Sodium-coupled sugar and amino acid transport in an acidic microenvironment

1. Nutrient transport mechanisms of lobster hepatopancreatic epithelial brush border membrane vesicles (BBMV) are strongly influenced by the acidic nature of the tubular lumen. 2. Sodium-dependent glucose uptake by BBMV was electrogenic and was stimulated at low pH by reducing sugar transport Ki, without affecting JM. 3. Glutamate was largely transported in zwitterionic form at pH 4.0 by an electrically silent cotransport mechanism with both Na and Cl. 4. Increased H+ concentration tripled the apparent membrane permeability to glutamate as well as the amino acid transport JM. 5. At pH 4.0 leucine was transported as a cation by two dissimilar carrier systems: a Na-independent process shared by polar amino acids, and an electroneutral Na-2Cl-dependent mechanism shared with non-polar amino acids. 6. A model is proposed for hepatopancreatic BBMV at acidic pH which employs ionic chemical gradients and membrane potential as nutrient transport driving forces.     

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.     

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

Summary 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 neutrall-amino acids including 2-amino-2-norbornanecarboxylic acid hemihydrate (BCH), but not -alanine or -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 -alanine carrier, and (2) no major proline/glycine interactions.     

Cation-dependent nutrient transport in shrimp digestive tract

Purified epithelial brush border membrane vesicles (BBMV) were produced from the hepatopancreas of the Atlantic White shrimp, Litopeneaus setiferus, using standard methods originally developed for mammalian tissues and previously applied to other crustacean and echinoderm epithelia. These vesicles were used to study the cation dependency of sugar and amino acid transport across luminal membranes of hepatopancreatic epithelial cells. ³H-d-glucose uptake by BBMV against transient sugar concentration gradients occurred when either transmembrane sodium or potassium gradients were the only driving forces for sugar accumulation, suggesting the presence of a possible coupled transport system capable of using either cation. ³H-l-histidine transport was only stimulated by a transmembrane potassium gradient, while ³H-l-leucine uptake was enhanced by either a sodium or potassium gradient. These responses suggest the possible presence of a potassium-dependent transporter that accommodates either amino acid and a sodium-dependent system restricted only to l-leucine. Uptake of ³H-l-leucine was significantly stimulated (P < 0.05) by several metallic cations (e.g., Zn²⁺, Cu²⁺, Mn²⁺, Cd²⁺, or Co²⁺) at external pH values of 7.0 or 5.0 (internal pH 7.0), suggesting a potential synergistic role of the cations in the transmembrane transfer of amino acids. ³H-l-histidine influxes (15 suptakes) were hyperbolic functions of external [zinc] or [manganese], following Michaelis–Menten kinetics. The apparent affinity constant (e.g., K _m) for manganese was an order of magnitude smaller (K _m = 0.22 μM Mn) than that for zinc (K _m = 1.80 μM Zn), while no significant difference (P > 0.05) occurred between their maximal transport velocities (e.g., J _max). These results suggest that a number of cation-dependent nutrient transport systems occur on the shrimp brush border membrane and aid in the absorption of these important dietary elements.     

Variable H+/substrate stoicheiometries in Rhodotorula gracilis are caused by a pH-dependent protonation of the carrier(s).

Two carrier-mediated systems transport sugars in the yeast Rhodotorula gracilis depending on the pH. One system, with higher affinity for sugars, catalyses a symport of protons with sugar, whereas the other system, having lower affinity, is independent of protons. This was shown in three different ways. (1) At low pH, where only the high-affinity system works, a H+/sugar stoicheiometry of 1 was found. An increase of the pH and of the sugar concentration, which allowed the low-affinity system to operate, brought about a drop of the stoicheiometry to values below 1. (2) During H+ symport the influx of positive charge was electrically compensated by an equivalent efflux of K+ from the cells. At high pH and high sugar concentrations this stoicheiometry of K+ and sugar decreased concomitant with the H+/sugar stoicheiometry. (3) At pH 7.5 both transport systems were operating, as shown by biphasic saturation kinetics. Under these conditions only the high-affinity transport was found to be electrogenic. These results agree with the theory of an electrogenic H+/sugar symport where changes in the affinity for substrate are brought about by reversible protonation and deprotonation of the carrier.     

Maturation of membrane function: transport of amino acid by rat erythroid cells.

The membrane changes which occur during cellular maturation of erythroid cells have been investigated. The transport of alpha-aminoisobutyric acid, alanine, and N-methylated-alpha-aminoisobutyric acid have been studied in the erythroblastic leukemic cell, the reticulocyte, and the erythrocyte of the Long-Evans rat. The dependence of amino acid transport on extracellular sodium concentration was investigated. Erythrocytes were found to transport these amino acids only by Na-independent systems. The steady state distribution ratio was less than 1. Reticulocytes were found to transport alpha-aminoisobutyric acid and alanine by Na-dependent systems, but only small amounts of N-methylated-alpha-aminoisobutyric acid. Small amounts of these amino acids were transported by Na-independent systems. The steady state distribution ratio was greater than one for Na-dependent transport. The erythroblastic leukemia cell, a model immature erythroid cell, showed marked Na-dependence (greater than 90%) for alpha-aminoisobutyric acid and alanine transport, and greater than 80% for the Na-dependent transport of N-methyl-alpha-aminoisobutyric acid. The steady state distribution ratio for the Na-dependent transport was greater than 4. In the erythroblastic leukemic cell, at least three Na-dependent systems are present: one includes alanine and alpha-aminoisobutyric acid, but excludes N-methyl-alpha-aminoisobutyric acid; one is for alpha-aminoisobutyric acid, alanine and also N-methyl-alpha-aminoisobutyric acid; and one is for N-methyl-alpha-aminoisobutyric acid alone. In the reticulocyte, the number of Na-dependent systems are reduced to two: one for alpha-aminoisobutyric acid and alanine; one for N-methyl-alpha-aminoisobutyric acid. In the erythrocytes, no Na-dependent transport was found. Therefore, maturation of the blast cell to the mature erythrocyte is characterized by a systematic loss in the specificity and number of transport system for amino acids.     

Effect of functional loading on the operation of the active transport system for organic acids in the proximal kidney tubules of rats after unilateral nephrectomy and in the early postnatal period

The influence of a prolonged introduction of exogenic organic acid penicillin (that is functional loading) on the level of accumulation of an anionic dye (fluorescein) in renal proxima tubules was studied after unilateral nephrectomy and early postnatal period. Injection of penicillin 2 days after unilateral nephrectomy slowly increased Na-independent and strongly increased Na-dependent component of active fluorescein transport in renal proximal tubules of randombred, but strongly decreased both Na-independent and Na-dependent transport in renal tubules of the Campbell rats. When newborn random-bred, Wistar and Campbell rats were pretreated with penicillin, we obtained a slow increase in Na-independent and a strong increase in Na-dependent component of fluorescein transport in renal tubules of random-bred and Wistar rats, but a significant reduction in both Na-independent and Na-dependent transport. It is concluded that the ability for adaptive (or substrate) stimulation of active transport of organic anion in renal proximal tubules is controlled genetically. Adaptive stimulation of organic acid transport in renal tubules referred to in literature as "carried induction", was accomplished apparently by the increase in driving force of the active transport, that is evidently the level of electrochemical Na+-gradient.     

Maturation of membrane function: Transport of amino acid by rat erythroid cells

The membrane changes which occur during cellular maturation of erythroid cells have been investigated. The transport of α-aminoisobutyric acid, alanine, and N-methylated-α-aminoisobutyric acid have been studied in the erythroblastic leukemic cell, the reticulocyte, and the erythrocyte of the Long-Evans rat. The dependence of amino acid transport on extracellular sodium concentration was investigated. Erythrocytes were found to transport these amino acids only by Na-independent systems. The steady state distribution ratio was less than 1. Reticulocytes were found to transport α-aminoisobutyric acid and alanine by Na-dependent systems, but only small amounts of N-methylated-α-aminoisobutyric acid. Small amounts of these amino acids were transported by Na-independent systems. The steady state distribution ratio was greater than one for Na-dependent transport. The erythroblastic leukemia cell, a model immature erythroid cell, showed marked Na-dependence (>90%) for α-aminoisobutyric acid and alanine transport, and >80% for the Na-dependent transport of N-methyl-α-aminoisobutyric acid. The steady state distribution ratio for the Na-dependent transport was >4. In the erythroblastic leukemic cell, at least three Na-dependent systems are present: one includes alanine and α-aminoisobutyric acid, but excludes N-methyl-α-aminoisobutyric acid; one is for α-aminoisobutyric acid, alanine and also N-methyl-α-aminoisobutyric acid; and one is for N-methyl-α-aminoisobutyric acid alone. In the reticulocyte, the number of Na-dependent systems are reduced to two: one for α-aminoisobutyric acid and alanine; one for N-methyl-α-aminoisobutyric acid. In the erythrocytes, no Na-dependent transport was found. Therefore, maturation of the blast cell to the mature erythrocyte is characterized by a systematic loss in the specificity and number of transport systems for amino acids.     

Characterization of a novel variant of amino acid transport system asc in erythrocytes from Przewalski's horse (Equus przewalskii).

In thoroughbred horses, red blood cell amino acid transport activity is Na(+)-independent and controlled by three codominant genetic alleles (h, l, s), coding for high-affinity system asc1 (L-alanine apparent Km for influx at 37 degrees C congruent to 0.35 mM), low-affinity system asc2 (L-alanine Km congruent to 14 mM), and transport deficiency, respectively. The present study investigated amino acid transport mechanisms in red cells from four wild species: Przewalski's horse (Equus przewalskii), Hartmann's zebra (Zebra hartmannae), Grevy's zebra (Zebra grevyi), and onager (Equus hemonius). Red blood cell samples from different Przewalski's horses exhibited uniformly high rates of L-alanine uptake, mediated by a high-affinity asc1-type transport system. Mean apparent Km and Vmax values (+/- SE) for L-alanine influx at 37 degrees C in red cells from 10 individual animals were 0.373 +/- 0.068 mM and 2.27 +/- 0.11 mmol (L cells.h), respectively. As in thoroughbreds, the Przewalski's horse transporter interacted with dibasic as well as neutral amino acids. However, the Przewalski asc1 isoform transported L-lysine with a substantially (6.4-fold) higher apparent affinity than its thoroughbred counterpart (Km for influx 1.4 mM at 37 degrees C) and was also less prone to trans-stimulation effects. The novel high apparent affinity of the Przewalski's horse transporter for L-lysine provides additional key evidence of functional and possible structural similarities between asc and the classical Na(+)-dependent system ASC and between these systems and the Na(+)-independent dibasic amino acid transport system y+. Unlike Przewalski's horse, zebra red cells were polymorphic with respect to L-alanine transport activity, showing high-affinity or low-affinity saturable mechanisms of L-alanine uptake. Onager red cells transported this amino acid with intermediate affinity (apparent Km for influx 3.0 mM at 37 degrees C). Radiation inactivation analysis was used to estimate the target size of system asc in red cells from Przewalski's horse. The transporter's in situ apparent molecular weight was 158,000 +/- 2500 (SE).     

Dibasic amino acid interactions with Na+-independent transport system asc in horse erythrocytes. Kinetic evidence of functional and structural homology with Na+-dependent system ASC

Amino acid transport in horse erythrocytes is regulated by three co-dominant allelomorphic genes coding for high-affinity transport activity (system asc1), low-affinity transport activity (system asc2) and transport-deficiency, respectively. The asc systems are selective for neutral amino acids of intermediate size, but unlike conventional system ASC, do not require Na+ for activity. In the present series of experiments we have used a combined kinetic and genetic approach to establish that dibasic amino acids are also asc substrates, systems asc1 and asc2 representing the only mediated routes of cationic amino acid transport in horse erythrocytes. Both transporters were found to exhibit a strong preference for dibasic amino acids compared with neutral amino acids of similar size. Apparent Km values (mM) for influx via system asc1 were L-lysine (9), L-ornithine (27), L-arginine (27), L-alanine (0.35). Corresponding Vmax estimates (mmol/l cells per h, 37 degrees C) were L-lysine (1.65), L-ornithine (2.15), L-arginine (0.54), L-alanine (1.69). Apparent Km values for L-lysine and L-ornithine influx via system asc2 were approximately 90 and greater than 100 mM, respectively, with Vmax values greater than 2 and greater than 1 mmol/l cells per h, respectively. Apparent Km and Vmax values for L-alanine uptake by system asc2 were 14 mM and 6.90 mmol/l cells per h. In contrast, L-arginine was transported by system asc2 with the same apparent Km as L-alanine (14 mM), but with a 77-fold lower Vmax. This dibasic amino acid was shown to cause cis- and trans-inhibition of system asc2 in a manner analogous to its interaction with system ASC, where the side-chain guanidinium group is considered to occupy the Na+-binding site on the transporter. Concentrations of extracellular L-arginine causing 50% inhibition of zero-trans L-alanine influx and half-maximum inhibition of L-alanine zero-trans efflux were 14 mM (extracellular L-alanine concentration 15 mM) and 3 mM (intracellular L-alanine concentration 15.5 mM), respectively. We interpret these observations as evidence of structural homology between the horse erythrocyte asc transporters and system ASC. Physiologically, intracellular L-arginine may function as an endogenous inhibitor of system asc2 activity.     

The effects of dietary phosphorus deficiency on surface pH and membrane composition of the mucosa epithelium in caprine jejunum

In ruminants, the uptake of inorganic phosphate (P_i) across the intestinal mucosa epithelium by Na-dependent and Na-independent mechanisms is a main regulatory factor in P homeostasis. The aim of the study was to elucidate to which extent Na-independent mechanisms, including pH effects or composition of mucosal brush-border membranes, could be involved in positive stimulation of P_i absorptive processes seen under the P deficient condition. Therefore, luminal, surface and intracellular pH of the jejunal epithelial cells in control and P depleted goats were compared and biochemical analyses of membrane phospholipids in the apical membrane of the jejunal epithelium were performed. Dietary P depletion resulted in decreased plasma P_i levels. While pH in jejunal ingesta was not significantly changed, P depletion resulted in a significantly lower surface pH in the crypt region compared to control animals (7.62 ± 0.02 vs. 7.77 ± 0.04, n = 4, P < 0.01). Inhibition of apical Na⁺/H⁺-exchange resulted in an increase of the jejunal surface pH in P depleted animals by 0.07 ± 0.01 (n = 6, P < 0.01) and 0.05 ± 0.01 (n = 6, P < 0.01) for the villus and the crypt region, respectively. This increase were inversely correlated with the initial surface pH prior to inhibition. In contrast to surface pH, intracellular pH of the jejunal epithelium and the phospholipid composition of the apical jejunal membrane were not affected by P depletion. Although the data suggest the existence of a Na⁺/H⁺-exchange mechanism at the luminal surface of goat jejunum they do not support the hypothesis that adaptational processes of active P_i absorption from goat jejunum in response to low dietary P could be based on “non P_i transporter events”.     

Functional and molecular characteristics of system L in human breast cancer cells

The functional and molecular properties of system L in human mammary cancer cells (MDA-MB-231 and MCF-7) have been examined. All transport experiments were conducted under Na(+)-free conditions. alpha-Aminoisobutyric acid (AIB) uptake by MDA-MB-231 and MCF-7 cells was almost abolished by BCH (2-amino-2-norbornane-carboxylic acid). AIB uptake by MDA-MB-231 cells was also inhibited by L-alanine (83.6%), L-lysine (75.6%) but not by L-proline. Similarly, L-lysine and L-alanine, respectively, reduced AIB influx into MCF-7 cells by 45.3% and 63.7%. The K(m) of AIB uptake into MDA-MB-231 and MCF-7 cells was, respectively, 1.6 and 8.8 mM, whereas the V(max) was, respectively, 9.7 and 110.0 nmol/mg protein/10 min. AIB efflux from MDA-MB-231 and MCF-7 cells was trans-stimulated by BCH, L-glutamine, L-alanine, L-leucine, L-lysine and AIB (all at 2 mM). In contrast, L-glutamate, L-proline, L-arginine and MeAIB had no effect. The interaction between L-lysine and AIB efflux was one of low affinity. The fractional release of AIB from MDA-MB-231 cells was trans-accelerated by D-leucine and D-tryptophan but not by D-alanine. MDA-MB-231 and MCF-7 cells expressed LAT1 and CD98 mRNA. MCF-7 cells also expressed LAT2 mRNA. The results suggest that AIB transport in mammary cancer cells under Na(+)-free conditions is predominantly via system L which acts as an exchange mechanism. The differences in the kinetics of AIB transport between MDA-MB-231 and MCF-7 cells may be due to the differential expression of LAT2.     

Folate Transport by Prawn Hepatopancreas Brush-Border Membrane Vesicles

The transport system of folic acid (Pte-Glu) by brush-border membrane vesicles (BBMV) isolated from prawn (Penaeus japonicm) hepatopancreas, was studied by measuring the uptake of Pte-Glu. This uptake was found to have two components, intravesicular transport and membrane binding. Membrane binding was not affected by the presence of a transmembrane pH-gradient at a short incubation period. However, a transmembrane pH-gradient increased membrane binding at 60 min. The transport of Pte-Glu appeared to be carrier-mediated, was stimulated by an inwardly proton gradient (pH 5.5 outside, 7.4 inside) and was unaffected by a sodium-gradient. The relationship between pH gradient-driven Pte-Glu uptake and medium Pte-Glu concentration followed saturating Michaelis–Menten kinetics. Eadie–Hofstee representation of the pH gradient-driven Pte-Glu uptake indicated a single transport system with a Km of 0.37 M and Vmax of 1.06 pmol/mg protein/15 s. These findings indicate that BBMV isolated from prawn hepatopancreas possesses a Pte-Glu transport system similar to that described in mammalian intestine.     

Sodium ion-dependent amino acid transport in membrane vesicles of Bacillus stearothermophilus.

Amino acid transport in membrane vesicles of Bacillus stearothermophilus was studied. A relatively high concentration of sodium ions is needed for uptake of L-alanine (Kt = 1.0 mM) and L-leucine (Kt = 0.4 mM). In contrast, the Na(+)-H(+)-L-glutamate transport system has a high affinity for sodium ions (Kt less than 5.5 microM). Lithium ions, but no other cations tested, can replace sodium ions in neutral amino acid transport. The stimulatory effect of monensin on the steady-state accumulation level of these amino acids and the absence of transport in the presence of nonactin indicate that these amino acids are translocated by a Na+ symport mechanism. This is confirmed by the observation that an artificial delta psi and delta mu Na+/F but not a delta pH can act as a driving force for uptake. The transport system for L-alanine is rather specific. L-Serine, but not L-glycine or other amino acids tested, was found to be a competitive inhibitor of L-alanine uptake. On the other hand, the transport carrier for L-leucine also translocates the amino acids L-isoleucine and L-valine. The initial rates of L-glutamate and L-alanine uptake are strongly dependent on the medium pH. The uptake rates of both amino acids are highest at low external pH (5.5 to 6.0) and decline with increasing pH. The pH allosterically affects the L-glutamate and L-alanine transport systems. The maximal rate of L-glutamate uptake (Vmax) is independent of the external pH between pH 5.5 and 8.5, whereas the affinity constant (Kt) increases with increasing pH. A specific transport system for the basic amino acids L-lysine and L-arginine in the membrane vesicles has also been observed. Transport of these amino acids occurs most likely by a uniport mechanism.     

Effects of Na+, H+, and Cl− on alanine transport by lobster hepatopancreatic brush border membrane vesicles

Summary Epithelial brush border membrane vesicles (BBMV) of lobster hepatopancreas were formed by a magnesium precipitation technique previously described (Ahearn et al. 1985).³H-l-alanine transport by these vesicles was sodium and potassium insensitive, in contrast to a strong Na-dependency exhibited by³H-d-glucose transport. Initial alanine entry rates (15 s uptake) were stimulated and transient alanine uptake overshoots were observed when external pH was acidic (e. g. pH 4.0, 5.0 or 6.0) and a Cl gradient was imposed across the vesicular wall; at pH_o=7.4 alanine uptake was reduced in rate and hyperbolic in character. Alanine uptake from an acidic extravesicular environment in the absence of Cl responded to a transmembrane electrical potential difference created by an outwardly-directed, valinomycin-induced, potassium diffusion potential, suggesting that the alanine molecule alone carried sufficient charge under these conditions to respond to the electrical gradient. External 5.0 mMl-lysine andl-serine similarly inhibited the influx and overshoot properties of 0.05 mM³H-l-alanine uptake, whereas 5.0 mMl-leucine had virtually no effect. Trans-stimulation of alanine initial uptake rates and an enhancement of alanine accumulation against a concentration gradient were observed by vesicles preloaded with 1 mMl-lysine, but not by vesicles lacking amino acids or those containing 1 mMl-leucine orl-serine.³H-l-alanine influx from acidic external environments in the presence of a Cl gradient occurred by a combination of carrier-mediated transfer and apparent diffusion. Decreasing pH_o from 6.0 to 4.0 elevated alanineK _t from 0.55 to 2.64 mM, while alanineJ _M increased from 55 to 550 pmol/mg protein· 15 s. Apparent diffusional permeability of the membranes to alanine under these conditions increased slightly. These results suggest, but do not conclusively prove, that alanine transport across BBMV of lobster hepatopancreas may occur by way of a classical y+ transprot protein at acidic pH. The extent of this transport is determined by the magnitude of the transmembrane chloride gradient which serves as a powerful driving force for cationic amino acids in this tissue.     

Effects of monovalent cations on the sodium-alanine interaction in rabbit ileum. Implication of anionic groups in sodium binding

H, K, Rb, and Li inhibit Na-dependent alanine influx across the brush border of rabbit ileum. Kinetic analysis indicates that H and K behave as competitive inhibitors of influx so that increasing the concentration of H or K in the mucosal solution is kinetically indistinguishable from decreasing the Na concentration. In addition the coupling between alanine and Na influxes is markedly reduced at pH 2.5. With the exception of H and Li, none of these monovalent cations significantly affects carrier-mediated alanine influx in the absence of Na indicating that their inhibitory effects are largely restricted to the Na-dependent fraction of influx. Increasing H concentration from 0.03 to 3 mM does not affect influx in the absence of Na but markedly inhibits influx in the presence of Na. Li significantly enhances alanine influx in the absence of Na. Ag, UO₂, and La also inhibit the Na-dependent fraction of alanine influx. These findings suggest that anionic groups having a pK_a of approximately 4 are involved in the interaction between Na and the alanine-carrier complex; present evidence implicates carboxylate groups however, phosphoryl residues cannot be ruled out. The previously proposed kinetic model for the Na-alanine interaction has been extended to accommodate these effects of H and other monovalent cations. The mechanistic and physiological implications of these findings are discussed.     

Kinetics of Myo-inositol transport in rat ocular lens

Myo-inositol (MI) influx as a function of concentration in rat lens consisted of a saturable component, fit by a rectangular hyperbola, and a linear component which was more distinct at high myo-inositol concentrations suggesting passive diffusion. The hyperbolic component was half-maximally saturated (K_t) at 61.3 μM and had a maximal transport rate (J_max) of 44.6 μMol/kg wet wt/h. The linear component had an apparent permeability coefficient of 1.44 × 10^?6 s^?1. Sorbitol, which distributed rapidly in the extracellular space (6.83 ml/100 g wet wt), also appeared to enter the intracellular space with a permeability coefficient of 1.37 × 10^?6 s^?1, similar to that of myo-inositol. The influx of myo-inositol was critically dependent on the concentration of extracellular sodium consistent with a sodium-myo-inositol contransport. The kinetics of influx activation by sodium suggested an apparent 2:1 coupling ratio for sodium and myo-inositol. When potassium was used as sodium substitute, a significantly stronger influx inhibition was observed than with nondepolarizing sodium substitutes, indicating that myoinositol was driven by the electrochemicl gradient of sodium rather than the chemical gradient only. Reducing the extracellular Na concentration increased the MI concentration at which transport was half-maximally activated, suggesting an ordered binding sequence of Na followed by MI. Myo-inositol influx was competitively inhibited by phlorizin with an inhibitory coefficient (K_i) of 35 μM. Phloretin also was capable of inhibition but with a much lesser efficacy. Myoinositol desaturates from the lens at a rate of 0.00862 h^?1. Approximately 19% of the efflux can be inhibited with phlorizin, suggesting that it represents carrier-mediated flux. The phlorizin insensitive flux has a rate of 0.00695 h^?1 or 1.93 × 10^?6 s^?1, similar to the Na-independent passive influx. MI influx is due to a Na-dependent, phlorizin-sensitive active transport while the efflux consists largely of a phlorizin-independent passive leakage. © 1995 Wiley-Liss, Inc.     

A cation counterflux supports lysosomal acidification

The profound luminal acidification essential for the degradative function of lysosomes requires a counter-ion flux to dissipate an opposing voltage that would prohibit proton accumulation. It has generally been assumed that a parallel anion influx is the main or only counter-ion transport that enables acidification. Indeed, defective anion conductance has been suggested as the mechanism underlying attenuated lysosome acidification in cells deficient in CFTR or ClC-7. To assess the individual contribution of counter-ions to acidification, we devised means of reversibly and separately permeabilizing the plasma and lysosomal membranes to dialyze the cytosol and lysosome lumen in intact cells, while ratiometrically monitoring lysosomal pH. Replacement of cytosolic Cl⁻ with impermeant anions did not significantly alter proton pumping, while the presence of permeant cations in the lysosomal lumen supported acidification. Accordingly, the lysosomes were found to acidify to the same pH in both CFTR- and ClC-7–deficient cells. We conclude that cations, in addition to chloride, can support lysosomal acidification and defects in lysosomal anion conductance cannot explain the impaired microbicidal capacity of CF phagocytes.     

Lysine and alanine transport in the perfused guinea-pig placenta

The characteristics of L-lysine transport were investigated at brush-border (maternal) and basal (fetal) sides of the syncytiotrophoblast in the term guinea-pig placenta artificially perfused either through the umbilical vessels in situ or through both circulations simultaneously. Cellular uptake, efflux and transplacental transfer were determined using a single-circulation paired-tracer dilution technique. Unidirectional L-[3H]lysine uptake (%) (perfusate lysine 50 microM) was high on maternal (M = 87 +/- 1) and fetal (F = 73 +/- 2) sides. L-[3H]Lysine efflux back into the ipsilateral circulation was asymmetrical (F/M ratio = 2.3) and transplacental flux occurred in favour of the fetal circulation. Unidirectional lysine influx kinetics (0.05-8.00 mM) gave Km values of 1.75 +/- 0.70 mM and 0.90 +/- 0.25 mM at maternal and fetal sides, respectively; corresponding Vmax values were 1.95 +/- 0.38 and 0.87 +/- 0.10 mumol.min-1.g-1. At both sides, lysine influx (50 microM) could be inhibited (about 60-80%) by 4 mM L-lysine and L-ornithine and less effectively (about 10-40%) by L-citrulline, L-arginine, D-lysine and L-histidine. At the basal side: (i) lysine influx kinetics were greatly modified in the presence of 10 mM L-alanine (Km = 6.25 +/- 3.27 mM; Vmax = 2.62 +/- 0.94 mumol.min-1.g-1), but unchanged by equimolar L-phenylalanine or L-tryptophan; (ii) in the converse experiments, lysine (10 mM) did not affect the kinetic characteristics for either L-alanine or L-phenylalanine; (iii) L-lysine and L-alanine influx kinetics were not dependent on the sodium gradient; (iv) the inhibition of L-[3H]lysine uptake by 4 mM L-homoserine was partially (60%) Na+-dependent. At the maternal side the kinetic characteristics for alanine influx were highly Na+-dependent, while lysine influx was partially Na+-dependent only at low concentrations (0.05-0.5 mM). Bilateral perfusion with 2,4-dinitrophenol (1 mM) reduced L-[3H]lysine uptake into the trophoblast and abolished transplacental transfer. It is suggested that lysine transport in the guinea-pig placenta is mediated by a specific transport system (y+) for cationic amino-acids. The asymmetry in the degree of sodium-dependency at both trophoblast membranes may in part explain the maternal-to-foetal polarity of placental amino-acid transfer in vivo.     

Transport of glycyl-L-proline in intestinal brush-border membrane vesicles of the suckling rat: characteristics and maturation

Transport of the dipeptide glycine-L-proline (Gly-L-Pro) in the developing intestine of suckling rats and its subsequent maturation in adult rats was examined using the brush-border membrane vesicles (BBMV) technique. Uptake of Gly-L-Pro by BBMV was mainly the result of transport into the intravesicular space with little binding to membrane surfaces. Transport of Gly-L-Pro in BBMV of suckling rats was: (1) Na+ independent; (2) pH dependent with maximum uptake at an incubation buffer pH of 5.0; (3) saturable as a function of concentration (apparent Km = 21.5 +/- 7.9 mM, Vmax = 8.6 +/- 1.5 nmol/mg protein per 10 s); (4) inhibited by other di- and tripeptides; and (5) stimulated and inhibited by inducing a negative and positive intravesicular membrane electrical potential, respectively. Similarly, transport of Gly-L-Pro in intestinal BBMV of adult rats was saturable as a function of concentration (apparent Km = 17.4 +/- 8.6 mM, Vmax = 9.1 +/- 2.1 nmol/mg protein per 10 s) and was stimulated and inhibited by inducing a relatively negative and positive intravesicular membrane potential, respectively. No difference in the transport kinetic parameters of Gly-L-Pro was observed in suckling and adult rats, indicating a similar activity (and/or number) and affinity of the transport carrier in the two age groups. These results demonstrate that the transport of Gly-L-Pro is by a carrier-mediated process which is fully developed at the suckling period. Furthermore, the process is H+-dependent but not Na+-dependent, electrogenic and most probably occurs by a Gly-L-Pro/H+ cotransport mechanism.