KINETICS OF INTESTINAL ACTIVE TRANSPORT OF FIVE NEUTRAL AMINO ACIDS

The Genetic Background Is Shaping Cecal Enlargement in the Absence of Intestinal Microbiota

Germ-free (GF) rodents have become a valuable tool for studying the role of intestinal microbes on the host physiology. The major characteristic of GF rodents is an enlarged cecum. The accumulation of mucopolysaccharides, digestion enzymes and water in the intestinal lumen drives this phenotype. Microbial colonization normalizes the cecum size in ex-GF animals. However, whether strain genetics influences the cecal enlargement is unknown. Here we investigated the impact of mouse genetic background on the cecal size in five GF strains frequently used in biomedical research. The cecal weight of GF mice on B6 background (B6J and B6N) represented up to 20% of total body weight. GF NMRI and BALBc mice showed an intermediate phenotype of 5-10%, and those on the C3H background of up to 5%. Reduced cecal size in GF C3H mice correlated with decreased water content, increased expression of water transporters, and reduced production of acidic mucins, but was independent of the level of digestive enzymes in the lumen. In contrast, GF B6J mice with greatly enlarged cecum showed increased water content and a distinct metabolic profile characterized by altered amino acid and bile acid metabolism, and increased acidic mucin production. Together, our results show that genetic background influences the cecal enlargement by regulating the water transport, production of acidic mucins, and metabolic profiles.

Intestinal absorption studies on peptide mimetic alpha-methyldopa prodrugs

Two dipeptide mimetic prodrugs, 1 and 2, and two tripeptide mimetic prodrugs, 3 and 4, of L-alpha-methyldopa were evaluated for intestinal absorption by in-situ single pass rat jejunal perfusion studies and by in-vitro uptake experiments in brush-border membrane vesicles (BBMVs) prepared from rat intestine. In the perfusion studies, compound 1 demonstrated a 3.5-fold increase in permeability (Pm* = 2.27) as compared with that of alpha-methyldopa (Pm* = 0.65), indicating that this prodrug was better absorbed in the intestine than its parent drug. Other prodrugs showed no significant improvement in intestinal permeability. The results correlated with the results of BBMV uptake studies. In the presence of an inward proton gradient, compound 1 showed Michaelis-Menton saturable kinetics of BBMV uptake with a low value of K(m) (0.06 +/- 0.13 mM) and a high value of Vmax/K(m)(36.38 nmol (mg protein)-1/30s mM-1) at a low concentration range and a linear uptake at high concentrations with Kd = 0.14 +/- 0.02 mM. Compounds 2 and 3 were mainly taken up in BBMVs via passive diffusion. Compound 4 was taken up in BBMVs basically via the carrier-mediated transport system, while the rate of uptake was much lower than that of compound 1. The uptake of compounds 1 and 4 was significantly inhibited by dipeptides L-Gly-L-Pro and L-Gly-L-Phe, and cephradine, a beta-lactam known to be transported via the dipeptide carrier system, indicating that both compounds were taken up in BBMVs via the H(+)-coupled dipeptide-mediated transport system. In contrast to the complicated uptake profile of alpha-methyldopa, the higher rate of BBMV uptake with less variation demonstrated on compound 1 suggested that the attached nonessential amino acid moiety, D-phenylglycine, is a feasible delivery tool in carrying the parent drug through the intestine.

Chloride-dependent amino acid transport in the small intestine: occurrence and significance

The unidirectional influx of amino acids, D-glucose and ions across the brush-border membrane of the small intestine of different species has been measured in vitro with emphasis on characterization of topographic and species differences and on chloride dependence. The regional differences in transport along the small intestine are outlined and shown to be caused by variation in transport capacity, while the apparent affinity constants are unchanged. Rabbit small intestine is unique by exhibiting maximal rates of transport in the distal ileum and a very steep decline in the oral direction from where tissues are normally harvested for preparation of brush-border membrane vesicles. Transport in the guinea pig and rat is much more constant throughout the small intestine. Since the capacity of nutrient carriers is regulated by their substrates it is possible that bacterial breakdown of peptides and proteins in rabbit distal ileum increases the concentration of amino acids leading to an upregulation of the carriers. Chloride dependence is a characteristics of the carrier rather than the transported amino acid, and is used to improve the classification of amino acid carriers in rabbit small intestine. In this species the imino acid carrier, the beta-amino acid carrier, and the beta-alanine carrier, which should be renamed the B0,+ carrier, are chloride-dependent. The steady-state mucosal uptake of classical substrates for these carriers in biopsies from the human duodenum is also chloride-dependent. The carrier of beta-amino acids emerges as ubiquitous and chloride-dependent, and evidence of cotransport with both sodium and chloride is reviewed. A sodium:chloride:2-methyl-aminoisobutyric acid coupling stoichiometry of approx. 2:1:1 is suggested by ion activation studies. Direct measurements of coupled ion fluxes in rabbit distal ileum confirm that sodium, chloride and 2-methyl-aminoisobutyric acid are cotransported on the imino acid carrier with an identical influx stoichiometry. Control experiments and reference to the literature on the electrophysiology of the small intestine exclude alterations of the membrane potential as a feasible explanation of the chloride dependence. Thus, it is concluded that chloride is cotransported with both sodium and 2-methyl-aminoisobutyric acid across the brush-border membrane of rabbit distal ileum.

Effect of olsalazine on sodium-dependent bile acid transport in rat ileum

Olsalazine (OLZ), a relatively new form of 5-aminosalicylic acid (5-ASA), is being used for the treatment of colitis. A major side effect of olsalazine is diarrhea, reported in 12-25% of patients. One suggested mechanism for this side effect is enhanced ileal water and electrolyte secretion. We propose that OLZ may also inhibit ileal bile acid (BA) transport, resulting in choleretic diarrhea. This would result in excess BAs reaching the colon, with consequent BA-induced secretory diarrhea. Therefore, we studied the effect of OLZ on rat ileal absorption of taurocholate. BA uptake was determined in rat ileal segments, everted sacs, brush border membrane vesicles (BBMV), and Xenopus laevis oocytes. Segments and everted sacs were treated with 5 mM OLZ for 30 min prior to and throughout 10-min taurocholate (Tc) uptake. Terminal ileal BBMV were used to study the effect of OLZ on sodium-dependent bile acid uptake independent of cellular metabolism. Direct effects on the bile acid carrier were examined using Xenopus laevis oocytes expressing the cloned apical rat ileal BA transporter. In ileal segments 5 mM OLZ inhibited 10-min Tc uptake by 69.4 +/- 8.8% (P < 0.01) (N = 10 animals). Increasing concentrations of OLZ resulted in a dose-dependent inhibition of Tc uptake. Ten-minute Tc uptake with 0.5, 1.0, 2.0, 2.5, and 5 mM OLZ was inhibited by 13.5, 39.6, 49.7, and 70.5%, respectively. In BBMV, OLZ inhibited 45-sec Tc uptake in a dose-dependent manner but did not effect Na-dependent L-alanine uptake.(ABSTRACT TRUNCATED AT 250 WORDS)

Transmucosal impedance of small intestine: correlation with transport of sugars and amino acids

Transmucosal impedances of isolated perfused segments of jejunum from mice and hamsters were measured at frequencies from 10-100,000 Hz in the presence and absence of sugars and amino acids. Na-coupled transport of organic substrates caused large decreases of transmucosal impedance, reflecting contraction of cytoskeletal proteins controlling permeability of tight junctions, functional surface of basolateral membranes, and width of extracellular pathways. The observed changes of impedance were closely correlated with molar rates of Na-coupled active transport rather than with molecular species. Thus amino acids and sugars having the same molar rates of active transport also have the same effects on transmucosal impedance. It is proposed that a nonspecific increase of intracellular osmotic pressure during active transport is the first step initiating cytoskeletal contraction. Cell volume regulatory responses, including increased basolateral K+ conductance and Ca2+ influx, may be subsequent steps leading to contraction of perijunctional actomyosin, formation of junctional dilatations, and exposure of lateral membranes. Enhancement of oxygen capacity of perfusion fluids (e.g., with fluorocarbon emulsion) is required to maintain viability of isolated intestinal epithelium; in plain oxygenated Ringer-HCO3 solution, the transmucosal impedance is abnormally low and cytoskeletal contractile responses to Na-coupled transport are attenuated. An electrical circuit analog is presented that simulates almost exactly the observed transmucosal impedances and provides quantitative evaluation of the effects of Na-coupled transport of sugars and amino acids on resistances of tight junctions, capacitance of basolateral membranes, and postjunctional resistances of lateral intercellular spaces and villus cores.

Jejunal water and electrolyte absorption from two proprietary enteral feeds in man: importance of sodium content

Jejunostomy losses of Na+ and water during enteral nutrition after massive intestinal resection may be severe. We have attempted to analyse this practical problem by using an in vivo perfusion technique in healthy volunteers to study Na+, water and nutrient absorption from a short (25 cm) segment of jejunum during perfusion of an isotonic solution of the elemental diet Vivonex. Further solutions made from the amino acid and carbohydrate components of Vivonex were also perfused in part I of the study in order to determine the causes of the marked Na+ and water secretion seen during Vivonex perfusion. Low initial Na+ concentration was found to be the major determinant of net Na+ secretion, initial Na+ concentration correlating significantly with Na+ absorption (r = 0.95, n = 7 p less than 0.001). Water absorption correlated with net absorption of NaCl (r = 0.82, n = 7 p less than 0.01). There was, however, a better correlation with total absorption of NaCl plus amino acids (r = 0.99, n = 7, p less than 0.01). In part II of the study separate isotonic solutions of NaCl, glucose, and the polymeric diet, Ensure were also studied. Net sodium secretion occurred during glucose and Ensure perfusion, as predicted from their low Na+ concentration. Owing to rapid sucrose absorption from Ensure there was substantial luminal disappearance of osmotically active particles and hence marked water absorption, which was accurately predicted using the regression equation for water absorption derived in part I, substituting sucrose absorption for amino acid absorption. We conclude that the marked Na+ and water secretion observed during Vivonex perfusion is not a unique property of this amino acid based diet but is due to its low Na+ content.

Concentration and pH dependency of alpha-methyldopa absorption in rat intestine

The intestinal absorption of alpha-methyldopa from perfused segments of rat intestine was determined. Jejunal and ileal segments were studied at pH 4.5, 6.0 and 7.4 at an alpha-methyldopa concentration of 0.01 mM, and at pH 6.0 at concentrations of 0.01, 0.10 and 10 mM. Intestinal absorption was found to depend on both concentration and pH but not on segment. A weak positive correlation (r = 0.5) was observed between net water absorption and alpha-methyldopa absorption, similar to that observed for amino acids. The low intestinal wall permeability of alpha-methyldopa under normal intestinal pH conditions (pH 7.4) is consistent with the incomplete oral bioavailability of this drug.

Comparative aspects of the apparent Michaelis constant for neutral amino acid transport in several animal tissues

The apparent Michaelis constant, Km, for transport of a number of neutral amino acids has been compared between intestine, heart, brain and erythrocytes among a variety of animals using values available in the literature. Neutral amino acids with side chains containing 3, 4, 7 and 9 carbon atoms had approximately equal mean Km values when tested for intestinal transport among a variety of species; alanine appeared to have a mean Km value that was larger than those found for the first group, and glycine had a significantly greater mean Km than all of the other compounds tested. Km values for phenylalanine and tryptophan measured in rat heart were found to be close to the means measured for these substrates in intestine. The mean Km values measured in mammalian brain for each of the neutral amino acid substrates were found not be significantly different from each other. When the means of Km values for the neutral amino acids tested were compared between intestine and brain, only the glycine means were shown to differ significantly between the organs. Based on data for several mammalian species, brain appears to have a greater average apparent affinity for glycine than does intestine. In the human erythrocytes and in a few other mammalian species, Km values for all neutral amino acids tested with exception of glycine were found to be similar in magnitude to each other and to the Km averages of neutral amino acids found in intestine for the series containing 3-9 carbon atoms. The Km value for glycine in the human erythrocyte was noted to be substantially lower in value than the averages for glycine in brain or intestine. Avian red blood cells appear to have high apparent affinity for neutral amino acid transport when compared with red cells of several mammalian species.

Sites of dipeptide hydrolysis in relation to sites of histidine and glucose active transport in hamster intestine

The effects of dipeptides and amino acids on the active transport of L-histidine and D-glucose by sacs of everted small intestine of the hamster have been used to determine the sites of final hydrolysis of the dipeptides in relation to the sites of active transport of L-histidine and D-glucose. The results, plus earlier observations (Wiseman, 1977), show that (a) dipeptide active transport occurs at a superficial site, followed by progressively deeper sites for (b) final hydrolysis of glycyl-phenylalanine and phenylalanyl-glycine, then deeper (c) L-histidine active transport, then (d) final hydrolysis of alanyl-alanine, alanyl-leucine, glycyl-alanine, glycyl-proline, leucyl-alanine and leucyl-leucine, then (e) D-glucose active transport, then (f) final hydrolysis of alanyl-glycine, alanyl-valine, glycyl-glycine, prolyl-glycine, valyl-alanine and valyl-valine. The site of D-glucose active transport (2e) and all the sites superficial to it (2a-d) lie in the intestinal epithelial cell's brush-border. The location within the cell of site(s) 2f is not known; it may lie in the cytosol. All the dipeptides appeared to inhibit L-histidine active transport by the release of free amino acid and not by action of intact dipeptide, supporting the view that dipeptides and free amino acids do not share a common transport pathway in the epithelium of the small intestine.

Simulation studies on the kinetics of intestinal absorption

1. A model has been used to simulate the absorption of solutes from perfused intestines. The model makes possible the numerical solution of the differential equations describing absorption processes along the length of the intestine which cannot be solved analytically. It allows for water absorption and the non-linear fall in solute concentration down the intestine. It can be modified easily to include other features, e.g. a change in V (maximum rate of absorption) or K (solute concentration at V/2) along the intestine. 2. 90 perfect data sets have been simulated using the model. The Michaelis-Menten equation was fitted to a quarter of them using different algebraic expressions for the apparent solute concentration. The fit of the equation was very good in every case and it was not possible to explain the poorness-of-fit encountered during an earlier survey (Atkins, G.L. and Gardner, M.L.G. (1977) Biochim, Biophys. Acta 468, 127--145) in terms of the fall in solute concentration described above. 3. The equation was also fitted to all the data sets in order to compare the use of several algebraic expressions for the apparent solute concentration. It has been shown that the current practice of using either the initial concentration or the effluent concentration can lead to estimates of V and K up to amost twice their true value. It has been shown that in one situation (glucose absorption by perfused rat intestine) it is possible to use an empirical expression that will reduce the errors considerably. 4. It is also possible, and perhaps preferable, to use a computer program to fit the model directly to data from the simulated experiments and obtain precise estimates of V and K. 5. In order to show that the model can be easily modified to incorporate other characteristics of perfused intestines, simulations were performed in which V decreased linearly down the intestine. In this example, it was concluded that an inhomogeneity due to non-constancy of V cannot be detected by single-pass perfusions.

[Constant level of intestinal amino acids as a function of the composition of same]

The present study was conducted with pigs. It was designed to investigate whether extreme amino acid proportions in the ration would influence the excretion of endogenic amino acids in the faeces. Addition of DL-methionine or L-Lysine produced slight changes in the composition of endogenic intestiornal protein, a fact that was practically of little or no importance for the method of faecal analysis used in determining the true digestibility of amino acids (classical technique).

Absorption of amino acids and peptides from a complex mixture in the isolated small intestine of the rat

Amino acid and peptide absorption from a pancreatic digest of casein at low concentration by an isolated preparation of perfused rat small intestine has been measured. 2. The rate of absorption of each amino acid (free or peptide-bound) is closely proportional to its concentration in the perfusate; this implies a constant Vmax/Km ration for all amino acids in the mixture. 3. There is a high correlation between the compositions of luminal perfusate and secretion into the tissue fluid (apart from the content of glutamic and aspartic acids and alanine). 4. The concentrations of each free amino acid are, on average, 9 times as great in secretion as in lumen; the total peptide-N concentration in secretion is approximately 4 times that in the lumen. 5. The rate of absorption of each free amino acid is highly negatively dependent on the rate of absorption of that amino acid in peptide-bound form, in addition to being positively dependent on the perfusate concentration of free amino acid. 6. While peptide-bound proline appears to be well absorbed, free proline liberated by hydrolysis appears to pass back into the lumen as well as into the tissue fluid. Substantial back flux of hydrolysis products may occur for all amino acids. 7. About one-third of the amino acids appearing in the secretion on the serosal surface are peptide-bound. 8. The rate of absorption of peptides appears to determine the rate of their hydrolysis which probably occurs mainly after entry into the mucosal cells.

Effect of microflora on the free amino acid distribution in various regions of the mouse gastrointestinal tract

The distribution of free amino acids in the contents of various regions of the gastrointestinal tract (stomach, upper small intestine, lower small intestine, cecum, upper colon and lower colon) was studied in germfree and conventionalized mice. Particular emphasis was placed on the conversion of tryptophan to indole as a probe for studying intermicrobial interactions and microbe-host interactions in vivo. Great differences were observed in the free amino acid content of the various regions of the digestive tract in each type of mouse and also in any one region between germfree and conventionalized mice. As would be expected, there were fewer differences in amino acid distribution between the types of mice in both regions of the small intestine. This correlates with a much lower population of microorganisms in these regions. The changes in free amino acid content and distribution produced by microflora are great enough to serve as a good probe for studying the interactions of a limited number of species of microbes in gnotobiotic animals and assign possible specific functions to each species.

Structure-affinity relationships of substrates for the neutral amino acid transport system in rabbit ileum

The apparent affinities of various amino acids for the neutral amino acid transport system in rabbit ileum were determined by measuring the inhibition of L-methionine-(14)C influx across the brush border membrane. The apparent affinity was very low for compounds lacking an alpha-amino group, compounds with the alpha-hydrogen substituted by a methyl group, D-compounds, compounds with tertiary branching in the side chain, compounds with either a positive or negative charge in the side chain, and in most cases, compounds with a hydrophilic moiety in the side chain. High apparent affinities were exhibited by compounds with unbranched carbon or carbon-sulfur side chains. Branched compounds such as valine and leucine exhibited affinities which correlate with binding of only the linear portion of the side chain. The calculated change in free energy of binding is 370 cal/mol/CH(2) group which suggests the binding region for the side chain is partially hydrophobic. The affinities of families of analogues, derivatives of cysteine, methionine, serine, alanine, valine, and phenylalanine, correlate with their calculated octanol/water partition coefficients and are also correlated with apparent structural and electronic differences between families. The data permit a preliminary description of the functional geometry of the neutral amino acid transport site. The site contains a region for binding the alpha-amino group, alpha-carboxyl group, and side chain. The regions about the alpha-amino group and alpha-hydrogen are quite sterically limited. The side chain binding region is hydrophobic in nature and appears to be shallow, binding only the linear portion of branched or ring compounds.

Functional differentiation of human jejunum and ileum: a comparison of the handling of glucose, peptides, and amino acids

The characteristics of glucose, glycine, L-alanine, and glycyl-L-alanine absorption from the jejunum and ileum have been compared in normal human subjects. A perfusion technique has been used, and correct positioning of the perfusion tube has been confirmed by measuring the differential jejunal and ileal handling of bicarbonate. Glucose and glycine were absorbed faster from the jejunum than from the ileum of all subjects studied, and L-alanine was absorbed faster from the jejunum than from the ileum in five out of six subjects studied. In contrast, the dipeptide glycyl-L-alanine was absorbed at comparable rates from the jejunum and ileum. Higher concentrations of free amino acids were detected in the luminal contents aspirated during the ileal dipeptide perfusions.These results emphasize the importance of oligopeptide transport in the absorption of protein digestion products, especially in the human ileum, and the practical implications of these findings are discussed.

Functional characterization of dipeptide transport system in human jejunum

The present studies were performed to determine whether dipeptide absorption in human jejunum exhibits the characteristics of carrier-mediated transport. 15-cm jejunal segments from human volunteers were perfused with test solutions containing varying amounts of either glycylglycine, glycylleucine, glycine, leucine, glycylglycine with leucine or glycine, glycylglycine with glycylleucine, or glycylleucine with an equimolar mixture of free glycine and leucine. Jejunal absorption rates of both glycylglycine and glycylleucine followed the kinetics of a saturable process. The K(m) value in millimoles/liter of glycylglycine was significantly greater than the K(m) value of glycylleucine (43.3+/-2.6 vs. 26.8+/-5.9, P < 0.05); and the K(m) value of glycine was also significantly greater than the K(m) value of leucine (42.7+/-7.5 vs. 20.4+/-5.4, P < 0.05). While overlapping occurred among the K(m) values of free amino acids and dipeptides, the transport kinetics of dipeptides were characterized by higher V(max) values (in micromoles per minute per 15 centimeters) than those of free amino acids. For example, the V(max) values for glycylglycine and glycine were 837+/-62 and 590+/-56, respectively (P < 0.02). While jejunal absorption rates of glycylglycine were not significantly affected by free leucine or free glycine, they were competitively inhibited by glycylleucine. The jejunal absorption rate of glycylleucine was not significantly altered by an equimolar mixture of free glycine and leucine. The selective absorption of dipeptides was investigated by infusing three equimolar mixtures, each containing two different dipeptides. Among the three dipeptides examined, glycylglycine was the least absorbed. There was no significant difference between the absorption of glycylleucine and leucylglycine. The above studies suggest that absorption of both glycylglycine and glycylleucine is mediated by a carrier which is not shared with free neutral amino acids; and that both COOH- and NH(2)-terminal amino acids appear to be influential in imposing the affinity of a dipeptide for the absorption sites.

Characteristics of the amino acid transport system in the mucosal border of rabbit ileum

The specificity of the neutral amino acid transport system in the brush border was examined by studying the ability of amino acid analogues to inhibit the unidirectional influx of phenylalanine from mucosal solution into the cells. Effects were evaluated in terms of the affinity of various substrates for the amino acid site in the transport system. The affinity of amino acids for the site was proportional to the number of carbon atoms in the side chain. Electron-withdrawing substituents in the ring of phenylalanine increased affinity and electron-releasing groups decreased affinity. Removal of the alpha-amino group from phenylalanine decreased affinity by a factor of approximately 50 and removal of the carboxyl group decreased affinity 12-fold. Effects on affinity of variations in the side chain of the amino acid can be comparable in magnitude to that of the carboxyl group. The effect of sodium ion on the transport system appears to be similar for all compounds tested.

Kinetic relations of the Na-amino acid interaction at the mucosal border of intestine

The relation between unidirectional influxes of Na and amino acids across the mucosal border of rabbit ileum was studied under a variety of conditions. At constant Na concentration in the mucosal bathing solution, amino acid influx followed Michaelis-Menten kinetics permitting determination of maximal influx and the apparent Michaelis constant, K(t). Reduction in Na concentration, using choline as substitute cation, caused an increase in K(t) for alanine but had no effect on maximal alanine influx. The reciprocal of K(t) was a linear function of Na concentration. Similar results were obtained for valine and leucine and these amino acids competitively inhibited alanine influx both in the presence and in the absence of Na. These results lead to a model for the transport system which involves combination of Na and amino acid with a single carrier or site leading to penetration of both solutes. The model predicts that alanine should cause an increase in Na influx and the ratio of this extra Na flux to alanine flux should vary with Na concentration. The observed relation agreed closely with predicted values for Na concentrations from 5 to 140 mM. These results support the hypothesis that interactions between Na and amino acid transport depend in part on a common entry mechanism at the mucosal border of the intestine.

Effect of amino acids on sugar absorption

1. Sacs of everted mid-small intestine of the hamster have been used to study the effect of amino acids on sugar absorption.2. The sugars employed were D-glucose, D-galactose, 3-O-methyl-D-glucose, D-fucose, L-glucose, alpha-glucoheptose, L-fucose, D-mannose and L-sorbose. The amino acids were L- and D-histidine, L- and D-methionine, L- and D-alanine, L- and D-valine, L- and D-glutamic acid, L-leucine, L-proline, L-ornithine and L-aspartic acid.3. Actively absorbed amino acids considerably inhibit the transport of actively absorbed sugars. The results give support for the view that D-histidine and L-glucose are actively transferred. Passively absorbed amino acids and sugars are not involved.4. As L-glutamic and L-aspartic acids in the mucosal fluid have no inhibitory effect on D-glucose absorption, although mucosal fluid L-alanine is quite potent, the step at which the latter exerts its inhibitory action must be before that at which the intracellular transamination of L-glutamic and L-aspartic acids occurs. It would seem likely, therefore, that L-alanine interferes with the process by which epithelial cells capture and concentrate sugars at the luminal border.5. More than one active transfer system may exist for D-glucose.6. The influence of actively absorbed L-amino acids on D-glucose active transport seems to be in some way related to the efficiency with which the amino acids are themselves concentrated.7. Inhibition of D-glucose active absorption by an amino acid may be a simple test of an amino acid's participation in an active transport system.

Active transport of 5,5-dimethyl-2,4-oxazolidinedione

5,5-Dimethyl-2,4-oxazolidinedinedione, a substance commonly used to estimate intracellular pH, moves against both a concentration gradient and a hydrogen-ion gradient in the everted gut sac. Furthermore, the value of the flux ratio for this substance under conditions of zero electrochemical potential across the bowel wall unequivocally demonstrates active transport.