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

Intestinal Amino Acid Transport and Metabolic Health

Amino acids derived from protein digestion are important nutrients for the growth and maintenance of organisms. Approximately half of the 20 proteinogenic amino acids can be synthesized by mammalian organisms, while the other half are essential and must be acquired from the nutrition. Absorption of amino acids is mediated by a set of amino acid transporters together with transport of di- and tripeptides. They provide amino acids for systemic needs and for enterocyte metabolism. Absorption is largely complete at the end of the small intestine. The large intestine mediates the uptake of amino acids derived from bacterial metabolism and endogenous sources. Lack of amino acid transporters and peptide transporter delays the absorption of amino acids and changes sensing and usage of amino acids by the intestine. This can affect metabolic health through amino acid restriction, sensing of amino acids, and production of antimicrobial peptides.

SLC transporters ASCT2, B0 AT1-like, y+ LAT1, and LAT4-like associate with methionine electrogenic and radio-isotope flux kinetics in rainbow trout intestine

Methionine (Met) is an important building block and metabolite for protein biosynthesis. However, the mechanism behind its absorption in the fish gut has not been elucidated. Here, we describe the fundamental properties of Met transport along trout gut at µmol/L and mmol/L concentration. Both electrogenic and unidirectional DL-[14 C]Met flux were employed to characterize Met transporters in Ussing chambers. Exploiting the differences in gene expression between diploid (2N) and triploid (3N) and intestinal segment as tools, allowed the association between gene and methionine transport. Specifically, three intestinal segments including pyloric caeca (PC), midgut (MG), and hindgut (HG) were assessed. Results at 0-150 µmol/L concentration demonstrated that the DL-Met was most likely transported by apical transporter ASCT2 (SLC1A5) and recycled by basolateral transporter y+ LAT1 (SLC7A7) due to five lines of observation: (1) lack of Na+ -independent kinetics, (2) low expression of B0 AT2-like gene, (3) Na+ -dependent, high-affinity (Km , µmol/L ranges) kinetics in DL-[14 C]Met flux, (4) association mRNA expression with the high-affinity kinetics and (5) electrogenic currents induced by Met. Results at 0.2-20 mmol/L concentration suggested that the DL-Met transport is likely transported by B0 AT1-like (SLC6A19-like) based on gene expression, Na+ -dependence and low-affinity kinetics (Km , mmol/L ranges). Similarly, genomic and gene expression analysis suggest that the basolateral exit of methionine was primarily through LAT4-like transporter (SLC43A2-like). Conclusively, DL-Met uptake in trout gut was most likely governed by Na+ -dependent apical transporters ASCT2 and B0 AT1-like and released through basolateral LAT4-like, with some recycling through y+ LAT1. A comparatively simpler model than that previously described in mammals.

Amino Acid Transport Across the Mammalian Intestine

The small intestine mediates the absorption of amino acids after ingestion of protein and sustains the supply of amino acids to all tissues. The small intestine is an important contributor to plasma amino acid homeostasis, while amino acid transport in the large intestine is more relevant for bacterial metabolites and fluid secretion. A number of rare inherited disorders have contributed to the identification of amino acid transporters in epithelial cells of the small intestine, in particular cystinuria, lysinuric protein intolerance, Hartnup disorder, iminoglycinuria, and dicarboxylic aminoaciduria. These are most readily detected by analysis of urine amino acids, but typically also affect intestinal transport. The genes underlying these disorders have all been identified. The remaining transporters were identified through molecular cloning techniques to the extent that a comprehensive portrait of functional cooperation among transporters of intestinal epithelial cells is now available for both the basolateral and apical membranes. Mouse models of most intestinal transporters illustrate their contribution to amino acid homeostasis and systemic physiology. Intestinal amino acid transport activities can vary between species, but these can now be explained as differences of amino acid transporter distribution along the intestine. © 2019 American Physiological Society. Compr Physiol 9:343-373, 2019.

Spatial mRNA Expression and Response to Fasting and Refeeding of Neutral Amino Acid Transporters slc6a18 and slc6a19a in the Intestinal Epithelium of Mozambique tilapia

The mRNA expressions of the epithelial neutral amino acid transporters slc6a18 and slc6a19a in the five segments (HL, PMC, GL, DMC, and TS) of the intestine of Mozambique tilapia, and their responses to fasting and refeeding were investigated for a better understanding of the functional and nutritional characteristics of slc6a18 and slc6a19a. Although both slc6a18 and slc6a19a were expressed mainly in the intestine, these genes showed opposing spatial distributions along the intestine. The slc6a18 was mainly expressed in the middle (GL) and posterior (DMC and TS) intestines, while slc6a19a was specifically expressed in the anterior intestine (HL and PMC). Large decreases of amino acid concentrations from the HL to GL imply that amino acids are mainly absorbed before reaching the GL, suggesting an important role of slc6a19a in the absorption. Moreover, substantial amounts of some neutral amino acids with the isoelectric point close to 6 remain in the GL. These are most likely the remaining unabsorbed amino acids or those from of amino acid antiporters which release neutral amino acids in exchange for uptake of its substrates. These amino acids were diminished in the TS, suggesting active absorption in the posterior intestine. This suggests that slc6a18 is essential to complete the absorption of neutral amino acids. At fasting, significant downregulation of slc6a19a expression was observed from the initial up to day 2 and became stable from day 4 to day 14 in the HL and PMC suggesting that slc6a19a expression reflects nutritional condition in the intestinal lumen. Refeeding stimulates slc6a19a expression, although expressions did not exceed the initial level within 3 days after refeeding. The slc6a18 expression was decreased during fasting in the GL but no significant change was observed in the DMC. Only a transient decrease was observed at day 2 in the TS. Refeeding did not stimulate slc6a18 expression. Results in this study suggest that Slc6a18 and Slc6a19 have different roles in the intestine, and that both of these contribute to establish the efficient neutral amino acid absorption system in the tilapia.

Seaweed proteins and amino acids as nutraceuticals

Seaweeds demonstrate original and interesting nutritional characteristics. Protein concentration ranges from 5% to 47% of dry basic. Its value depends particularly on species and the environmental conditions. Seaweed protein is a source of all amino acids, especially glycine, alanine, arginine, proline, glutamic, and aspartic acids. In algae, essential amino acids (EAAs) represent almost a half of total amino acids and their protein profile is close to the profile of egg protein. In case of non-EAAs, all three groups (green, brown, and red seaweeds) contain the similar amount. Red seaweed seems to be a good source of protein because its value reaches 47%. The issue of protein malnutrition supports the trend to find a new and cheap alternative source of protein. Algae could play an important role in the above-mentioned challenge because of relatively high content of nitrogen compounds. Algae may be used in the industry as a source of ingredients with high nutritional quality.

Amino acid transport across mammalian intestinal and renal epithelia

The transport of amino acids in kidney and intestine is critical for the supply of amino acids to all tissues and the homeostasis of plasma amino acid levels. This is illustrated by a number of inherited disorders affecting amino acid transport in epithelial cells, such as cystinuria, lysinuric protein intolerance, Hartnup disorder, iminoglycinuria, dicarboxylic aminoaciduria, and some other less well-described disturbances of amino acid transport. The identification of most epithelial amino acid transporters over the past 15 years allows the definition of these disorders at the molecular level and provides a clear picture of the functional cooperation between transporters in the apical and basolateral membranes of mammalian epithelial cells. Transport of amino acids across the apical membrane not only makes use of sodium-dependent symporters, but also uses the proton-motive force and the gradient of other amino acids to efficiently absorb amino acids from the lumen. In the basolateral membrane, antiporters cooperate with facilitators to release amino acids without depleting cells of valuable nutrients. With very few exceptions, individual amino acids are transported by more than one transporter, providing backup capacity for absorption in the case of mutational inactivation of a transport system.

Mechanism and putative structure of B(0)-like neutral amino acid transporters

The Na(+)-dependent transport of neutral amino acids in epithelial cells and neurons is mediated by B(0)-type neutral amino acid transporters. Two B(0)-type amino acid transporters have been identified in the neurotransmitter transporter family SLC6, namely B(0)AT1 (SLC6A19) and B(0)AT2 (SLC6A15). In contrast to other members of this family, B(0)-like transporters are chloride-independent. B(0)AT1 and B(0)AT2 preferentially bind the substrate prior to the Na(+)-ion. The Na(+)-concentration affects the K ( m ) of the substrate and vice versa. A kinetic scheme is proposed that is consistent with the experimental data. An overlapping binding site of substrate and cosubstrate has been demonstrated in the bacterial orthologue LeuT( Aa ) from Aquifex aeolicus, which elegantly explains the mutual effect of substrate and cosubstrate on each other's K ( m )-value. LeuT( Aa ) is sequence-related to transporters of the SLC6 family, allowing homology modeling of B(0)-like transporters along its structure.

Na+ gradient-dependent transport of hypoxanthine by calf intestinal brush border membrane vesicles

The properties of hypoxanthine transport were investigated in purified brush border membrane vesicles isolated from calf proximal and distal jejunum. Hypoxanthine uptake in the vesicles was stimulated by a transmembrane Na(+) gradient and an inside negative potential resulting in a transient accumulation of intravesicular hypoxanthine, especially in the proximal jejunum. Na(+)-dependent hypoxanthine uptake at this site seemed to occur by two saturable transport systems, a high affinity (K(m)=0.33 micromol/l) and a low affinity (K(m)=165 micromol/l) transporter. Guanine, hypoxanthine, thymine and uracil inhibited intravesicular hypoxanthine uptake, whereas adenine and the nucleosides inosine and thymidine were without effect. These findings represent the first demonstration of active Na(+) gradient-dependent nucleobase transport in intestinal brush border membrane vesicles.

Nucleosides are efficiently absorbed by Na(+)-dependent transport across the intestinal brush border membrane in veal calves

In previous work, a comparatively high capacity for Na(+)-dependent transport of nucleosides across the intestinal brush border membrane (BBM) was observed in dairy cows, which might be related to digestion of the large amount of nucleic acids present in ruminal microorganisms in the ruminant small intestine. If this were the case, the capacity for Na(+)-dependent intestinal nucleoside transport should be much lower in veal calves, in which only small amounts of nucleic acids, nucleotides, and nucleosides reach the small intestine via the milk replacer. To test this hypothesis, we investigated Na(+)-dependent transport of 3H-labeled thymidine and guanosine across the BBM using BBM vesicles (BBMV) isolated from the small intestine of veal calves. In the presence of a transmembrane Na+ gradient both substrates were transported against a concentration gradient. Inhibitory studies showed that thymidine and guanosine are transported by two different transporters with overlapping substrate specificity, one accepting predominantly pyrimidine nucleosides (N2) and one accepting particularly purine nucleosides (N1). Nucleoside transport was inhibited by glucose along the whole small intestine. Maximal transport rates similar to those in dairy cows were obtained for the proximal, mid-, and distal small intestine. These findings suggest that the high absorptive capacity for nucleosides is a genetically fixed property in the bovine small intestine, which is already present in the preruminant state of veal calves. It may contribute to the high digestibility of nucleic acids observed by others in veal calves receiving milk replacer supplemented with RNA. Its main function may be the efficient absorption of nucleosides resulting from the digestion of nucleic acids associated with desquamated enterocytes. Due to the limited de novo synthesis of nucleotides in enterocytes intracellular uptake of nucleosides across the BBM may contribute to nucleic acid synthesis in enterocytes and thus may have a trophic effect on the intestinal epithelium.

Amino acid transport by intestinal brush border vesicles of a marine fish, Boops salpa

The transport of glycine, alanine, methionine and alpha amino-isobutyric acid (AIB) was studied on brush border membrane vesicles of Boops salpa, a marine fish. This transport was Na(+)-, Cl(-)- and pH-dependent. In the presence of NaCl, the uptake decreased as the pH increased from 5.5 to 8.5. With Na2SO4, the transport of the four amino acids was strongly reduced and the pH optimum was 7-8. In the presence of NaCl, amino acid transport was described by high and low affinity kinetics. The K(t) of the high-affinity component was comparable for glycine, alanine and methionine (0.1 mM), and was significantly enhanced for AIB (0.6 mM). The J(max) of the low affinity component was significantly lower for methionine and AIB than for glycine and alanine. Lowering the sodium concentration from 80 to 20 mM significantly increased K(t) and J(max) of the high-affinity component of glycine transport. Moreover, the kinetics of AIB transport under 100 mM Na(+) were similar to glycine kinetics under 40 mM Na(+) and the two amino acids competed for the same carrier(s). These results suggest that chloride ions are essential in neutral amino acid transport in Boops, that multiple saturable components are involved in this process, and that sodium plays an important role in the differences between the transport kinetics of amino acids.

Multiple transport pathways for dibasic amino acids in the larval midgut of the silkworm Bombyx mori

The transport pathways for dibasic amino acids were investigated in brush border membrane vesicles (BBMV) from the anterior-middle (AM) and posterior (P) regions of Bombyx mori midgut. In the absence of K(+), a low-affinity saturable transport of arginine in both AM- and P-BBMV (K(m) 1.01 mM, V(max) 4.07 nmol/7s/mg protein and K(m) 1.38 mM, V(max) 2.26 nmol/7s/mg protein, respectively) was detected. Arginine influx was dependent on the membrane electrical potential (Deltapsi) and increased raising the alkalinity of the external medium from pH 7.2 to 10.6. Competition experiments indicated the following order of substrate affinity: arginine, homoarginine, N(G)-monomethylarginine, N(G)-nitroarginine>lysine>>ornithine>cysteine>methionine. Leucine, valine and BCH (2-amino-2-norbornanecarboxylic acid) did not inhibit arginine influx. In the presence of external K(+), the influx of arginine as a function of arginine concentration fitted to a complex saturation kinetics compatible with both a low-affinity and a high-affinity component. The latter (K(m) 0.035 mM, V(max) 2.54 nmol/7s/mg protein) was fully characterized. The influx rate had an optimum at pH 8.8, was strongly affected by Deltapsi and was homogeneous along the midgut. The substrate affinity rank was: homoarginine>arginine, N(G)-monomethylarginine>>cysteine, lysine>>N(G)-nitroarginine>ornithine>methionine. Leucine and amino acids with a hydrophobic side chain were not accepted. This system is also operative in the absence of potassium, with the same order of specificity but a very low activity. Lysine influx is mediated by two more transport systems, the leucine uniport and the K(+)/leucine symport specific for amino acids with a hydrophobic side chain that recognizes lysine at extravesicular pH values (pH(out)) exceeding 9. Both the uniport and the symport differ from the cationic transport systems so far identified in mammals because they are unaffected by N-ethylmaleimide, have no significant affinity for neutral amino acids in the presence of the cation and show a striking difference in their optimum pH.

Active intestinal absorption of nucleosides by Na+-dependent transport across the brush border membrane in cows

Transport of 3H-labeled nucleosides across the bovine intestinal brush border membrane (BBM) was characterized with BBM vesicles (BBMV) isolated from mid-jejunum of cows because large amounts of nucleic acids are digested in the small intestine of ruminants. Two Na+-dependent electrogenic nucleoside transporters with overlapping substrate specificity were shown to be present in the jejunal BBM, one for pyrimidine nucleosides and one for purine nucleosides. As indicated by inhibitory studies, thymidine seemed to be a specific substrate for the pyrimidine nucleoside transporter, while this applied to guanosine and deoxyguanosine for the purine nucleoside transporter. Uridine and adenosine appear to have an affinity to both transporters. This also applies to deoxyadenosine and deoxyuridine. Nucleobases (uracil, hypoxanthine) did not affect transport of nucleosides. The kinetic constants (Km and Vmax) for Na-dependent thymidine and guanosine transport were 29 and 24 micromol/L and 78 and 51 pmol (mg protein)(-1) s(-1), respectively. These values are much higher than those reported for monogastric species.

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

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

Effects of pH changes on systems ASC and B in rabbit ileum

Influx of D-aspartate (D-Asp), L-glutamate (L-Glu), and serine (Ser) across the brush-border membrane of the intact mucosa from rabbit ileum has been examined. L-Glu influx is chloride independent and completely sodium dependent. D-Asp and L-Glu share a transport system with a maximum transport rate of 1 micromol. cm-2. h-1 and an apparent affinity constant (K1/2) of approximately 0.3 mM. The function of this transport system is pH insensitive between pH 5.65 and 8.2, and bipolar amino acids do not affect the way in which the transport system handles D-Asp and L-Glu. The characteristics of this transport system match those of system X-AG. L-Glu and Ser share a transporter for which the inhibitor constant (Ki) of L-Glu against Ser decreases from 54 to 10 mM when pH is reduced from 7.2 to 5.65, while the maximum rate of transport remains unaffected at approximately 10 micromol. cm-2. h-1. The Ki values (5 mM) of Ser against L-Glu influx and the L-Glu-sensitive contribution to Ser influx (0.8 micromol. cm-2. h-1 at 1 mM Ser) are the same at both pH values. The L-Glu-sensitive transport of Ser together with the contribution of system bo,+ account for approximately 50% of Ser influx at pH 7.2. The remaining 50% can be ascribed to system B. Transport of Ser by system B is reduced by >95% at pH 5.65. At pH 7. 2 Ki of Ser against transport of leucine (Leu) by system B is 18 mM and Ki of Leu against transport of Ser is 1.7 mM. The low-affinity transport of L-Glu and the L-Glu-sensitive transport of Ser are performed by an equivalent of system ASC. Supplementary experiments using the jejunum confirm the validity of these results for a major portion of the rabbit small 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.

Phenylalanine transport in rabbit small intestine

1. The proposal that rabbit small intestine possesses a separate, sodium-dependent carrier of phenylalanine has been examined by measurements of the unidirectional influx of amino acids across the brush-border membrane of the intact epithelium of the rabbit small intestine. 2. We demonstrate that, like alanine, glycine and leucine, phenylalanine shares sodium-dependent as well as sodium-independent transport with lysine. 3. Using the distal ileum we applied the A (phenylalanine)-B (leucine)-C (alanine) test on the sodium-dependent, lysine-resistant transport of phenylalanine. For phenylalanine, K1/2 (concentration required for half-maximal transport) was 3.1 +/- 0.2 mM (n = 7) and Ki (inhibitor constant) against leucine transport was 3.1 +/- 0.2 mM (n = 4). For leucine, K1/2 was 1.1 +/- 0.1 mM (n = 4) and Ki against transport of phenylalanine was 1.1 +/- 0.1 mM (n = 4). For alanine, K1/2 was 12.6 +/- 1.1 mM (n = 3), Ki against phenylalanine was 13.1 +/- 1.8 mM (n = 4) and Ki against leucine was 11.0 +/- 0.4 mM (n = 4). 4. Using the jejunum we applied the A (phenylalanine)-B (alanine)-C (methionine) test on the lysine-resistant, sodium-dependent transport of phenylalanine. For phenylalanine, K1/2 was 4.7 +/- 0.2 mM (n = 7) and Ki against alanine was 4.8 +/- 0.2 mM (n = 4). For alanine, K1/2 was 15.6 +/- 0.8 mM (n = 7) and Ki against phenylalanine was 18.1 +/- 0.9 mM (n = 5).(ABSTRACT TRUNCATED AT 250 WORDS)

Epithelial properties of human intestinal Caco-2 cells cultured in a serum-free medium

Human intestinal Caco-2 cells were cultured under serum-free conditions on an insoluble collagen and FCS matrix (Caco-2-SF), and a comparison was made between several characteristics of Caco-2 and Caco-2-SF cells. Their morphological appearance was identical. Slight differences were found in cell growth and expression of brush border enzymes between Caco-2 and Caco-2-SF cells. Similar levels of activity of Gly-Gly transport were expressed in both types of cell. Caco-2 cells cultured on permeable filters showed high transepithelial electrical resistance (TEER), indicating the high monolayer integrity. The transepithelial transport activity for glucose, alanine and Gly-Gly was detected by measuring the change in short-circuit current (delta Isc) after adding each of these nutrients to the apical chamber. In Caco-2-SF cells, such parameters as TEER and delta Isc were reduced drastically, suggesting that the monolayer integrity and cell polarity that are important for transepithelial transport were not attained. These parameters, however, could be restored by adding FCS or by milk whey. The result suggested that FCS and milk whey contain factors which regulate the formation of the tight junctions and, consequently, the development of cell polarity. Thus the Caco-2-SF cell-culture system will provide a useful model for studying factors which regulate the intestinal transepithelial transport functions.

Branched-Chain Amino Acid Transport in Cytoplasmic Membranes of Leuconostoc mesenteroides subsp. dextranicum CNRZ 1273

Membrane vesicles of Leuconostoc mesenteroides subsp. dextranicum fused with proteoliposomes prepared from Escherichia coli phospholipids containing beef heart cytochrome c oxidase were used to study the transport of branched-chain amino acids in a strain isolated from a raw milk cheese. At a medium pH of 6.0, oxidation of an electron donor system comprising ascorbate, N,N,N′,N′ -tetramethyl- p -phenylenediamine, and horse heart cytochrome c resulted in a membrane potential (Δψ) of −60 mV, a pH gradient of −36 mV, and an l -leucine accumulation of 76-fold (Δμ Leu / F = 108 mV). Leucine uptake in hybrid membranes in which a Δψ, ΔpH, sodium ion gradient, or a combination of these was imposed artificially revealed that both components of the proton motive force (Δp) could drive leucine uptake but that a chemical sodium gradient could not. Kinetic analysis of leucine (valine) transport indicated three secondary transport systems with K t values of 1.7 (0.8) mM, 4.3 (5.9) μM, and 65 (29) nM, respectively. l -Leucine transport via the high-affinity leucine transport system ( K t = 4.3 μM) was competitively inhibited by l -valine and l -isoleucine ( K i and K t values were similar), demonstrating that the transport system translocates branched-chain amino acids. Similar studies with these hybrid membranes indicated the presence of high-affinity secondary transport systems for 10 other amino acids.

Affinity of antineoplastic amino acid drugs for the large neutral amino acid transporter of the blood-brain barrier

The relative affinity of six anticancer amino acid drugs for the neutral amino acid carrier of the blood-brain barrier was examined in rats using an in situ brain perfusion technique. Affinity was evaluated from the concentration-dependent inhibition of L-[14C]-leucine uptake into rat brain during perfusion at tracer leucine concentrations and in the absence of competing amino acids. Of the six drugs tested, five, including melphalan, azaserine, acivicin, 6-diazo-5-oxo-L-norleucine, and buthionine sulfoximine, exhibited only low affinity for the carrier, displaying transport inhibition constants (Ki, concentrations producing 50% inhibition) ranging from 0.09 to 4.7 mM. However, one agent - D,L-2-amino-7-bis[(2-chloroethyl)amino]- 1,2,3,4-tetrahydro-2-naphthoic acid (D,L-NAM) - demonstrated remarkably high affinity for the carrier, showing a Ki value of approximately 0.2 microM. The relative affinity (1/Ki) of D,L-NAM was greater than 100-fold that of the other drugs and greater than 10-fold that of any compound previously tested. As the blood-brain barrier penetrability of most endogenous neutral amino acids is related to their carrier affinity, the results suggest that D,L-NAM may be a promising agent which may show enhanced uptake and distribution to brain tumors.

Kinetics and specificity of L-alanine transport across the basolateral cell surface in isolated oxyntic glands

The time course, kinetics, specificity and sodium-dependence of alanine uptake by isolated oxyntic glands were studied. The uptake of alanine by the hydrolyzed cells was measured directly, after incubation of the glands with L-[3H]alanine. L-Alanine total influx was saturable and apparently mediated by a single entry system (Kt = 7.93 mM and Vmax = 8.0 mumol.mg-1.30s-1). The Kt was comparable to previously reported values for L-alanine transport in other epithelial cells. Kinetic studies performed in the presence and absence of Na+ suggest L-alanine uptake is mainly mediated by a Na(+)-dependent carrier system, but in addition, a minor diffusional component has been detected. Cross inhibition experiments performed over a wide range of concentrations (1 to 100 mM) suggest that the Na(+)-dependent transport system for alanine resembles system A and displays higher affinity for L-serine (Ki = 1.81 mM) than for L-alanine (K't = 4.86 mM); a lower affinity was found for L-cysteine (Ki = 16.30 mM). Results obtained with MeAIB support the hypothesis that system A is present at the basolateral membrane of the gland cells.

Determination of intrinsic membrane transport parameters from perfused intestine experiments: a boundary layer approach to estimating the aqueous and unbiased membrane permeabilities

A boundary layer approach is developed for estimating the aqueous resistance in a perfused rat intestine experiment. Knowing the aqueous resistance allows the membrane surface concentration to be calculated as a function of the perfusate inlet concentration and perfusional flow rate. Determination of membrane uptake as a function of the membrane surface concentration rather than the perfusate concentration gives the intrinsic, unbiased membrane parameters for the uptake mechanism of Michaelis-Menten-type kinetics in parallel with passive diffusion. The aqueous resistance derived in the analysis is verified by comparison with flux data for 1-leucine and progesterone measured at various flow rates and intestinal lengths. The approach allows for a direct estimate to be made of the unbiased membrane permeability parameters.

Transport of tyrosine and phenylalanine across the rat nasal mucosa

Transport of tyrosine (Tyr) and phenylalanine (Phe) across the rat nasal mucosa was studied using an in situ perfusion technique. It was found that both amino acids were absorbed by active, saturable transport processes. The Km and Vmax values were calculated to be 0.68 mM and 0.44 mM/hr for L-Tyr, and 0.40 mM and 0.39 mM/hr for L-Phe, respectively. The values for L-Tyr agreed well with the results previously reported. When D-Tyr and D-Phe were used as substrates, the extent of nasal absorption was significantly reduced indicating the specific affinity of the carrier for the L-amino acids. When mixtures of L-Tyr and L-Phe were used as perfusates, both amino acids were found to be concomitantly absorbed in a competitive manner. This implied that at least one common carrier system was present in the nasal mucosa. In addition the transport appears to be Na+-dependent and may require metabolic energy as a driving force as seen from the inhibition of the L-Phe uptake by ouabain and 2,4-dinitrophenol.

Characteristics of transmural potential changes associated with the proton-peptide co-transport in toad small intestine

1. Ionic dependence and kinetic properties of the peptide-evoked potentials across everted toad intestine were investigated with eighteen dipeptides and four tripeptides. All peptides evoked saturable increases in the mucosal negativity regardless of the presence of Na+. 2. The peptide-evoked potentials recorded in the absence of Na+ were sensitive to external pH (pHo); lowering pHo from 7.4 to 6.5 and 5.5 caused stepwise increases in their amplitude. 3. Loading epithelial cells with 9-aminoacridine or acetate caused a significant increase or decrease in amplitude of the Gly-Gly-evoked potential, suggesting intracellular alkalinization or acidification also has a great influence on the peptide-evoked potential. 4. Kinetically, Na+-independent peptide-evoked potentials conformed to simple Michaelis-Menten kinetics, and lowering pHo caused a decrease of the half-saturation concentration (Kt) for Gly-Gly without changing the maximum potential difference increase. Similar affinity-type kinetic effect was also seen for Gly-Gly influx. 5. Simultaneous measurements of Gly-Gly-induced increase in short-circuit current and Gly-Gly influx revealed that the coupling ratio of H+ and Gly-Gly flows was 1.78 +/- 0.12, suggesting the stoichiometry of the H+-peptide co-transport being 2:1. 6. Kinetic analyses of the peptide-evoked potentials indicated that all glycyl-dipeptides tested (Gly-Gly, Gly-Pro, Gly-Sar, Gly-Leu, Gly-Phe) and other dipeptides (Ala-Ala, Ala-Phe, Phe-Ala) shared a common carrier. Gly-Gly-Gly and Ala-Ala-Ala were also found to share the same carrier, while Phe-Phe, Leu-Leu and Phe-Leu appeared to be transported by a different carrier. 7. Kt values for di- and tripeptides, which apparently shared a common carrier, fell in a narrow range (0.5-2.2 mM). There was no clear correlation between 1/Kt value and molecular weight.

Transport of branched-chain amino acids in membrane vesicles of Streptococcus cremoris

The kinetics, specificity, and mechanism of branched-chain amino acid transport in Streptococcus cremoris were studied in a membrane system of S. cremoris in which beef heart mitochondrial cytochrome c oxidase was incorporated as a proton motive force (delta p)-generating system. Influx of L-leucine, L-isoleucine, and L-valine can occur via a common transport system which is highly selective for the L-isomers of branched chain amino acids and analogs. The pH dependency of the kinetic constants of delta p-driven L-leucine transport and exchange (counterflow) was determined. The maximal rate of delta p-driven transport of L-leucine (Vmax) increased with increasing internal pH, whereas the affinity constant increased with increasing external pH. The affinity constant for exchange (counterflow) varied in a similar fashion with pH, whereas Vmax was pH independent. Further analysis of the pH dependency of various modes of facilitated diffusion, i.e., efflux, exchange, influx, and counterflow, suggests that H+ and L-leucine binding and release to and from the carrier proceed by an ordered mechanism. A kinetic scheme of the translocation cycle of H+-L-leucine cotransport is suggested.

Kinetics of neutral amino acid transport across the blood-brain barrier

Neutral amino acid (NAA) transport across the blood-brain barrier was examined in pentobarbital-anesthetized rats with an in situ brain perfusion technique. Fourteen of 16 plasma NAAs showed measurable affinity for the cerebrovascular NAA transport system. Values of the transport constants (Vmax, Km, KD) were determined for seven large NAAs from saturation studies, whereas Km values for five small NAAs were estimated from inhibition studies. These data, together with our previous work, provide a complete set of constants for prediction of NAA influx from plasma. Among the NAAs, Vmax varied at least fivefold and Km varied approximately 700 fold. The apparent affinity (1/Km) of each NAA was related linearly (r = 0.910) to the octanol/water partition coefficient, a measure of NAA side-chain hydrophobicity. Predicted influx values from transport constants and average plasma concentrations agree well with values measured using plasma perfusate. These results provide accurate new estimates of the kinetic constants that determine NAA transport across the blood-brain barrier. Furthermore, they suggest that affinity of a L-alpha-amino acid for the transport system is determined primarily by side-chain hydrophobicity.

Transport of the alpha-amino-mono-carboxylic acid L-alanine by the beta-alanine carrier of the rabbit ileum

The proposal that the beta-alanine carrier of the rabbit ileum is a high affinity carrier of the neutral amino acids was examined by means of measurements of influx across the brush border membrane of the intact epithelium using L-alanine as a representative of the neutral amino acids. Confirming the proposal, evidence was provided for mutual competitive inhibition between beta-alanine and L-alanine; and it was also demonstrated that a process contributes to the influx of L-alanine, which is characterized by a maximum rate of transport equal to that of beta-alanine and a Kt, which is equal to the Ki of L-alanine against the influx of beta-alanine. In the concentration range 0.01 to 0.125 mM the influx of L-alanine was found to be linearly related to the concentration indicating a significant unstirred layer influence on present and previous estimates of the Kt values for influx of amino acids across the brush-border membrane of intact intestinal epithelia.

Effect of enzymatic digestion on phenylalanine transport across the small intestine

Phenylalanine accumulation in mucosal strips isolated from rat small intestine was significantly inhibited (P less than 0.01) after preincubation with trypsin, chymotrypsin, phospholipase D and neuraminidase. Unidirectional phenylalanine influx across the small intestine was significantly reduced (P less than 0.01) when the mucosal strips were preincubated with the above mentioned enzymes. Intestinal cell water and volume were not significantly changed (P greater than 0.6) when the intestinal tissues were preincubated with these enzymes.

Electrical properties of a Na+-dependent phenylalanine transport in lizard (Lacerta galloti) duodenum

The unidirectional transepithelial fluxes of L-phenylalanine across lizard duodenum were determined in flux chambers. Phenylalanine was preferentially transferred from the mucosal to the serosal fluid. This transport was accompanied by an accumulation of substrate from the mucosal medium into the tissue to a similar level and against a concentration gradient. There was no net movement of phenylalanine when the sodium was substituted by choline. The influx of L-phenylalanine into the epithelial cells of lizard duodenum was examined by incubating slices of intestine in radioactively-labelled solutions of the substrate for 2 min. The steady-state uptake was assessed after similar incubations lasting 45 min. Phenylalanine influx obeys the Michaelis-Menten equation with a Km of 5.1 and is dependent on the presence of sodium ions in the incubation medium. Phenylalanine has been used to induce changes in short-circuit current (delta Isc) across intestine. delta Isc was a hyperbolic function of amino acid concentration characterized by the parameters Jm (maximum change in delta Isc) and Km (concentration needed to attain an delta Isc equal to half the Jm). delta Isc determined Km constants showed good agreement with values obtained from direct measurements of phenylalanine uptake into tissue.

Inhibitory effects of probenecid on the individual transport routes which mediate the influx and efflux of methotrexate in L1210 cells

L1210 cells contain a single transport system which mediates the influx of methotrexate and at least three routes for drug efflux [G. B. Henderson and E. M. Zevely, J. biol. Chem. 259, 1526 (1984)]; each of these processes is sensitive to probenecid. The influx carrier was inhibited reversibly and completely by probenecid with a Ki of 0.25 mM, while efflux via the same system was relatively unaffected by this compound (50% inhibition above 2.0 mM). The two remaining efflux routes (which do not contribute to methotrexate influx) showed a much higher sensitivity to probenecid. Efflux via these components was reduced half-maximally at probenecid concentrations of 0.08 and 0.22 mM, respectively, and a complete block was achieved with excess amounts (2.0 mM) of the inhibitor. Intracellular levels of ATP, glucose metabolism, and the membrane potential were also reduced by probenecid, indicating that the mechanism for inhibiting methotrexate efflux may involve the ability of probenecid to act as a metabolic inhibitor. Probenecid may have a broad capacity for inhibiting anion transport processes since it also reduced sulfate influx and efflux via the general anion carrier system.

Substrate specificity of the intestinal brush-border proline/sodium (IMINO) transporter

L-proline uptake via the intestinal brush-border IMINO carrier was tested for inhibition by 41 compounds which included sugars, N-methylated, alpha-, beta-, gamma- and epsilon- amino and imino acids, and heterocyclic analogs of pyrrolidine, piperidine and pyridine. Based on competitive inhibitor constants (apparent Ki' 's) we find that the IMINO carrier binding site interacts with molecules which possess a well-defined set of structural prerequisites. The ideal inhibitor must 1) be a heterocyclic nitrogen ring, 2) have a hydrophobic region, 3) be the L-stereoisomer of 4) an electronegative carbonyl group which is 5) separated by a one-carbon atom spacer from 6) an electropositive tetrahedral imino nitrogen with two H atoms. Finally, 7) the inhibitor conformation determined by dynamic ring puckering must position all these features within a critical domain. The two best inhibitors are L-pipecolate (apparent Ki' 0.2 mM) and L-proline (apparent Ki' 0.3 mM).

Transport of imino acids and non-alpha-amino acids across the brush-border membrane of the rabbit ileum

The transport of beta-alanine and MeAIB and their effects as inhibitors of the transport of alanine, leucine and lysine across the brush-border membrane of the intact epithelium from the rabbit's distal ileum has been examined. Two separate transport systems have been characterized: 1) A sodium-dependent, beta-alanine-accepting system, which is a high-affinity transport system for alpha-amino-monocarboxylic acids (neutral a.a.) and for cationic a.a., accepts non-alpha-amino acids as well as non-alpha-imino acids, is moderately stereospecific, and for which the affinity of a neutral a.a. is greatly reduced by N-methylation. 2) A sodium-dependent transport system for imino acids, which is inaccessible to cationic amino acids and non-alpha-amino acids but accepts cyclic, non-alpha-imino acids, is moderately stereospecific, and for which neutral a.a. have much lower affinities than their N-methylated derivatives. On the basis of the observations of this and the preceding paper five transport systems for amino acids are ascribed to the rabbit ileum. Some discrepancies between the present results and those obtained with brush-border membrane microvesicles from the rabbit small intestine are discussed.

Transport of neutral and cationic amino acids across the brush-border membrane of the rabbit ileum

The transport of sugars and amino acids across the brush-border membrane of the distal rabbit ileum has been studied. The kinetics of the transport of glucose demonstrated that the data obtained with the present technique are less distorted by unstirred layers than those obtained with the same technique adapted to the use of magnetic stirring. The role of depolarization of the electrical potential difference across the brush-border membrane in mutual inhibition between different classes of amino acids was estimated by measurements of the effects of high concentrations of alanine and lysine on the transport of galactose. It was found that this role would be insignificant in the present study. By measurements of the transport of alanine, leucine and lysine and the inhibitory interactions between these amino acids the function of three transport systems has been delineated. The transport of lysine is resolved in a high- and a low-affinity contribution. At 140 mM sodium these transport systems may also function as respectively high- and low-affinity contributors to the transport of neutral amino acids. At 0 mM sodium the high-affinity system remains a high-affinity system for cationic and neutral amino acids with reduced capacity especially for the neutral amino acids. At 0 mM sodium the low-affinity system's affinity for lysine is reduced and it is inaccessible to neutral amino acids. In addition to the two systems for lysine transport the existence of a lysine-resistant, sodium-dependent, high-affinity system for the transport of neutral amino acids has been confirmed. It seems unlikely that the distal ileum is equipped with a low-affinity, sodium-independent system for the transport of neutral amino acids.

Imino acid transport across the brush-border membrane of the guinea-pig small intestine

The transport of imino and non-alpha-amino acids across the brush-border membrane of the guinea-pig small intestine has been examined. It was found that the guinea pig is without a transport system for non-alpha-amino acids. The transport of imino acids was characterized using methylaminoisobutyrate (MeAIB) as a substrate. This choice was validated by lack of kinetic evidence that more than one transport system was involved in the transport of MeAIB, by the identical values of the estimates of the passive permeability of MeAIB, the magnitude of its proline-resistant transport, and the permeability of mannitol. The transport system for MeAIB is moderately stereospecific. It does not accept cationic amino acids. It accepts alpha-amino-monocarboxylic acids but N-methylation increases the affinities of these amino acids by an order of magnitude. The length of the side-chain of the aliphatic imino acids seems of little importance for the affinity for the transport system, but the data on inhibition of the transport of MeAIB by proline and piperidine-2-carboxylic acid indicate that it is sharply increased by ring formation.

A comparison of methods for estimating the kinetic parameters of two simple types of transport process

Sets of experimental data, with known characteristics and error structures, have been simulated for the Michaelis-Menten equation plus a second term, either for linear transport or for competitive inhibition. The Michaelis-Menten equation plus linear term was fitted by several methods and the accuracy and the precision of the parameter estimates from the several methods were compared. The model-fitting methods were: three for least-squares non-linear regression, computer versions of two graphical methods and of two non-parametric methods. The most precise and accurate method was that of D.W. Marquardt (J. Soc. Ind. Appl. Math. 11 (1963) 431--441). The Michaelis-Menten equation with competitive inhibition was also fitted by several methods, viz., two for least-squared non-linear regression, non-parametric method and four variants of the Preston-Schaeffer-Curran plot (Preston, R.L. et al. (1974) J. Gen. Physiol. 64, 443--467). The most precise and accurate of these was the non-linear regression method of W.W. Cleland (Adv. Enzymol. 29 (1967) 1--32). For both these models, the various graphical methods and non-parametric methods gave poor results and are not recommended.

Multiple transport pathways for neutral amino acids in rabbit jejunal brush border vesicles

Amino acids enter rabbit jejunal brush border membrane vesicles via three major transport systems: (1) simple passive diffusion; (2) Na-independent carriers; and (3) Na-dependent carriers. The passive permeability sequence of amino acids is very similar to that observed in other studies involving natural and artificial membranes. Based on uptake kinetics and cross-inhibition profiles, at least two Na-independent and three Na-dependent carrier-mediated pathways exist. One Na-independent pathway, similar to the classical L system, favors neutral amino acids, while the other pathway favors dibasic amino acids such as lysine. One Na-dependent pathway primarily serves neutral L-amino acids including 2-amino-2-norbornanecarboxylic acid hemihydrate (BCH), but not beta-alanine or alpha-methylaminoisobutyric acid (MeAIB). Another Na-dependent route favors phenylalanine and methionine, while the third pathway is selective for imino acids and MeAIB. Li is unable to substitute for Na in these systems. Cross-inhibition profiles indicated that none of the Na-dependent systems conform to classical A or ACS paradigms. Other notable features of jejunal brush border vesicles include (1) no beta-alanine carrier, and (2) no major proline/glycine interactions.

Effects of unstirred layers on the kinetics of carrier-mediated solute transport by two systems

Equations describing the effect of unstirred layers on the simultaneous transport of a solute via two mediated systems have been derived. It has been previously established that unstirred layers cause convex curvature of Eadie-Hofstee transformations of kinetic data for single mediated transport systems. On the other hand, multiple transport systems produce concave curvature in this plot. A numerical example is presented which illustrates the fact that under some circumstances the opposing effects can nearly cancel out, producing an erroneous apparently linear relation. Furthermore, statistical analysis of these curves suggests that the normal error encountered in physiological transport measurements can readily obscure evidence of unstirred layers. It is suggested that kinetic analysis of transport, particularly in epithelial tissues, should whenever possible, include independent measurements of unstirred layers, employ a broad range of substrate concentrations, and vary the stirring rate of the medium. Evaluation of the 'true' kinetic constants can then be made with the relations derived here. In addition, several limiting cases have been discussed which reduce to simpler kinetic descriptions published earlier.

Single-site uptake of neutral amino acids into guinea-pig intestinal rings

1. A kinetic analysis of amino acid influx into guinea-pig small intestinal rings has been performed in an attempt to ascertain whether one or more transport sites for these substrates exists in the luminal membrane of the enterocyte. 2. No indirect correction for uptake into the extracellular space was applied, but it was assumed in the analysis that the total uptake included a diffusion term. This procedure was vindicated by the results obtained. 3. All analyses were performed by non-linear regression techniques. In many experiments, both substrate and inhibitor concentrations were varied within the same experiment, thus giving rise to three-dimensional diagrams describing transport processes. 4. All results pointed to the existence of a single transport agency shared by all amino acids tested. The kinetic constants, Km and Vmax, were independent of the concentration range used for their estimation; such behaviour would not be expected if several sites with different kinetic constants were available for transport. The value of KD, the constant describing the diffusive component of uptake, was the same when estimated from the uptake of an amino acid alone or when determined as the asymptote of the curve describing the inhibition of this uptake by an analogue. Finally, the Ki for an amino acid when used as an inhibitor was identical to its Km when used as substrate. This property was maintained even when the most disparate pair of amino acids, threonine and isoleucine, was examined.

Electrogenic responses induced by neutral amino acids in endoderm cells from Xenopus embryo

1. Membrane potential measurements were carried out on endoderm cells from early Xenopus embryos in order to study neutral amino acid transport in non-excitable cells. 2. The electrical properties of the cell membrane were studied under normal conditions, then in the presence of various Na/K-pump inhibitors and at different Na, K and Cl concentrations in Ringer solution. Blockade of the Na/K-pump by ouabain, Li, cooling to 10 degrees C or low [Na]0 induces similar depolarizations of about 40 mV. 3. External application of various neutral L-amino acids induces reversible membrane depolarizations. The D-isomeric forms are found to be ineffective. The amino acid induced depolarizations are not accompanied by changes in membrane resistance. They do not show voltage dependence for potential changes of less than 40 mV. 4. The amino acid depolarization increases with increasing concentration and follows first order Michaëlian kinetics. Both the size and the time course of the amino acid depolarization depend on [Na]0. Increasing [Na]0 markedly increases the apparent affinity of the membrane receptor for amino acid. 5. Increasing [k]0 reduces the size of the amino acid response. Short exposures to either ouabain or Li do not alter the amino acid depolarization. However, p time course of the amino acid depolarization depend on [Na]0. Increasing [Na]0 markedly increases the apparent affinity of the membrane receptor for amino acid. 5. Increasing [k]0 reduces the size of the amino acid response. Short exposures to either ouabain or Li do not alter the amino acid depolarization. However, p time course of the amino acid depolarization depend on [Na]0. Increasing [Na]0 markedly increases the apparent affinity of the membrane receptor for amino acid. 5. Increasing [k]0 reduces the size of the amino acid response. Short exposures to either ouabain or Li do not alter the amino acid depolarization. However, prolonged exposure to pump inhibitors or marked alteration of the Na concentration gradient leads to a complete inhibition of amino acid responses. 6. The results are in good agreement with the notion that the amino acid induced responses reflect the activation of an electrogenic amino acid carrier, very likely co-transporting Na and amino acid.