www.cantinesanpancrazio.it/components/cojocima/649-come-localizzare-un.php The importance of SGLT1 phosphorylation will be discussed below. SGLT1 is found in brush border membrane of mature enterocytes in the small intestine, with very small amounts detectable in the kidneys and the heart. Recently, SGLT1 has also been detected in the luminal membrane of intracerebral capillary endothelial cells, where it may participate in the transport of glucose across the blood-brain barrier[ 16 ].
The process of intestinal sugar transport has been reviewed by Wright et al [ 17 ]. Another conformational change allows the substrates to enter the enterocyte. This up-regulation may influence the functioning of SGLT1 and subsequently alter intestinal sugar uptake in these conditions. Wright et al evaluated the role of SGLT1 phosphorylation[ 24 ].
The change in maximal transport rates Vmax was accompanied by alterations in the number of transporters in the plasma membrane, as well as changes in the surface area of the membrane. Since endocytosis and exocytosis alter the membrane surface area, the findings of the effects of PKA and PKC on SGLT1 suggest that these proteins may be involved in the regulation of glucose transport.
The effects of PKC, however, may depend on the sequence of the co-transporter, as there are conflicting reports of the effect of PKC on glucose transport. Veyhl et al demonstrated the presence of an intracellular regulatory protein RS1 that may modify the activity of SGLT1[ 28 ]. To investigate the role of intracellular trafficking in sugar transport, oocytes were injected either with cRNA of wild type, or mutant dynamin.
Dynamin is a motor protein involved in receptor-mediated endocytosis, vesicle recycling, caveolae internalization and vesicle trafficking from the Golgi[ 29 ]. The inhibition of glucose uptake by RS1 was largely reduced after co-expression of the mutant dynamin protein. These researchers speculated that therapeutic strategies aimed at reducing glucose uptake by increasing RS1 might potentially be used to treat obesity.
A study done in renal epithelial cells showed that treatment with hsp70 increased glucose transport, but not the abundance of SGLT1 protein[ 31 ]. The researchers speculated that hsp70 might stabilize SGLT1 expression in the membrane. In this model, the absence of vimentin, an intermediate filament component, decreased glucose transport and caused a reduction in the amount of SGLT1 protein in these membrane microdomains.
Furthermore, fluidization of the plasma membrane, or depleting the membrane of cholesterol, dramatically decreased glucose transport. This suggests that the activity of SGLT1 is optimal in a microenvironment characterized by low fluidity. Further research is required to determine if SGLT1 is localized to lipid rafts in the intestinal BBM, if this localization is mandatory for the functioning of SGLT1, and what are the factors that may regulate the localization of SGLT1 to these specialized microdomains.
Martin et al, characterized the promoter region of the human SGLT1 gene by transfecting reporter constructs into Caco-2 cells[ 33 ]. Some members of the Sp1 family have been implicated in tissue- and developmental- specific regulation of genes[ 34 , 35 ]. HNF-1 alters the expression of many small intestinal genes, including sucrase-isomaltase SI and lactase. It has also been implicated in the diurnal regulation of SGLT1 in rodents[ 36 ].
Of interest, HNF-1 knockout mice experience life-threatening effects on the hepatic and renal systems, but no adverse effects on the gastrointestinal tracts were reported. Katz et al identified a link between a mesenchymal factor and the regulation of a specific epithelial transport process[ 37 ]. Foxl1 is a winged-helix transcription factor expressed in the mesenchymal cells bordering the crypts in the small intestine. Using the everted sleeve method coupled with Western blotting, the researchers showed that homozygous Foxl1 null mice had decreased intestinal glucose uptake and decreased levels of SGLT1 protein.
Growth retardation and abnormal small intestinal architecture were observed, characterized by short, broad and irregular villi. The transport of water across the intestinal epithelia has always been a subject of curiosity. The discovery of aquaporins by Preston et al renewed interest in this topic[ 38 ]. Overexpression of human or rabbit SGLT1 in Xenopus oocytes revealed that activation of the transporter was associated with an increase in volume of the cell reflecting water transport , and this effect was blocked by phlorizin. If oocytes expressing SGLT1 were incubated in a sugar-free solution, no change in oocyte volume was observed.
A channel formed by five C-terminal transmembrane helices of SGLT1 is thought to transport not only water, but also urea[ 22 , 40 ]. It is important to note, however, that others have suggested that local osmotic gradients fully account for water fluxes. Lapointe et al present evidence from experiments using Xenopus oocytes expressing human SGLT1 that contradicts the water cotransport hypothesis and suggests the passive movement of water across the plasma membrane[ 41 ].
The introduction of this very simple treatment has reduced mortality due to diarrhea in children under five years of age from 5 million in , to 1. This success led to the proclamation that ORT was the most important medical advance of the 20 th century, and earned Dr Hirschhorn and colleagues the first Pollin prize for Pediatric Research. The goals of ORT are to replace fluids and minimize malnutrition. Starting in , solutions containing a mixture of glucose, sodium, chloride, potassium and citrate were being commonly distributed by the World Health Organization.
In fact, million packets of ORT were distributed worldwide in [ 43 ]. Interestingly, controversy now exists over the optimal formulation, with reduced osmolarity formulas, rice-based formulas, or formulas containing amylase-resistant starch being favored by some researchers. For example, hypoosmolar rice-based formulas produced better results in cholera patients when compared to standard formulas[ 44 ]. The advantages of this rice-based formula are that it is cheap, offers more calories than standard ORT, and rice is readily available in many cholera-stricken regions.
These effects counteract the fluid losses and hypersecretion seen with infectious diarrhea. Several features of carbohydrate digestion contribute to the efficacy of ORT. This life-saving therapy is based on the ability of SGLT1 to co-transport water.
Since ORT is commonly administered to infants, it is important to utilize a transport system that is expressed and functional early in life. SGLT1 is expressed prenatally[ 48 ], and is functional at birth, making it an ideal candidate. Glucose-galactose malabsorption GGM is a very rare autosomal recessive disease characterized by severe life-threatening diarrhea in the neonate, that resolves when the offending sugars glucose, galactose and lactose are removed from the diet[ 15 , 50 ]. Normal intestinal mucosal histology is observed, while phlorizin binding studies show reductions in SGLT1 protein in the BBM[ 51 , 52 ].
Electrophysiological studies and freeze fracture electron microscopy showed that this disease is due to a failure of the SGLT1 protein to traffic normally to the BBM[ 53 ]. Approximately cases of GGM have been identified worldwide, affecting all racial and ethnic groups. Unlike genetic diseases like cystic fibrosis, in which a single mutation accounts for most cases, in GGM each patient appears to have a unique mutation, ranging from missense mutations, to frame-shift mutations, to split-site- conservative mutations which produce truncated protein and mistrafficking of SGLT1 to the BBM[ 53 - 56 ].
This variety of mutations limits the usefulness of genetic testing for GGM, although prenatal diagnosis in a family at risk may be possible. GGM is a difficult condition to diagnose. Oral glucose tolerance tests in GGM patients produce a flat glucose response in the blood, as glucose is malabsorbed in the intestine. GGM is treated by using glucose-, galactose- and lactose-free formulas, and by eliminating the offending sugars from the diet[ 17 ]. Normal growth and neurological development are possible if infants receive fructose-based formula, and if dietary counselling is available[ 15 , 57 ].
It was cloned by Burant and colleagues in GLUT5 was expressed in Xenopus oocytes, and its substrate specificity and kinetic properties were determined using radiolabelled substrates. Northern and Western blotting demonstrated the presence of GLUT5 in human small intestine and testis. Further work by Davidson et al focused on the developmental expression of GLUT5 in the human and fetal small intestine[ 58 ]. Immunohistochemical techniques confirmed this finding, and further localized GLUT5 to only the mature enterocytes populating the upper half of the villus. This luminal localization provided further support for the notion that GLUT5 played a role in the intestinal uptake of dietary sugars.
Rand et al, characterized the expression of GLUT5 in rats[ 49 ]. GLUT5 mRNA was detected in the small intestine, kidney and brain by Northern blotting, and in the small intestine, testis, adipose and skeletal muscle using in situ hybridization. In the intestine, a proximal-distal gradient was observed, with GLUT5 mRNA levels being higher in the proximal small intestine when compared to the distal small intestine. A distinct pattern of expression was seen along the crypt-villous axis, with mRNA being highest in midvillus region. This enabled the researchers to conclude that the regions necessary for fructose transport lie between the amino terminus and the third transmembrane domain, and between the 5 th and 11 th transmembrane domain.
This was consistent with the suggestion that GLUT5 was a high affinity fructose transporter. Subsequent work by David et al showed that in 16 d old rats, feeding fructose but not glucose increased fructose uptake[ 60 ]. Furthermore, while both fructose and sucrose feeding enhanced absorption in older d old animals, glucose alone had no effect.
An interesting study by Castello et al demonstrated that GLUT5 mRNA in rats followed a circadean rhythm, with a fold increase in mRNA at the end of the light cycle as compared to early in the light cycle[ 61 ]. Although this pattern was thought to be a reflection of rodent feeding patterns, Corpe et al found that gene expression is hard-wired, because GLUT5 is up-regulated prior to the onset of feeding, even in the absence of dietary fructose[ 63 ]. Shu et al noted that this circadean rhythm was not developed at the time of weaning, possibly because the feeding patterns of suckling rats do not follow the same adult nocturnal patterns[ 64 ].
This diurnal variation in adult animals needs to be carefully considered when designing experiments in which levels of GLUT5 are measured, by performing studies in the morning in the early post-prandial period. Treatment of the cells with forskolin, which stimulates adenylate cyclase and raises intracellular cAMP levels, increased fructose uptake 2-fold, and increased GLUT5 protein and mRNA 5-fold and 7-fold, respectively.
Matosin-Matekalo et al used Caco2 cells transfected with a GLUT5 promoter inserted up-stream of the luciferase reporter gene[ 66 ]. Helliwell et al[ 67 ] looked at the regulation of GLUT5 by a number of signals that have well-established roles in the regulation of sugar transport. Extensive cross-talk occurs between the pathways. For example, inhibiting the ERK pathway with PD increased the sensitivity to anisomysin, which stimulates the p38 pathway.
The authors concluded that the three pathways have the potential to regulate fructose transport during the digestion and absorption of a meal. They suggested that future work should focus on determining the hormones that influence these pathways, and the molecular mechanisms that regulate the levels and activities of the sugar transporters. Gouyon et al[ 68 ] used Caco-2 cells to investigate the mechanism by which fructose increases GLUT5 expression.
Although both glucose and fructose increased the activity of the GLUT5 promoter, the effect of fructose was stronger and associated with higher cAMP concentrations.
A sugar response element was identified in the GLUT5 promoter. Infection may also regulate fructose transport. Adaptive immunity also influences the expression of a number of developmentally regulated genes. Recent advances have been made in understanding the signalling pathways involved in the regulation of GLUT5. Cui et al[ 73 ] have demonstrated that cAMP stimulates fructose transport in the neonatal rat intestine. Perfusing fructose mM plus 8-bromo-cAMP in d-old rats increased fructose uptake rates, while an inhibitor of adenylate cyclase abolished this effect. GLUT2 is a low affinity, high capacity facilitative transporter in the BLM that transports glucose, fructose, galactose and mannose[ 10 , 63 , 75 - 77 ].
It has 12 transmembrane domains, with intracellular N and C terminals. Using immunohistochemistry, Thorens et al[ 75 ] showed that GLUT2 expression increases as enterocytes migrate up from the crypt to the villous tip. Amino acid sequences in transmembrane segments are primarily responsible for GLUT2's distinctive glucose affinity, whereas amino acid sequences in transmembrane segments enable GLUT2 to transport fructose[ 78 ].
Luminal sugars[ 23 , 79 ] or vascular infusions of glucose or fructose[ 80 , 81 ] stimulate GLUT2 expression and activity. In this study, they fed sugar-enriched diets to male Sprague Dawley rats for 5 d. GLUT2 modulation required intracellular metabolism of the sugar, as it was unaffected by 3-O-methyglucose, a non-metabolized glucose analog. This traditional view has been challenged, with the suggestion that the kinetic characteristics of sugar uptake could also be described by a second high affinity, high capacity BBM transporter[ 83 ].
This finding suggests the presence of two components: an active, phloridzin-sensitive component, and a phloridzin-insensitive, possibly passive component that does not appear to be saturable. The passive component of glucose transport was characterized by Pappenheimer and Reiss [ 88 ]. The observation that high rates of water absorption accompany glucose absorption[ 89 ] provided a rationale for proposing that glucose in the intercellular spaces provided an osmotic force that resulted in bulk flow of nutrients.
Structural studies using electron microscopy and freeze fracture analysis revealed large dilatations within junctions following glucose perfusion. Madara and Pappenheimer further demonstrated that the transport of glucose via SGLT1 caused dilatation of the tight junctions[ 90 ].
They concluded that passive glucose absorption is a result of paracellular solvent drag, and is indeed SGLT1 dependent. Therefore, like the more recent model suggested by Loo et al [ 39 ], these investigators suggest that the transport of water is SGLT1-dependent. However, this theory suggests the presence of a non-specific route, which could potentially allow passage of several solutes. Ferraris and Diamond proposed an alternative theory, in which paracellular flow is negligible[ 91 , 92 ].
Based on the determination of up-dated kinetic constants for glucose absorption, and the determination of the usual free glucose concentrations in the intestinal lumen, they concluded that SGLT1 fully accounts for glucose absorption. Much of their work is based on studies examining long-term dietary adaptations, from which they concluded that BBM transporters are matched to dietary intake.
Much of the controversy surrounding the role of the paracellular pathway stems from the discrepancies between the estimated concentrations of glucose in the intestinal lumen.
Pappenheimer[ 94 ] used the rate of membrane hydrolysis of maltose to indirectly measure luminal glucose concentrations. They also point out that the techniques used by Ferraris et al [ 92 ], which involve glucose analysis of luminal contents, will underestimate the concentration found at the membrane following hydrolysis by disaccharidases. The actual physiological levels of glucose in the lumen remain a subject of debate.
The concept of more than one transport system for glucose was suggested by Malo[ 95 ]. Using human fetal and adult BBM vesicles, curvilinear Eadie-Hofstee plots and sodium activation curves were obtained when glucose concentrations were varied in the medium. These findings, coupled with determinations of phloretin-sensitive and -insensitive components, and the ability of the BBM vesicles to transport 3-O-methylglucose, suggested the presence of two transport systems: a high-affinity low-capacity system and a low-affinity high-capacity system[ 95 , 96 ].
Although this concept was proposed many years ago, it was not until recently that interest in the area has re-emerged due to an alternative model of intestinal glucose transport proposed by George Kellett and his colleagues at the University of York, and by Edith Brot-Laroche and her colleagues at the University of Paris. Several years ago, GLUT2 was detected in the BBM of enterocytes in diabetic animals, although at the time this was interpreted to be a pathological event[ 63 ].
Interestingly, these are the same cells that are responsible for glucose absorption. Once a meal is ingested and BBM enzymes hydrolyse disaccharides, luminal glucose concentrations increase. A rounding of the apical surface, due to a contraction of the peri-junctional actomyosin ring, allows luminal glucose to have increased access to the BBM enzymes and transporters.
Helliwell et al[ 67 ] investigated the role of several signalling pathways in intestinal fructose absorption. Using an in vivo perfusion model, they showed that inhibitors of these pathways wortmannin and rapamycin, respectively block GLUT2 trafficking to the BBM and inhibit sugar absorption. A role for insulin in the regulation of intestinal sugar absorption is suggested. This observation may help to explain why the passive component was more apparent in the in vivo studies, as compared to in vitro experiments. Helliwell and Kellett [ ] looked at perfusion conditions in order to determine if the so called passive component was SGLT1-dependent, as suggested by their work, or was SGLT1-independent, as suggested by earlier work by Debnam and Levin[ ].
They concluded that the passive component is independent of the active component in high mechanical stress perfusions, suggesting that SGLT1-dependent recruitment of GLUT2 did not occur under these conditions. This may be related to the restrictions in blood flow and supply of endogenous nutrients and hormones caused by the high stress perfusions. Clearly, the perfusion conditions affect the results of the experiment, and this may explain the discrepancies between various studies. Currently, there are two commercially available antibodies that recognize GLUT2: one that recognizes the extracellular loop between transmembrane 1 and 2 Biogenesis, Poole, England ; and another that recognizes a portion of the C- terminus Research Diagnostics, Flanders, NJ.
The choice of antibody is important, as Au et al demonstrated that the biotinylation procedure they used to detect surface proteins interfered with the ability of the GLUT2 antibody to recognize the extracellular loop, forcing them to use the C-terminus antibody[ ]. In agreement with this notion, different APN-associated complexes have been isolated previously [ 19 , 20 , 44 , 45 ]. Likewise, isolation of proteins from lipid rafts has been performed using a diversity of experimental conditions and tissue sources [ 47 ].
In addition, rafts have been isolated using Triton X at different temperatures [ 49 ], or with different detergents from both primary tissue and cultured epithelial cell lines [ 50 — 52 ]. As APN is heavily glycosylated [ 14 , 53 ] and is a major component of the glycolipid-rich rafts [ 20 , 29 , 44 ], it is tempting to speculate that the complexes identified in the present study are stabilized and trafficked by a galectindependent mechanism. B 0 AT1 has also been shown to contain N-glycosylation sites in its extracellular loops [ 54 ].
To our knowledge, no study isolating apical complexes have yet identified B 0 AT1, APN or ACE2 as co-components of a brush-border complex, either in the apical membrane or as part of a stable trafficking unit. The combination of these three proteins is restricted to the intestine. There is limited overlap of ACE2 and B 0 AT1 expression in the kidney, where the transporter is mainly associated with collectrin [ 55 ].
Recently, alterations in substrate affinity for alanine, a major B 0 AT1 substrate, were also demonstrated over widespread sections of rat small intestine incubated in more proximal sections with the same amino acids that displayed affinity alterations [ 56 ]. Such a mechanism is not uncommon in nature, and is reminiscent of numerous periplasmic binding proteins and ABC ATP-binding cassette primary transporters in archeal and bacterial species [ 57 — 61 ].
Homology modelling and structural data of E. Several large neutral amino acids have been crystallized in the E. From sequence identity, structural homology and biochemical analysis, we demonstrate that the APN active site is homologous with LAP. This basic geometry of the E. The increase of the local concentration is expected to become more relevant as the peptidase to transporter ratio increases. This was confirmed by the oocyte experiments, where peptidase-induced currents were additive to the currents induced by bulk leucine at low surface concentrations of B 0 AT1 Figure 6 A.
When the B 0 AT1 concentration was increased relative to a constant amount of APN, the currents were no longer additive. This model does not rule out the possibility that APN increases the transporter's substrate affinity by altering the thermodynamics of substrate binding, whether through direct B 0 AT1 binding site or allosteric means. In the present study we have demonstrated the presence of digestive protein complexes in the intestinal brush-border containing the peptidases APN and ACE2 and the neutral amino acid transporter B 0 AT1. On the basis of our results, we propose these complexes to function as a metabolon, which optimises protein digestive and absorption processes at the brush-border.
We envision a scenario where B 0 AT1 trafficking and expression in the membrane is largely dependent on ACE2, whereas optimal, or adaptive, functioning to changing dietary conditions requires association with APN. It appears likely that these complexes contain a wider array of proteins than shown in the present study, and the full elucidation of their components, functional significance, and prevalence in main absorptive epithelial surfaces of the body are subjects for further study. Stephen Fairweather performed and designed the experiments and wrote the paper.
We are indebted to Cathy Gillespie Australian National University for assistance with confocal microscopy, and to Salik Ali and Teresa Neeman for support with statistical analysis. National Center for Biotechnology Information , U. Biochemical Journal. Biochem J. Published online Jul Prepublished online Jun 7.
Author information Article notes Copyright and License information Disclaimer. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. This article has been cited by other articles in PMC. Associated Data Supplementary Materials Supplementary data. Abstract The brush-border membrane of the small intestine and kidney proximal tubule are the major sites for the absorption and re-absorption of nutrients in the body respectively.
Keywords: aminopeptidase N, angiotensin-converting enzyme 2 ACE2 , broad neutral 0 amino acid transporter 1 B 0 AT1 , brush-border membrane, nutrient absorption, protein complex. Complex Solubilization conditions Proteins detected M. Open in a separate window. Expression of transporters in Xenopus laevis oocytes Isolation and preparation of X. Electrophysiological recordings and flux measurements Electrical recordings of amino-acid-induced currents and uptake of radiolabelled amino acids were performed as described previously [ 17 , 32 ]. Co-localization of membrane proteins using the Venus fly trap The original Venus fly trap vectors encode antiparallel leucine-zipper motifs, with each zipper fused to one half of GFP green fluorescent protein from Aequorea v ictoria in pcDNA3.
Calculations, statistics and data analysis For all electrophysiological recordings, results were averaged for 6—8 oocytes, unless otherwise indicated. Figure 1. Isolation of intestinal brush-border protein complexes Murine intestinal BBMVs were prepared using MgCl 2 precipitation and centrifugation. Figure 2. Figure 3. Figure 4. Table 3 Validation of experimental apparent K m values for B 0 AT1 and APN co-expression The kinetic parameters apparent K m and I max were calculated using electrophysiological recordings of the proteins indicated expressed in X.
Mechanism of increased substrate affinity in B 0 AT1 Having established a close physical and functional interaction between B 0 AT1 and APN, we hypothesized that APN may act as a molecular funnel, channelling hydrolysed N -terminal amino acids into the extracellular binding site of the transporter. Figure 5. Figure 6. Figure 7. Figure 8. Figure 9. Online data Supplementary data: Click here to view. References 1. Boron W. Medical Physiology. Philadelphia: Saunders; Tyska M. Myosin-1a is critical for normal brush border structure and composition. Mentlein R. Cell-surface peptidases.
Alpers D. Digestion and absorption of carbohydrates and proteins. In: Johnson L.
Physiology of The Gastro-intestinal Tract. New York: Raven Press; Broer S. Amino acid transport across mammalian intestinal and renal epithelia.
Seow H. Hartnup disorder is caused by mutations in the gene encoding the neutral amino acid transporter SLC6A Scriver C. Hartnup Disease: a genetic modification of intestinal and renal transport of certain neutral alpha-amino acids. Matthews D. Protein Absorption. New York: Wiley-Liss; Camargo S. Tissue-specific amino acid transporter partners ACE2 and collectrin differentially interact with hartnup mutations. Kowalczuk S.
A protein complex in the brush-border membrane explains a Hartnup disorder allele. Singer D. Plakidou-Dymock S. Riemann D. CD13—not just a marker in leukemia typing. Sjostrom H. Structure and function of aminopeptidase N.
Olsen J. Structure and expression of aminopeptidase N. Bohmer C. Characterization of mouse amino acid transporter B0AT1 slc6a19 Biochem. Mina-Osorio P. The moonlighting enzyme CD old and new functions to target. Trends Mol. Babusiak M. Native proteomic analysis of protein complexes in murine intestinal brush border membranes. Danielsen E. Galectin-4 and small intestinal brush border enzymes form clusters. Alfalah M. Intestinal dipeptidyl peptidase IV is efficiently sorted to the apical membrane through the concerted action of N - and O -glycans as well as association with lipid microdomains.
O -linked glycans mediate apical sorting of human intestinal sucrase-isomaltase through association with lipid rafts. Spodsberg N. Characteristics and structural requirements of apical sorting of the rat growth hormone through the O-glycosylated stalk region of intestinal sucrase-isomaltase. Biber J.
Isolation of renal proximal tubular brush-border membranes. Redondo F.
J Nutr Biochem —25 Google Scholar. Cell Biol. The current fluctuations are due to the opening and closing of ion channels, and the power spectrum yields information on channel density and occupancy probabilities in the open and closed states of the channel. We have try to classify, at least partially, data that we found in the literature on the mechanisms and localization of transporters for various classes of substances in the intestine of mammals. While in the dog, a commonly used model of in vivo motor activity, phase 3 complexes can be identified at the ileocecal sphincter and even in the proximal colon Figure 3 , MMC activity tends to peter out in the distal ileum in man. The duration and severity of postoperative ileus appear to be directly proportional to the extent of the surgical procedure and to manipulation of the intestines, in particular. NZB download transport phenomena membrane digestion in small intestinal mucosa an Agreement, Planning two ethical complaints; request and file.
L-alaninenitroanilide as a substrate for microsomal aminopeptidase. Lottenberg R. Solution composition dependent variation in extinction coefficients for p-nitroaniline.
Schagger H. Blue native electrophoresis for isolation of membrane protein complexes in enzymatically active form. Involvement of detergent-insoluble complexes in the intracellular transport of intestinal brush border enzymes. Share This Paper. Tables from this paper.
References Publications referenced by this paper. Transport phenomena and membrane digestion in small intestinal mucosa.
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