L-lysine Transport in Chicken Jejunal Brush Border Membrane Vesicles

The properties of l-lysine transport in chicken jejunum have been studied in brush border membrane vesicles isolated from 6-wk-old birds. l-lysine uptake was found to occur within an osmotically active space with significant binding to the membrane. The vesicles can accumulate l-lysine against a concentration gradient, by a membrane potential-sensitive mechanism. The kinetics of l-lysine transport were described by two saturable processes: first, a high affinity-transport system (K _mA= 2.4 ± 0.7 μmol/L) which recognizes cationic and also neutral amino acids with similar affinity in the presence or absence of Na⁺ (l-methionine inhibition constant K_iA, NaSCN = 21.0 ± 8.7 μmol/L and KSCN = 55.0 ± 8.4 μmol/L); second, a low-affinity transport mechanism (K_mB= 164.0 ± 13.0 μmol/L) which also recognizes neutral amino acids. This latter system shows a higher affinity in the presence of Na⁺ (K_iB for l-methionine, NaSCN = 1.7 ± 0.3 and KSCN = 3.4 ± 0.9 mmol/L). l-lysine influx was significantly reduced with N-ethylmaleimide (0.5 mmol/L) treatment. Accelerative exchange of extravesicular labeled l-lysine was demonstrated in vesicles preloaded with 1 mmol/L l-lysine, l-arginine or l-methionine. Results support the view that l-lysine is transported in the chicken jejunum by two transport systems, A and B, with properties similar to those described for systems b ^0,+ and y⁺, respectively. Received: 14 August 1995/Revised: 2 April 1996     

Sequential Amino Acid Exchange across b0,+-like System in Chicken Brush Border Jejunum

In the small intestine, cationic amino acids are transported by y⁺-like and b^0,+-like systems present in the luminal side of the epithelium. Here, we report the characterization of a b^0,+-like system in the apical membrane of the chicken jejunum, and its properties as an amino acid exchanger. Analysis of the brush border membrane by Western blot points out the presence of rBAT (protein related to b^0,+ amino acid transport system) in these membranes. A functional mechanism for amino acid exchange across this system was established by kinetic analysis measuring fluxes at varying substrate concentrations both in internal (in) and external (out) vesicle compartments. This intestinal b^0,+-like system functions for l-arginine as an obligatory exchanger since its transport capacity increases 100–200 fold in exchange conditions, thus suggesting an important role in the intestinal absorption of cationic amino acids. The kinetic analysis of Arg_in efflux velocities is compatible with the formation of a ternary complex and excludes a model involving a ping-pong mechanism. The binding affinity of Arg_out is higher than that of Arg_in, suggesting a possible order of binding (Arg_out first) for the formation of the ternary complex during the exchange cycle. A model of double translocation pathways with alternating access is discussed. Received: 23 March 2000/Revised: 29 December 2000     

Identification of a Region Critically Involved in the Interaction of Phlorizin with the Rabbit Sodium-D-Glucose Cotransporter SGLT1

In order to define potential interaction sites of SGLT1 with the transport inhibitor phlorizin, mutagenesis studies were performed in a hydrophobic region of loop 13 (aa 604–610), located extracellularly, close to the C-terminus. COS 7 cells were transiently transfected with the mutants and the kinetic parameters of α-methyl-d-glucopyranoside (AMG) uptake into the cells were investigated. Replacement of the respective amino acids with lysine reduced the maximal uptake rate: Y604K showed 2.2%, L606K 48.4%, F607K 15.1%, C608K 13.1%, G609K 14.1%, and L610K 17.2% of control. In all mutants the apparent K _i for phlorizin increased at least by a factor of 5 compared to the wild-type K _i of 4.6 ± 0.7 μmol/l; most striking changes were observed for Y604K (K _i = 75.3 ± 19.0 μmol/l) and C608K (K _i = 83.6 ± 13.9 μmol/l). Replacement of these amino acids with a nonpolar amino acid instead of lysine such as in Y604F, Y604G and C608A showed markedly higher affinities for phlorizin. In cells expressing the mutants the apparent affinity of AMG uptake for the sugar was not statistically different from that of the wild type (K _m = 0.8 ± 0.2 mmol/l). These studies suggest that the region between amino acids 604 and 610 is involved in the interaction between SGLT1 and phlorizin, probably by providing a hydrophobic pocket for one of the aromatic rings of the aglucone moiety of the glycoside. Received: 29 March 2001/Revised: 15 June 2001     

Facilitated Transport of Lactate by Rat Jejunal Enterocyte

L-lactate transport mechanism across rat jejunal enterocyte was investigated using isolated membrane vesicles. In basolateral membrane vesicles l-lactate uptake is stimulated by an inwardly directed H⁺ gradient; the effect of the pH difference is drastically reduced by FCCP, pCMBS and phloretin, while furosemide is ineffective. The pH gradient effect is strongly temperature dependent. The initial rate of the proton gradient-induced lactate uptake is saturable with respect to external lactate with a K _m of 39.2 ± 4.8 mm and a J _max of 8.9 ± 0.7 nmoles mg protein⁻¹ sec⁻¹. A very small conductive pathway for l-lactate is present in basolateral membranes. In brush border membrane vesicles both Na⁺ and H⁺ gradients exert a small stimulatory effect on lactate uptake. We conclude that rat jejunal basolateral membrane contains a H⁺-lactate cotransporter, whereas in the apical membrane both H⁺-lactate and Na⁺-lactate cotransporters are present, even if they exhibit a low transport rate. Received: 22 October 1996/Revised: 11 March 1997     

Phenylalanine-Binding RNAs and Genetic Code Evolution

We isolated RNAs by selection–amplification, selecting for affinity to Phe–Sepharose and elution with free l-phenylalanine. Constant sequences did not contain Phe condons or anticodons, to avoid any possible confounding influence on initially randomized sequences. We examined the eight most frequent Phe-binding RNAs for inclusion of coding triplets. Binding sites were defined by nucleotide conservation, protection, and interference data. Together these RNAs comprise 70% of the 105 sequenced RNAs. The K _D for the strongest sites is ≈50 μM free amino acid, with strong stereoselectivity. One site strongly distinguishes free Phe from Trp and Tyr, a specificity not observed previously. In these eight Phe-binding RNAs, Phe codons are not significantly associated with Phe binding sites. However, among 21 characterized RNAs binding Phe, Tyr, Arg, and Ile, containing 1342 total nucleotides, codons are 2.7-fold more frequent within binding sites than in surrounding sequences in the same molecules. If triplets were not specifically related to binding sites, the probability of this distribution would be 4.8 × 10⁻¹¹. Therefore, triplet concentration within amino acid binding sites taken together is highly likely. In binding sites for Arg, Tyr, and Ile cognate codons are overrepresented. Thus Arg, Tyr, and Ile may be amino acids whose codons were assigned during an era of direct RNA–amino acid affinity. In contrast, Phe codons arguably were assigned by another criterion, perhaps during later code evolution.     

Reconstitution of a Regulated Transepithelial Water Pathway in Cells Transfected with AQP2 and an AQP1/AQP2 Hybrid Containing the AQP2-C Terminus

Transepithelial water permeability was measured in LLC-PK1 cells stably transfected with aquaporins (AQPs): AQP1, AQP2, and a chimera of AQP1 and AQP2 containing 41 amino acids of the C-terminus of AQP2. Transepithelial water fluxes (Jw) were not previously reported in cells transfected with aquaporins. Jw were now recorded each minute using a specially developed experimental device. A significant increase in Posm after forskolin (FK) plus vasopressin (VP) was found in AQP2 transfected cells (39.9 ± 8.2 vs. 12.5 ± 3.3 cm · sec⁻¹· 10⁻³), but not in cells transfected with AQP1 (15.3 ± 3.6 vs. 13.4 ± 3.6 cm · sec⁻¹· 10⁻³). In the case of the AQP1/2 cells (chimera) the FK plus VP induced Posm was smaller than in AQP2 cells but significantly higher than in mock cells at rest (18.1 ± 4.8 vs. 6.7 ± 1.0 cm · sec⁻¹· 10⁻³). The increases in Posm values were not paralleled by increases in ¹⁴C-Mannitol permeability. HgCl₂ inhibited the hydrosmotic response to FK plus VP in AQP2 transfected epithelia. Results were comparable to those observed, in parallel experiments, in a native ADH-sensitive water channel containing epithelial barrier (the toad urinary bladder). Electron microscopy showed confluent LLC-PK1 cells with microvilli at the mucosal border. The presence of spherical or elongated intracellular vacuoles was observed in AQP2 transfected cells, specially after FK plus VP stimulus and under an osmotic gradient. These results demonstrate regulated transepithelial water permeability in epithelial cells transfected with AQP2. Received: 24 June 1997/Revised: 16 September 1997     

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.     

Mutants of the Lactose Carrier of Escherichia coli Which Show Altered Sugar Recognition Plus a Severe Defect in Sugar Accumulation

Lactose and melibiose are actively accumulated by the wild-type Escherichia coli lactose carrier, which is an integral membrane protein energized by the proton motive force. Mutants of the E. coli lactose carrier were isolated by their ability to grow on minimal plates with succinate plus IPTG in the presence of the toxic lactose analog β-thio-o-nitrophenylgalactoside (TONPG). TONPG-resistant mutants were streaked on melibiose MacConkey indicator plates, and red clones were picked. These melibiose positive mutants were then streaked on lactose MacConkey plates, and white clones were picked. Transport assays indicated that the mutants had altered sugar recognition and a defect in sugar accumulation. The mutants had a poor apparent K _m for both lactose and melibiose in transport. One mutant had almost no ability to take up lactose, but melibiose downhill transport was 58% (V _max ) of normal. All of the mutants accumulated methyl-α-d-galactopyranoside (TMG) to only 8% or less of normal, and two failed to accumulate. Immunoblot analysis of the mutant lactose carrier proteins indicated that loss of sugar transport activity was not due to loss of expression in the membrane. Nucleotide sequencing of the lacY gene from the mutants revealed changes in the following amino acids of the lactose carrier: M23I, W151L, G257D, A295D and G377V. Two of the mutants (G257D and G377V) are novel in that they represent the first amino acids in periplasmic loops to be implicated with changes in sugar recognition. We conclude that the amino acids M23, W151, G257, A295 and G377 of the E. coli lactose carrier play either a direct or an indirect role in sugar recognition and accumulation. Received: 12 October 1999/Revised: 21 December 1999     

l-Arginine Effects on Na+ Transport in M-1 Mouse Cortical Collecting Duct Cells—A Cationic Amino Acid Absorbing Epithelium

The effect of l-arginine on transepithelial ion transport was examined in cultured M-1 mouse renal cortical collecting duct (CCD) cells using continuous short circuit current (I _SC ) measurements in HCO₃ ⁻/CO₂ buffered solution. Steady state I _SC averaged 73.8 ± 3.2 μA/cm² (n= 126) and was reduced by 94 ± 0.6% (n= 16) by the apical addition of 100 μm amiloride. This confirms that the predominant electrogenic ion transport in M-1 cells is Na⁺ absorption via the epithelial sodium channel (ENaC). Experiments using the cationic amino acid l-lysine (radiolabeled) as a stable arginine analogue show that the combined activity of an apical system y⁺ and a basal amino acid transport system y⁺L are responsible for most cationic amino acid transport across M-1 cells. Together they generate net absorptive cationic amino acid flux. Application of l-arginine (10 mm) either apically or basolaterally induced a transient peak increase in I _SC averaging 36.6 ± 5.4 μA/cm² (n= 19) and 32.0 ± 7.2 μA/cm² (n= 8), respectively. The response was preserved in the absence of bath Cl⁻ (n= 4), but was abolished either in the absence of apical Na⁺ (n= 4) or by apical addition of 100 μm amiloride (n= 6). l-lysine, which cannot serve as a precursor of NO, caused a response similar to that of l-arginine (n= 4); neither L-NMMA (100 μm; n= 3) nor L-NAME (1 mm; n= 4) (both NO-synthase inhibitors) affected the I _SC response to l-arginine. The effects of arginine or lysine were replicated by alkalinization that mimicked the transient alkalinization of the bath solution upon addition of these amino acids. We conclude that in M-1 cells l-arginine stimulates Na⁺ absorption via a pH-dependent, but NO-independent mechanism. The observed net cationic amino acid absorption will counteract passive cationic amino acid leak into the CCD in the presence of electrogenic Na⁺ transport, consistent with reports of stimulated expression of Na⁺ and cationic amino acid transporters by aldosterone. Received: 11 September 2000/Revised: 6 December 2000     

Isolation and Characterization of Five Actin cDNAs from the Cestode Diphyllobothrium dendriticum: A Phylogenetic Study of the Multigene Family

Five cDNAs (pDidact2–pDidact6), representing different actin genes, were isolated from a Diphyllobothrium dendriticum cDNA library, and the DNA as well as the putative amino acid sequences were determined. The corresponding Didact2 and Didact4 genes code for peptides 376 amino acids long, with molecular weights 41,772 and 41,744 Da, respectively, while the deduced Didact3 protein is 377 amino acids long and weighs 41,912 Da. The pDidact5 and -6 cDNAs lack nucleotides corresponding to three to six amino acids at the amino-terminus. Two of the five cDNAs contain the conventional AATAAA as the putative polyadenylation signal, one has the common variant ATTAAA, whereas the hexanucleotide AATAGA is found 15 and 18 nucleotides, respectively, upstream of the poly(A) site in two of the cDNAs. Phylogenetic studies including 102 actin protein sequences revealed that there are at least four different types of cestode actins. In this study three of these types were found to be expressed in the adult D. dendriticum tapeworm. Structurally the cestode actin groupings differ from each other to an extent seen only among the metazoan actins between the vertebrate muscle and cytoplasmic isoforms. In the phylogenetic trees constructed, cestode actins were seen to map to two different regions, one on the border of the metazoan actins and the other within this group. It is, however, difficult to say whether the cestode actins branched off early in the metazoan evolution or if this position in the phylogenetic tree only reflects upon differences in evolutionary rate. Received: 19 June 1996 / Accepted: 20 August 1996     

A Thiamine/H+ Antiport Mechanism for Thiamine Entry into Brush Border Membrane Vesicles from Rat Small Intestine

Outwardly oriented H⁺ gradients greatly enhanced thiamine transport rate in brush border membrane vesicles from duodenal and jejunal mucosa of adult Wistar rats. At a gradient pH_in5:pH_out7.5, thiamine uptake showed an overshoot, which at 15 sec was three times as large as the uptake observed in the absence of the gradient. Under the same conditions, the binding component of uptake accounted for only 10–13% of intravesicular transport. At the same gradient, the K _m and J _max values of the saturable component of the thiamine uptake curve after a 6 sec incubation time were 6.2 ± 1.4 μm and 14.9 ± 3 pmol · mg⁻¹ protein · 6 sec⁻¹ respectively. These values were about 3 and 5 times higher, respectively, than those recorded in the absence of H⁺ gradient. The saturable component of the thiamine antiport had a stoichiometric thiamine: H⁺ ratio of 1:1 and was inhibited by thiamine analogues, guanidine, guanidine derivatives, inhibitors of the guanidine/H⁺ antiport, and imipramine. Conversely, the guanidine/H⁺ antiport was inhibited by unlabeled thiamine and thiamine analogues; omeprazole caused an approximately fourfold increase in thiamine transport rate. In the absence of H⁺ gradient, changes in transmembrane electrical potential did not affect thiamine uptake. At equilibrium, the percentage membrane-bound thiamine taken up was positively correlated with the pH of the incubation medium, and increased from about 10% at pH 5 to 99% at pH 9. Received: 17 July 1997/Revised: 16 September 1997     

Evolution of Chitin-Binding Proteins in Invertebrates

Analysis of a group of invertebrate proteins, including chitinases and peritrophic matrix proteins, reveals the presence of chitin-binding domains that share significant amino acid sequence similarity. The data suggest that these domains evolved from a common ancestor which may be a protein containing a single chitin-binding domain. The duplication and transposition of this chitin-binding domain may have contributed to the functional diversification of chitin-binding proteins. Sequence comparisons indicated that invertebrate and plant chitin binding domains do not share significant amino acid sequence similarity, suggesting that they are not coancestral. However, both the invertebrate and the plant chitin-binding domains are cysteine-rich and have several highly conserved aromatic residues. In plants, cysteines have been elucidated in maintaining protein folding and aromatic amino acids in interacting with saccharides [Wright HT, Sanddrasegaram G, Wright CS (1991) J Mol Evol 33:283–294]. It is likely that these residues perform similar functions in invertebrates. We propose that the invertebrate and the plant chitin-binding domains share similar mechanisms for folding and saccharide binding and that they evolved by convergent evolution. Furthermore, we propose that the disulfide bonds and aromatic residues are hallmarks for saccharide-binding proteins. Received: 2 March 1998 / Accepted: 17 July 1998     

Volume-regulatory Amino Acid Release from the Protozoan Parasite Crithidia luciliae

The unicellular protozoan parasite, Crithidia luciliae, responded to osmotic swelling by undergoing a regulatory volume decrease. This process was accompanied by the efflux of amino acids (predominantly alanine, proline and glycine). The relative loss of the electroneutral amino acids proline, valine, alanine and glycine was greater than that for the anionic amino acid, glutamate; there was negligible loss of the cationic amino acids, lysine, arginine and ornithine. The characteristics of amino acid release were investigated using a radiolabeled form of the nonmetabolized alanine analogue α-aminoisobutyrate. α-Aminoisobutyrate efflux was activated within a few seconds of a reduction of the osmolality, and inactivated rapidly (again within a few seconds) on restoration of isotonicity. The initial rate of efflux of α-aminoisobutyrate from cells in hypotonic medium was unaffected by the extracellular amino acid concentration. Hypotonically activated α-aminoisobutyrate efflux (as well as the associated regulatory volume decrease) was inhibited by the sulfhydryl reagent N-ethylmaleimide but was not inhibited by a range of anion transport blockers. As in the efflux experiments, unidirectional influx rates for α-aminoisobutyrate increased markedly following reduction of the osmolality, consistent with the swelling-activated amino acid release mechanism allowing the flux of solutes in both directions. Hypotonically activated α-aminoisobutyrate influx showed no tendency to saturate up to an extracellular concentration of 50 mm. The functional characteristics of the amino acid release mechanism are those of a channel, with a preference for electroneutral and anionic amino acids over cationic amino acids. However, the pharmacology of the system differs from that of the anion-selective channels that are thought to mediate the volume-regulatory efflux of organic osmolytes from vertebrate cells. Received: 13 May 1996/Revised: 9 July 1996     

Characterization of Fumarate Transport in Helicobacter pylori

The fumarate transport system of the bacterium Helicobacter pylori was investigated employing radioactive tracer analysis. The transport of fumarate at micromolar concentrations was saturable with a K _M of 220 ± 21 μm and V _max of 54 ± 2 nmole/min/mg protein at 20°C, depended on temperature between 4 and 40°C, and was susceptible to inhibitors, suggesting the presence of one or more fumarate carriers. The release of fumarate from cells was also saturable with a K _M of 464 ± 71 μm and V _max of 22 ± 2 nmol/min/mg protein at 20°C. The rates of fumarate influx at millomolar concentrations increased linearly with permeant concentration, and depended on the age of the cells. The transport system was specific for dicarboxylic acids suggesting that fumarate is taken up via dicarboxylate transporters. Succinate and fumarate appeared to form an antiport system. The properties of fumarate transport were elucidated by investigating the effects of amino acids, monovalent cations, pH and potential inhibitors. The results provided evidence that influx and efflux of fumarate at low concentrations from H. pylori cells was a carrier-mediated secondary transport with the driving force supplied by the chemical gradient of the anion. The anaerobic C₄-dicarboxylate transport protein identified in the genome of the bacterium appeared to be a good candidate for the fumarate transporter. Received: 11 December 1997/Revised: 7 May 1998     

Transient Swelling of Salivary Acinus Induced by Acetylcholine Stimulation: Water Secretion Pathway in Rat Submandibular Gland

The volume changes of isolated acini and acinar cells from rat submandibular glands were measured from digitized images recorded upon stimulation of acetylcholine (ACh) or reduction of the perfusate osmolarity and water secretion pathway in salivary gland was studied. When acinus is exposed to a hyposmotic solution, water flows into the acinar cells and into the lumen via acinar epithelia. If the water enters the lumen chiefly via the cells, the swelling of the lumen would follow the same time course as the cell swelling or slower. The results show that reduction of the perfusate osmolarity evoked a transient increase followed by a gradual increase in the volume of unstimulated acinus, while it evoked only a gradual increase in the volumes of unstimulated acinar cells. Thus, the time course of the acinar swelling is faster than that of the acinar cell swelling. Reduction of the perfusate osmolarity also evoked a transient swelling in ACh stimulated acini. When acinus is stimulated by ACh, water also flows into the lumen via acinar epithelia according to the osmotic gradient which was generated by the active electrolyte transport of acinar cells. If the water enters the lumen chiefly from the cells, there would be no overall change in acinar volume. The results show that stimulation of ACh (5 μm) evoked a transient increase followed by a gradual decrease in the volume of the acinus, while it evoked only a decrease in the volume of acinar cells. Video-enhanced optical microscopy exhibited that ACh stimulation caused transient swelling of the luminal space, prior to causing the volume of acinar cells to decrease and the transient swelling of the lumen followed the same time course as that of acinus. Thus, the transient acinar swelling is explained by the transient swelling of luminar volume. These results suggest that water is probably drawn into the lumen from interstitial space directly in the salivary acinus. Received: 3 February 1997/Revised: 29 October 1997     

The Evolution of Codon Preferences in Drosophila: A Maximum-Likelihood Approach to Parameter Estimation and Hypothesis Testing

Synonymous codon usage in related species may differ as a result of variation in mutation biases, differences in the overall strength and efficiency of selection, and shifts in codon preference—the selective hierarchy of codons within and between amino acids. We have developed a maximum-likelihood method to employ explicit population genetic models to analyze the evolution of parameters determining codon usage. The method is applied to twofold degenerate amino acids in 50 orthologous genes from D. melanogaster and D. virilis. We find that D. virilis has significantly reduced selection on codon usage for all amino acids, but the data are incompatible with a simple model in which there is a single difference in the long-term N _e, or overall strength of selection, between the two species, indicating shifts in codon preference. The strength of selection acting on codon usage in D. melanogaster is estimated to be |N _e s|≈ 0.4 for most CT-ending twofold degenerate amino acids, but 1.7 times greater for cysteine and 1.4 times greater for AG-ending codons. In D. virilis, the strength of selection acting on codon usage for most amino acids is only half that acting in D. melanogaster but is considerably greater than half for cysteine, perhaps indicating the dual selection pressures of translational efficiency and accuracy. Selection coefficients in orthologues are highly correlated (ρ= 0.46), but a number of genes deviate significantly from this relationship. Received: 20 December 1998 / Accepted: 17 February 1999     

The Molecular Evolution of Visual Pigments of Freshwater Crayfishes (Decapoda: Cambaridae)

This study examines the diverse maximum wavelength absorption (λ_max) found in crayfishes (Decapoda: Cambaridae and Parastacidae) and the associated genetic variation in their opsin locus. We measured the wavelength absorption in the photoreceptors of six species that inhabit environments of different light intensities (i.e., burrows, streams, standing waters, and subterranean waters). Our results indicate that there is relatively little variation in λ_max (522–530 nm) among species from different genera and families. The existing variation did not correlate with the habitat differences of the crayfishes studied. We simultaneously sequenced the rhodopsin gene to identify the amino acid replacements that affect shifts in maximum wavelength absorption. We then related these to changes that correlated with shifts in λ_max by reconstructing ancestral character states using a maximum-likelihood approach. Using amino acid sequences obtained from five species (all were 301 amino acids in length), we identified a number of candidates for producing shifts of 4 to 8 nm in λ_max. These amino acid replacements occurred in similar regions to those involved in spectral shifts in vertebrates. Received: 12 March 1997 / Accepted: 3 June 1997     

Identification of the site of glucocorticoid action on neutral amino acid transport in superficial nephrons of rat kidney

Summary. Glucocorticoid hormones enhance the reabsorptive capacity of filtered amino acids in rat kidney, as it was shown in previous in vivo clearance experiments. In the present study, the site of glucocorticoid action on neutral amino acid transport in superficial nephrons of rat kidney was investigated using in vivo micropuncture technique. Adult female Wistar rats were treated with dexamethasone (DEX), and fractional excretion of L-glutamine (L-Gln) and L-leucine (L-Leu) were determined and related to inulin after microinfusion into different nephron segments. DEX reduced fractional excretion of both neutral amino acids as a sign of enhanced reabsorptive capacity. The site of main DEX action on L-Leu reabsorption has been localized in the proximal straight tubule. However, in the case of L-Gln, the inhibition of γ-glutamyltranspeptidase (γ-GT) by administration of acivicin indicated the importance of this brush border enzyme in reduced L-Gln excretion. DEX enhanced γ-GT activity by tubular acidification. It can be presumed a DEX-inducible transport system for neutral amino acids mainly localized in proximal straight tubules of rat kidney. Received July 8, 1999     

Citrate Uptake into Tonoplast Vesicles from Acid Lime (Citrus aurantifolia) Juice Cells

Citrate transport into the vacuoles of acid lime juice cells was investigated using isolated tonoplast vesicles. ATP stimulated citrate uptake in the presence or in the absence of a ΔμH⁺. Energization of the vesicles only by an artificial K⁺ gradient (establishing an inside-positive Δψ) also resulted in citrate uptake as was the case of a ΔpH dominated ΔμH⁺. Addition of inhibitors to endomembrane ATPases showed no direct correlation between the inhibition to the tonoplast bound H⁺/ATPase and citrate uptake. The data indicated that, although some citrate uptake can be accounted for by Δψ and by a direct primary active transport mechanism involving ATP, under in vivo conditions of vacuolar pH of 2.0, citrate uptake is driven by ΔpH. Received: 27 April 1998/Revised: 8 September 1998     

Characteristics of rat jejunal transport of tryptophan.

The parameters of rat jejunal transport of tryptophan have been examined. The interactions between tryptophan and lysine or methionine have been reexamined, and some aspects of the trans effects of cellularly accumulates amino acids have been studied. It has been demonstrated that: (1) The influx of tryptophan across the jejunal brush border (Jmc-Trp) can be accounted for by the carrier of alpha-aminomonocarboxylic acids alone. (2) Tryptophan competes with lysine for the carrier of basic amino acids across the brush border membrane without itself being transported by this carrier. (3) Lysine has neither cis nor trans effects on Jmc-Trp, whereas intracellular tryptophan is highly inhibitory to Jmd-Lys. (4) The intracellular concentration of lysine and of tryptophan, [Lys]c and [Trp]c, are unaffected by tryptophan and lysine, respectively, although the transmural fluxes, from the mucosal side to the serosal side, Jms, of lysine, Jms-Lys, and of tryptophan, Jms-Trp, are inhibited by tryptophan and lysine, respectively. The latter effects thus represent inhibitory interactions at the basolateral membrane. (5) Methionine is a potent cis and transinhibitor of Jmc-Trp, but stimulated Jms-Trp and reduces [Trp]c. (6) Methionine causes trans acceleration of the influx of lysine across the brush border membrane, Jmc-Lys, but has no effect on the influx of galactose, Jmc-Gal. (7) Leucine causes trans inhibition of Jmc-Leu. (8) Tryptophan does not cause cis inhibition of Jmc-Gal, but is a strongtransinhibitor of Jmc-Gal. (9) Cellularly accumulated tryptophan appears to accelerate the eventual decline in transepithelial potential difference and short-circuit current. These results are consistent with the conclusions that: (1) Tryptophan is transported across the brush border membrane by the carrier of neutral amino acids alone, but leaves the cell across the basolateral membrane by a mechanism used by lysine also. (2) Leucine, methionine and probably tryptophan have a transeffect on the transport of neutral amino acids across the brush border membrane which may represent a phenomenon which can appropriately be termed decelerating exchange diffusion. (3) Cellularly accumulated tryptophan has a strong and indiscriminate depressive effect on all transport functions of rat jejunal epithelium.