue preparation for immunohistochemistry. Human and murine tissues were fixed with 4% PFA in PBS at pH 7.4 for 2 h and processed for cryoprotection and freezing as described for the uptake studies. Animals were killed with pentobarbital sodium (40 mg/kg body wt ip) and perfused retrogradely through the abdominal aorta with 4% PFA in PBS at pH 7.4 for 5 min. Cryoprotection and freezing were carried out as described for the uptake studies.
Immunohistochemistry. Immunohistochemistry was performed as described previously (25). Briefly, 10-µm cryostat sections were mounted onto gelatin-chromalum-coated slides, air-dried for 30 min, and washed in PBS. Preincubation with 2% low-fat milk powder in Tris-buffered saline containing 1% Tween 20, pH 7.4, for 1 h at room temperature was followed by washes in PBS and incubation with a polyclonal anti-rabbit-PEPT1 serum raised against the COOH-terminal 15 amino acids of PEPT1 (sequence: H2N-FRHRSKAYPKREHWC-COOH), diluted 1:1,000 in the preincubation solution overnight. For detection of the primary antibody, anti-rabbit indocarbocyanin antiserum (1:1,000, Dianova) was used. Control incubations without the primary antibody or preabsorption of the primary antibody with the corresponding antigenic peptide (concentration, 20 µg protein/ml diluted antiserum) in parallel sections were carried out to specify the reaction. Slides were mounted in carbonate-buffered glycerol (pH 8.6) and viewed using epifluorescence microscopy and confocal laser scanning microscopy (LSM 10, Zeiss, Jena, Gemany).
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RESULTS |
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Fluorophore-conjugated dipeptide uptake ex vivo.
Localization of peptide transport activity in the murine intestine was
performed by the novel fluorescent reporter molecule D-Ala-Lys-AMCA. Significant transmembrane transport of the
reporter was detected by the accumulation of fluorescence in all murine small intestinal segments. Absorptive enterocytes displayed a strong
uptake activity (Fig. 2), and
high-magnification microscopy allowed us to determine the histological
pattern of the uptake activity in the various sections of the jejunum.
The fluorescence signal was restricted to the cytoplasm of absorptive
epithelial cells (Fig. 2b). There was no visible
accumulation of the reporter molecule in the nuclei (Fig.
2c). The intensity was maximal in cells at the tip of the
villi and decreased toward the crypt compartment. Goblet cells and
vascular structures within the villi did not show any staining (Fig.
3a). Cells of the lamina
propria and muscularis also did not reveal any evidence for specific
transport activity apart from a nonspecific background fluorescence.
Maximal fluorescence accumulation was achieved after 10 min of
incubation. For a more detailed analysis of the longitudinal
distribution of transport activity along the gut samples of duodenum,
we analyzed the jejunum, ileum, and colon (Fig. 3). Absorptive
enterocytes of all small intestine segments showed uptake of
D-Ala-Lys-AMCA, whereas there was a complete lack of
fluorescence in colonic samples (Fig. 3).
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Immunohistochemistry.
Histochemistry performed with a polyclonal antibody raised against
PEPT1 detected specific fluorescence in the lamina mucosa but not in
the lamina muscularis, tela submucosa, and serosa in human samples of
ileum (Fig.
5a). Along
the murine intestine, duodenal, jejunal, and ileal segments showed
PEPT1 immunoreactivity (Fig. 5). There was no detectable PEPT1 signal
present in sections of the human and murine colon. PEPT1
immunoreactivity was confined to absorptive enterocytes of the villi.
The fluorescence signal was intense and of a nongranular type with
labeling of the cytoplasm and was especially strong at the luminal
brush-border membrane (Fig. 5). Confocal laser scanning microscopy
revealed a prominent signal at the luminal membrane (Fig.
5g). Specific intracellular organelles such as nuclei,
lysosomes, and Golgi apparatus were not stained. Epithelial
mucus-secreting goblet cells, connective tissue, and vascular
structures also did not show specific PEPT1 immunoreactivity. Some
cells of the lamina propria were labeled, but this signal was also
present when the primary antibody was omitted, indicating nonspecific
staining. The specific staining was abolished when the anti-PEPT1 serum
was preabsorbed with the antigenic peptide (Fig.
5f).
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DISCUSSION |
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Various physiological aspects of intestinal peptide handling and transport have been characterized in recent years (1). Enzymatic hydrolysis of dietary proteins to short-chain peptides by pancreatic and intestinal brush-border peptidases can cause the release of up to 400 different dipeptides and up to 8,000 different tripeptides. These peptides, which cover a range of molecular masses from 96.2 (diglycine) to 522.6 Da (tritryptophan), are all substrates of the intestinal high-capacity low-affinity peptide transporter PEPT1. The diversity of substrates recognized by PEPT1 is even greater taking into account the large number of peptidomimetics or the great variety of artificial substrates that we used (12) to elucidate the minimal molecular requirements for PEPT1-mediated transport.
The tissue distribution of PEPT1 in rat intestine has previously been determined by immunohistochemistry, and immunoreactivity was localized to absorptive enterocytes (30, 31). In situ hybridization studies (16) performed in rabbit small intestine revealed PEPT1 mRNA in the cytoplasm of mature enterocytes. In the human small intestine, a previous study (49) revealed a distribution of PEPT1 immunoreactivity similar to the pattern in rat and rabbit tissues, and functional studies (41) reported PEPT1-specific kinetics. For the murine intestine, carnosine-uptake studies (35) with brush-border membrane vesicles demonstrated a PEPT1-mediated transport. However, studies uniting both morphological and functional aspects had not been performed until now. We combined ex vivo uptake studies with immunohistochemistry to colocalize immunoreactive PEPT1 protein with its transport function.
In the current study, we unequivocally demonstrated that 1) human and murine enterocytes express a functional uptake system for peptides in their luminal brush-border membranes that is shared by classical H+/ peptide transporter specific substrates; 2) the peptide transport system is located in different sections of the small intestine but not in the colon; and 3) the peptide transporter PEPT1 is localized to the same structures that display an uptake activity in situ.
The newly developed technique of intestinal ex vivo uptake studies allows the identification and visualization of the physiologically relevant translocation process of peptides and derivatives. This technique may be of special interest for further pharmacological studies in human tissue preparations. D-Ala-Lys-AMCA was chosen as a reporter substrate for the uptake studies because of its specificity for the peptide transporter (10, 32). To prevent or reduce hydrolysis by brush-border membrane peptidases, an NH2-terminal D-Ala-residue was incorporated into the reporter to render it hydrolysis resistant. Uptake of fluorescent D-Ala-Lys-AMCA fragments by the transport systems for free amino acids was, in addition, prevented by the use of a specific medium (MEM 21011) containing various single amino acids that could compete with uptake of reporter fragments eventually produced by hydrolysis.
Different control studies were carried out to verify the specificity of reporter uptake by the transporter. It was shown that the transport of D-Ala-Lys-AMCA is competitively inhibited by higher concentrations of Gly-L-Gln, cefadroxil, and captopril. These compounds have been previously characterized as PEPT1 substrates and are of pharmacological interest because of their action as antibiotics (50) or ACE inhibitors (48). Moreover, the transport was blocked by diethylpyrocarbonate, which is known to act as an inhibitor for PEPT1 (23, 44). Passive diffusion of D-Ala-Lys-AMCA into the cells was ruled out by the lack of staining of the enterocytes surrounding the cells and the lack of tissue fluorescence in the cells when tissues were incubated at 4°C as demonstrated previously (18).
The finding that transport activity and PEPT1 are present in all mature murine absorptive enterocytes parallels the importance of PEPT1 as a delivery system for dietary amino acids in the small intestine. Transport activity as well as PEPT1 immunoreactivity declined toward the base of the villi and the crypt compartment, providing functional evidence for a previously (30) suggested distributional pattern of the transporter. In this region of the human and murine intestinal mucosa, stem cells move up the villus axis and differentiate to mature enterocytes with absorptive function and to excretory goblet cells (7). A similar distribution has been described for the two glucose transporters sodium-glucose cotransporter-1 (20, 43) and facilitated glucose transporter-2 (47), which were characterized as markers of the state of enterocyte cell differentiation. The expression of the PEPT1 protein and its transport activity therefore display a very similar developmental pattern.
Our previous findings that -ALA is a substrate of PEPT1
(11) is of particular importance for human small
intestinal epithelial cells.
-ALA is used in photodynamic therapy
(PDT; 36) as a substrate for intracellular porphyrine synthesis
(13). The accumulation of porphyrins after administration
of
-ALA promotes apoptosis and tissue necrosis on
photoactivation, and therefore epithelial neoplasms can successfully be
treated by PDT (33). Because
-ALA is used in the
treatment of a variety of cancers, including esophageal and small
intestinal neoplasms, we provide data for the possible uptake mechanism
and the sites for its uptake into absorptive epithelia.
In conclusion, we have demonstrated and visualized mammalian intestinal peptide transport ex vivo and identified the cellular sites of PEPT1 expression in the intestinal tract. Together with recent findings (46) on the dietary regulation of PEPT1 mRNA and protein expression and its minimal molecular requirements for substrate specificity (12), our results provide new insight into the intestinal uptake mechanism of dietary amino acids in peptide-based form as well as for peptide-based drugs and related xenobiotics. The D-Ala-Lys-AMCA uptake visualization assay might be a useful tool to characterize compounds that utilize PEPT1 for uptake into epithelial cells or that inhibit transport activity specifically.
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ACKNOWLEDGEMENTS |
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We thank J. Springer and Q. T. Dinh for helpful discussions and S. Wiegand and R. Strozynski for expert technical assistance.
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FOOTNOTES |
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This study was supported by Deutscher Akademischer Austauschdienst (DAAD) Grant D-00-10559 (D. A. Groneberg).
Address for reprint requests and other correspondence: D. A. Groneberg, Biomedical Research Center, Dept. of Pediatrics, Charité Campus Virchow, Augustenburger Platz 1, 13353 Berlin, Germany.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Received 23 February 2001; accepted in final form 11 May 2001.
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REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Adibi, SA.
The oligopeptide transporter (PEPT-1) in human intestine: biology and function.
Gastroenterology
113:
332-340,
1997[ISI][Medline].
2.
Adibi, SA,
and
Mercer DW.
Protein digestion in human intestine as reflected in luminal, mucosal, and plasma amino acid concentrations after meals.
J Clin Invest
52:
1586-1594,
1973[ISI][Medline].
3.
Berger, UV,
and
Hediger MA.
Distribution of peptide transporter PEPT2 mRNA in the rat nervous system.
Anat Embryol (Berl)
199:
439-449,
1999[ISI][Medline].
4.
Boll, M,
Herget M,
Wagener M,
Weber WM,
Markovich D,
Biber J,
Clauss W,
Murer H,
and
Daniel H.
Expression cloning and functional characterization of the kidney cortex high-affinity proton-coupled peptide transporter.
Proc Natl Acad Sci USA
93:
284-289,
1996
5.
Boll, M,
Markovich D,
Weber WM,
Korte H,
Daniel H,
and
Murer H.
Expression cloning of a cDNA from rabbit small intestine related to proton-coupled transport of peptides, -lactam antibiotics and ACE inhibitors.
Pflügers Arch
429:
146-149,
1994[ISI][Medline].
6.
Bretschneider, B,
Brandsch M,
and
Neubert R.
Intestinal transport of -lactam antibiotics: analysis of the affinity at the H+/peptide symporter (PEPT1), the uptake into Caco-2 cell monolayers and the transepithelial flux.
Pharm Res
16:
55-61,
1999[ISI][Medline].
7.
Cheng, H,
and
Leblond CP.
Origin, differentiation and renewal of the four main epithelial cell types in the mouse small intestine. I. Columnar cell.
Am J Anat
141:
461-479,
1974[ISI][Medline].
8.
Daniel, H.
Function and molecular structure of brush border membrane peptide/H+ symporters.
J Membr Biol
154:
197-203,
1996[ISI][Medline].
9.
Daniel, H,
and
Herget M.
Cellular and molecular mechanisms of renal peptide transport.
Am J Physiol Renal Physiol
273:
F1-F8,
1997
10.
Dieck, ST,
Heuer H,
Ehrchen J,
Otto C,
and
Bauer K.
The peptide transporter PepT2 is expressed in rat brain and mediates the accumulation of the fluorescent dipeptide derivative beta-Ala-Lys-N-AMCA in astrocytes.
Glia
25:
10-20,
1999[ISI][Medline].
11.
Döring, F,
Walter J,
Will J,
Focking M,
Boll M,
Amasheh S,
Clauss W,
and
Daniel H.
-Aminolevulinic acid transport by intestinal and renal peptide transporters and its physiological and clinical implications.
J Clin Invest
101:
2761-2767,
1998
12.
Döring, F,
Will J,
Amasheh S,
Clauss W,
Ahlbrecht H,
and
Daniel H.
Minimal molecular determinants of substrates for recognition by the intestinal peptide transporter.
J Biol Chem
273:
23211-23218,
1998
13.
Dougherty, TJ,
Gomer CJ,
Henderson BW,
Jori G,
Kessel D,
Korbelik M,
Moan J,
and
Peng Q.
Photodynamic therapy.
J Natl Cancer Inst
90:
889-905,
1998
14.
Dringen, R,
Hamprecht B,
and
Broer S.
The peptide transporter PepT2 mediates the uptake of the glutathione precursor CysGly in astroglia-rich primary cultures.
J Neurochem
71:
388-393,
1998[ISI][Medline].
15.
Fei, YJ,
Kanai Y,
Nussberger S,
Ganapathy V,
Leibach FH,
Romero MF,
Singh SK,
Boron WF,
and
Hediger MA.
Expression cloning of a mammalian proton-coupled oligopeptide transporter.
Nature
368:
563-566,
1994[ISI][Medline].
16.
Freeman, TC,
Bentsen BS,
Thwaites DT,
and
Simmons NL.
H+/di-tripeptide transporter (PepT1) expression in the rabbit intestine.
Pflügers Arch
430:
394-400,
1995[ISI][Medline].
17.
Ganapathy, ME,
Huang W,
Wang H,
Ganapathy V,
and
Leibach FH.
Valacyclovir: a substrate for the intestinal and renal peptide transporters PEPT1 and PEPT2.
Biochem Biophys Res Commun
246:
470-475,
1998[ISI][Medline].
18.
Groneberg, DA,
Nickolaus M,
Springer J,
Döring F,
Daniel H,
and
Fischer A.
Localization of the peptide transporter PEPT2 in the lung: implications for pulmonary oligopeptide uptake.
Am J Pathol
158:
707-714,
2001
19.
Guo, A,
Hu P,
Balimane PV,
Leibach FH,
and
Sinko PJ.
Interactions of a nonpeptidic drug, valacyclovir, with the human intestinal peptide transporter (hPEPT1) expressed in a mammalian cell line.
J Pharmacol Exp Ther
289:
448-454,
1999
20.
Hwang, ES,
Hirayama BA,
and
Wright EM.
Distribution of the SGLT1 Na+/glucose cotransporter and mRNA along the crypt-villus axis of rabbit small intestine.
Biochem Biophys Res Commun
181:
1208-1217,
1991[ISI][Medline].
21.
Inui, K,
Okano T,
Maegawa H,
Kato M,
Takano M,
and
Hori R.
H+ coupled transport of p.o. cephalosporins via dipeptide carriers in rabbit intestinal brush-border membranes: difference of transport characteristics between cefixime and cephradine.
J Pharmacol Exp Ther
247:
235-241,
1988[Abstract].
22.
Inui, K,
Tomita Y,
Katsura T,
Okano T,
Takano M,
and
Hori R.
H+ coupled active transport of bestatin via the dipeptide transport system in rabbit intestinal brush-border membranes.
J Pharmacol Exp Ther
260:
482-486,
1992[Abstract].
23.
Kramer, W,
Girbig F,
Petzoldt E,
and
Leipe I.
Inactivation of the intestinal uptake system for -lactam antibiotics by diethylpyrocarbonate.
Biochim Biophys Acta
943:
288-296,
1988[ISI][Medline].
24.
Liang, R,
Fei YJ,
Prasad PD,
Ramamoorthy S,
Han H,
Yang-Feng TL,
Hediger MA,
Ganapathy V,
and
Leibach FH.
Human intestinal H+/peptide cotransporter. Cloning, functional expression, and chromosomal localization.
J Biol Chem
270:
6456-6463,
1995
25.
Lim, S,
Groneberg D,
Fischer A,
Oates T,
Caramori G,
Mattos W,
Adcock I,
Barnes PJ,
and
Chung KF.
Expression of heme oxygenase isoenzymes 1 and 2 in normal and asthmatic airways: effect of inhaled corticosteroids.
Am J Respir Crit Care Med
162:
1912-1918,
2000
26.
Liu, W,
Liang R,
Ramamoorthy S,
Fei YJ,
Ganapathy ME,
Hediger MA,
Ganapathy V,
and
Leibach FH.
Molecular cloning of PEPT 2, a new member of the H+/peptide cotransporter family, from human kidney.
Biochim Biophys Acta
1235:
461-466,
1995[ISI][Medline].
27.
Matthews, DM,
and
Burston D.
Uptake of a series of neutral dipeptides including L-alanyl-L-alanine, glycylglycine and glycylsarcosine by hamster jejunum in vitro.
Clin Sci (Colch)
67:
541-549,
1984[ISI][Medline].
28.
Miller, PM,
Burston D,
Brueton MJ,
and
Matthews DM.
Kinetics of uptake of L-leucine and glycylsarcosine into normal and protein malnourished young rat jejunum.
Pediatr Res
18:
504-508,
1984[Abstract].
29.
Miyamoto, K,
Shiraga T,
Morita K,
Yamamoto H,
Haga H,
Taketani Y,
Tamai I,
Sai Y,
Tsuji A,
and
Takeda E.
Sequence, tissue distribution and developmental changes in rat intestinal oligopeptide transporter.
Biochim Biophys Acta
1305:
34-38,
1996[ISI][Medline].
30.
Ogihara, H,
Saito H,
Shin BC,
Terado T,
Takenoshita S,
Nagamachi Y,
Inui K,
and
Takata K.
Immunolocalization of H+/peptide cotransporter in rat digestive tract.
Biochem Biophys Res Commun
220:
848-852,
1996[ISI][Medline].
31.
Ogihara, H,
Suzuki T,
Nagamachi Y,
Inui K,
and
Takata K.
Peptide transporter in the rat small intestine: ultrastructural localization and the effect of starvation and administration of amino acids.
Histochem J
31:
169-174,
1999[ISI][Medline].
32.
Otto, C,
tom Dieck S,
and
Bauer K.
Dipeptide uptake by adenohypophysial folliculostellate cells.
Am J Physiol Cell Physiol
271:
C210-C217,
1996
33.
Peng, Q,
Warloe T,
Berg K,
Moan J,
Kongshaug M,
Giercksky KE,
and
Nesland JM.
5-Aminolevulinic acid-based photodynamic therapy. Clinical research and future challenges.
Cancer
79:
2282-2308,
1997[ISI][Medline].
34.
Poschet, JF,
Hammond SM,
and
Fairclough PD.
Characterisation of penicillin G uptake in human small intestinal brush border membrane vesicles.
Gut
44:
620-624,
1999
35.
Rajendran, VM,
Berteloot A,
Ishikawa Y,
Khan AH,
and
Ramaswamy K.
Transport of carnosine by mouse intestinal brush-border membrane vesicles.
Biochim Biophys Acta
778:
443-448,
1984[ISI][Medline].
36.
Rowe, PM.
Photodynamic therapy begins to shine.
Lancet
351:
1496,
1998[ISI][Medline].
37.
Saito, H,
Okuda M,
Terada T,
Sasaki S,
and
Inui K.
Cloning and characterization of a rat H+/peptide cotransporter mediating absorption of -lactam antibiotics in the intestine and kidney.
J Pharmacol Exp Ther
275:
1631-1637,
1995[Abstract].
38.
Saito, H,
Terada T,
Okuda M,
Sasaki S,
and
Inui K.
Molecular cloning and tissue distribution of rat peptide transporter PEPT2.
Biochim Biophys Acta
1280:
173-177,
1996[ISI][Medline].
39.
Shen, H,
Smith DE,
Yang T,
Huang YG,
Schnermann JB,
and
Brosius FC, III.
Localization of PEPT1 and PEPT2 proton-coupled oligopeptide transporter mRNA and protein in rat kidney.
Am J Physiol Renal Physiol
276:
F658-F665,
1999
40.
Smith, DE,
Pavlova A,
Berger UV,
Hediger MA,
Yang T,
Huang YG,
and
Schnermann JB.
Tubular localization and tissue distribution of peptide transporters in rat kidney.
Pharm Res
15:
1244-1249,
1998[ISI][Medline].
41.
Steinhardt, HJ,
and
Adibi SA.
Kinetics and characteristics of absorption from an equimolar mixture of 12 glycyl-dipeptides in human jejunum.
Gastroenterology
90:
577-582,
1986[ISI][Medline].
42.
Swaan, PW,
Stehouwer MC,
and
Tukker JJ.
Molecular mechanism for the relative binding affinity to the intestinal peptide carrier. Comparison of three ACE inhibitors: enalapril, enalaprilat, and lisinopril.
Biochim Biophys Acta
1236:
31-38,
1995[ISI][Medline].
43.
Takata, K,
Kasahara T,
Kasahara M,
Ezaki O,
and
Hirano H.
Immunohistochemical localization of Na+-dependent glucose transporter in rat jejunum.
Cell Tissue Res
267:
3-9,
1992[ISI][Medline].
44.
Terada, T,
Saito H,
and
Inui K.
Interaction of -lactam antibiotics with histidine residue of rat H+/peptide cotransporters, PEPT1 and PEPT2.
J Biol Chem
273:
5582-5585,
1998
45.
Terpend, K,
Boisgerault F,
Blaton MA,
Desjeux JF,
and
Heyman M.
Protein transport and processing by human HT29-19A intestinal cells: effect of interferon gamma.
Gut
42:
538-545,
1998
46.
Thamotharan, M,
Bawani SZ,
Zhou X,
and
Adibi SA.
Functional and molecular expression of intestinal oligopeptide transporter (Pept-1) after a brief fast.
Metabolism
48:
681-684,
1999[ISI][Medline].
47.
Thorens, B,
Cheng ZQ,
Brown D,
and
Lodish HF.
Liver glucose transporter: a basolateral protein in hepatocytes and intestine and kidney cells.
Am J Physiol Cell Physiol
259:
C279-C285,
1990[Medline].
48.
Thwaites, DT,
Cavet M,
Hirst BH,
and
Simmons NL.
Angiotensin-converting enzyme (ACE) inhibitor transport in human intestinal epithelial (Caco-2) cells.
Br J Pharmacol
114:
981-986,
1995[Abstract].
49.
Walker, D,
Thwaites DT,
Simmons NL,
Gilbert HJ,
and
Hirst BH.
Substrate upregulation of the human small intestinal peptide transporter, hPepT1.
J Physiol (Lond)
507:
697-706,
1998
50.
Wenzel, U,
Thwaites DT,
and
Daniel H.
Stereoselective uptake of -lactam antibiotics by the intestinal peptide transporter.
Br J Pharmacol
116:
3021-3027,
1995[Abstract].