Kinetics of leucine transport in brush border membrane vesicles from lepidopteran larvae midgut

Antioxidant defenses preserve membrane transport activity in Chironomus riparius larvae exposed to anoxia

Changes in enzyme activities, metabolite concentrations, and membrane transport activity underlying the Chironomus riparius larvae adaptive response to anoxia were investigated. Trehalose, malate, and aspartate degradation and alanine accumulation were recorded. During anoxia exposure, there was a boost of antioxidant defenses as shown by an increase of the specific activity of the enzymes catalase, glutathione-S-transferase, glutathione peroxidase, glutathione-synthase, malic enzyme, and NADP-dependent isocitrate dehydrogenase. The ratio, glutathione reduced over glutathione oxidized, decreased. Except for alanine and catalase, the parameters return to their basal value when larvae are transferred to normoxic conditions. To test whether antioxidant defenses had protective effects on membrane functionality, L-leucine uptake into brush border membrane vesicles and membrane lipid peroxidation was measured. No difference between membranes prepared from larvae exposed to anoxia and control larvae was found. The amino acid alanine, when present inside the vesicles, trans-stimulated leucine uptake. This effect could represent a mechanism to stimulate amino acid uptake and catabolism in vivo when free alanine concentration increases during hypoxic periods.

Leucine Transport Is Affected by Bacillus thuringiensis Cry1 Toxins in Brush Border Membrane Vesicles from Ostrinia nubilalis Hb (Lepidoptera: Pyralidae) and Sesamia nonagrioides Lefebvre (Lepidoptera: Noctuidae) Midgut

The pore-forming activity of Cry1Ab, Cry1Fa and Cry1Ca toxins and their interaction with leucine transport mediated by the K(+)/leucine cotransporter were studied in brush border membrane vesicles (BBMVs) isolated from the midgut of Ostrinia nubilalis and Sesamia nonagrioides. In both species, as in other Lepidoptera, leucine uptake by BBMVs can take place in the absence of cations, but it can also be driven by a K(+) gradient. Experiments with the voltage-sensitive fluorescent dye 3,3'-diethylthiacarbocyanine iodide proved that Cry1Ab, a Bacillus thuringiensis toxin active in vivo, enhanced the membrane permeability to potassium in O. nubilalis BBMVs. This result is in agreement with similar effects observed in S. nonagrioides BBMV incubated with various Cry1 toxins active in vivo. The effect of the above toxins was tested on the initial rate of 0.1 mM: leucine influx. Instead of an increase in leucine influx, a reduction was observed with the Cry1 toxins active in vivo. Cry1Ab and Cry1Fa, but not the inactive toxin Cry1Da, inhibited in a dose-dependent manner leucine uptake both in the absence and in the presence of a K(+) gradient, a clear indication that their effect is independent of the channel formed by the toxins and that this effect is exerted directly on the amino acid transport system.

Leucine transport in brush border membrane vesicles from freshwater insect larvae

Leucine transport across brush border membrane vesicles prepared from four insect species common to European freshwater streams has been characterized. The species studied were: Ephemera danica (Ephemeroptera: Ephemeridae), Isoperla grammatica (Plecoptera: Perlodidae), Hydropsyche pellucidula (Trichoptera: Hydropsychidae), and Hybomitra bimaculata (Diptera: Tabanidae). The transport differed among the studied taxa for several features, including pH and sodium dependence, substrate affinity and specificity, and efficiency. In H. pellucidula and E. danica, leucine uptake was higher at pH 7.4 than at more alkaline or acidic pH values, whereas in I. grammatica and H. bimaculata, the uptake was rather constant when pH varied from 5.0 to 7.4, then strongly decreased at pH 8.8. All but E. danica displayed a transient intravescicular leucine accumulation in the presence of sodium, suggesting the existence of a cation-leucine symport mechanism. The sodium dependence ranged according to the following order: H. pellucidula > I. grammatica > H. bimaculata > E. danica. Moreover, in H. pellucidula and I. grammatica, the sodium-dependence was stronger at pH 8.8 than at pH 7.4. In E. danica, leucine uptake was sodium-independent at all pH values. The highest value of V(max) (45.3 pmol.s(-1).mg proteins(-1)) was in E. danica, which, however, displayed the lowest affinity (K(m) 137 muM) when compared to the kinetic parameters of other taxa. The V(max) and K(m) values were: 40 and 52.5, 32.1 and 12.5, and 4.5 and 230 for H. bimaculata, H. pellucidula, and I. grammatica, respectively. The obtained results are discussed within our current knowledge of amino acid transport systems in insects.

Changes in leucine transport activity in Chironomus riparius larvae after short-term exposure to potassium dichromate and fenitrothion

The effect of sublethal concentrations of potassium dichromate and fenitrothion on sodium-leucine cotransport in brush border membrane vesicles from Chironomus riparius larvae has been investigated. Exposure to potassium dichromate and fenitrothion caused a dose- and time-dependent inhibition of leucine uptake. Transport inhibition is easily detectable at doses 100-fold lower than LD50. Kinetic experiments showed that inhibition was mainly caused by a decrease of the Vmax (680 +/- 53 vs. 382 +/- 23 and 555 +/- 27 nmol/15s/mg protein in control and exposed larvae to K2Cr2O7 and fenitrothion, respectively). Inhibition is possibly related to a variation of sodium ions permeability as evidenced by increased membrane lipid peroxidation. Appropriate control experiments ruled out that the observed differences could be due to changes in general features of membrane preparations. Transport inhibition observed in larvae exposed to potassium dichromate was accompanied by changes in ascorbate peroxidase and dehydroascorbate reductase activities, whereas those exposed to fenitrothion displayed an increase in transaminase activity. The possible value of leucine uptake as biochemical biomarker is briefly discussed. Arch. Insect Biochem. Physiol. 55:90-101, 2004.

Identification, functional characterization and expression of a LAT type amino acid transporter from the mosquito Aedes aegypti

We isolated two cDNAs from the mosquito Aedes aegypti, an L-amino acid transporter (AeaLAT) and a CD98 heavy chain (AeaCD98hc). Expression of AeaCD98hc or AeaLAT alone in Xenopus oocyte did not induce amino acid transport activity. However, co-expression of AeaCD98hc and AeaLAT, which are postulated to form a heterodimer protein linked through a disulfide bond, showed significant increase in amino acid transport activity. This heterodimeric protein showed uptake specificity for large neutral and basic amino acids. Small acidic neutral amino acids were poor substrates for this transporter. Neutral amino acid (leucine) uptake activity was partially Na+ dependent, because leucine uptake was approximately 44% lower in the absence of Na+ than in its presence. However, basic amino acid (lysine) uptake activity was completely Na+ independent at pH of 7.4. Extracellular amino acid concentration could be the main factor that determined amino acid transport. These results suggest the heteromeric protein is likely a uniporter mediating diffusion of amino acids in the absence of ions. The AeaLAT showed high level expression in the gastric caeca, Malpighian tubules and hindgut of larvae. In caeca and hindgut expression was in the apical cell membrane. However, in Malpighian tubules and in midgut, the latter showing low level expression, the transporter was detected in the basolateral membrane. This expression profile supports the conclusion that this AeaLAT is a nutrient amino acid transporter.

Leucine methyl ester is a powerful allosteric activator of the neutral amino acid cotransport system in Bombyx mori larval midgut

We have identified three methyl esters that have a potent stimulatory effect on the cotransport system responsible for the absorption of most essential amino acids in the silkworm Bombyx mori. L-Leucine methyl ester, the most powerful activator, determined a large dose-dependent, K(+)-independent increase of leucine uptake into midgut brush border membrane vesicles. Kinetic experiments revealed non-essential mixed-type activation, with K(a) values of 27+/-2 and 47+/-8 microM in the presence and in the absence of K(+), respectively. The activation increased K(m) twofold, and V(max) up to 18-fold depending upon the experimental conditions. Leucine uptake mediated by the amino acid uniport appears to be unaffected by the activator.

A novel regulatory mechanism for amino acid absorption in lepidopteran larval midgut

A number of methyl and ethyl esters of naturally occurring amino acids exert a potent stimulatory effect on the cotransport system responsible for the absorption of most essential amino acids along the midgut of the silkworm Bombyx mori. L-Leucine methyl ester (Leu-OMe), one of the most effective activators, induces a large increase of the initial rate of leucine uptake in midgut brush border membrane vesicles (BBMV) from the anterior-middle (AM) region, and a small effect in BBMV from the posterior (P) region. Nonetheless, the methyl ester causes in both regions a relevant K(+)-, Deltapsi- and pH-independent increase of the intravesicular accumulation of the amino acid. The activation by Leu-OMe proves that amino acid absorption can be modulated all along the B. mori larval midgut and that the AM region, where the ability to transport and concentrate the substrate is very low, is more susceptible than the P region. Leucine uptake in AM-BMMV can be activated by amino acid methyl esters with definite structural requisites, with the following order of potency: L-leucine>L-phenylglycine>L-methionine>L-phenylalanine>L-norleucinez.Gt;L-isoleucine. The activation is stereospecific and occurs also with some ethyl esters (e.g. leucine and phenylalanine). No activation was observed with esters of amino acids with short hydrophobic or polar side-chains. The activation mechanism here described plays a fundamental role in larval growth since silkworms reared on artificial diets supplemented with leucine or methionine methyl esters reach maximum body weight 12-18 h before control larvae and spin cocoons with a larger shell weight. This novel regulatory mechanism of an amino acid transport protein appears to be widespread among lepidopteran larvae.

Role of specific activators of intestinal amino acid transport in Bombyx mori larval growth and nutrition

Nutrient absorption and its modulation are critical for animal growth. In this paper, we demonstrate that leucine methyl ester (Leu-OMe) can greatly increase the activity of the transport system responsible for the absorption of most essential amino acids in the larval midgut of the silkworm Bombyx mori. We investigated leucine uptake activation by Leu-OMe in brush border membrane vesicles and in the apical membrane of epithelial cells in the midgut incubated in vitro. Moreover, the addition of this strong activator of amino acid absorption to diet significantly affected larval growth. Silkworms fed on artificial diet supplemented with Leu-OMe reached maximum body weight 12-18 h before control larvae, and produced cocoon shells up to 20% heavier than those of controls. The activation of amino acid absorption plays an essential role in larval development so that larval growth and cocoon production similar to controls reared on an artificial diet with 25% of dry mulberry leaf powder were observed in silkworms fed on an artificial diet with only 5% of mulberry powder. Arch.

Leucine transport in membrane vesicles from Chironomus riparius larvae displays a mélange of crown-group features

Leucine uptake into membrane vesicles from larvae of the midge Chironomus riparius was studied. The membrane preparation was highly enriched in typical brush border membrane enzymes and depleted of other membrane contaminants. In the absence of cations, there was a stereospecific uptake of l-leucine, which exhibited saturation kinetics. Parameters were determined both at neutral (Km 33 +/- 5 microM and Vmax 22.6 +/- 6.8 pmol/7s/mg protein) and alkaline (Km 46 +/- 5 microM and Vmax 15.5 +/- 2.5 pmol/7s/mg protein) pH values. At alkaline pH, external sodium increased the affinity for leucine (Km 17 +/- 1 microM) and the maximal uptake rate (Vmax 74.0 +/- 12.5 pmol/7s/mg protein). Stimulation of leucine uptake by external alkaline pH agreed with lumen pH measurements in vivo. Competition experiments indicated that at alkaline pH, the transport system readily accepts most L-amino acids, including branched, unbranched, and alpha-methylated amino acids, histidine and lysine, but has a low affinity for phenylalanine, beta-amino acids, and N-methylated amino acids. At neutral pH, the transport has a decreased affinity for lysine, glycine, and alpha-methylleucine. Taken together, these data are consistent with the presence in midges of two distinct leucine transport systems, which combine characters of the lepidopteran amino acid transport system and of the sodium-dependent system from lower neopterans.

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.

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.

Artoindonesianin C, a new xanthone derivative from Artocarpus teysmanii

A new xanthone derivative, artoindonesianin C (1), was isolated from Artocarpus teysmanii, together with two known prenylated flavonoids, cycloartobiloxanthone and artonin J. The structure of artoindonesianin C (1) was determined on the basis of MS and NMR evidence and by comparison with known related compounds.

K(+)-neutral amino acid symport of Bombyx mori larval midgut: a system operative in extreme conditions

The K(+)-dependent symporter for leucine and other neutral amino acids expressed along the midgut of the silkworm Bombyx mori operates with best efficiency in the presence of a steep pH gradient across the brush-border membrane, with external alkaline pH values up to 11, and an electrical potential difference (delta psi) of approximately 200 mV. Careful determinations of leucine kinetics as a function of external amino acid concentrations between 50 and 1,000 microM, performed with brush-border membrane vesicles (BBMV) obtained from the middle and posterior midgut regions, revealed that the kinetic parameter affected by the presence of a delta pH was the maximal rate of transport. The addition of delta psi caused a further marked increase of the translocation rate. At nonsaturating leucine concentrations in the solution bathing the external side of the brush-border membrane, leucine accumulation within BBMV and midgut cells was not only driven by the gradient of the driver cation K+ and delta psi but occurred also in the absence of K+. The ability of the symporter to translocate the substrate in its binary form allows the intracellular accumulation of leucine in the absence of K+, provided that a pH gradient, with alkaline outside, is present. The mechanisms involved in this accumulation are discussed.

Evidence for a low-affinity, high-capacity uniport for amino acids in Bombyx mori larval midgut

We investigated the kinetics of leucine influx as a function of external substrate concentration between 0.03 and 16 mM in brush-border membrane vesicles (BBMV) prepared from the middle region of Bombyx mori larval midgut. A detailed kinetic analysis of leucine uptake led to the identification, in parallel with the K(+)-dependent symporter for neutral amino acids, of a K(+)-independent, low-affinity, high-capacity system. The parameter values of the Michaelis constant (7.12 mM) and maximal rate of transport (4.48 nmol.7 s-1.mg protein-1) were not influenced by an external alkaline pH nor by a transmembrane electrical potential difference. The uniporter is poorly specific, as it displayed the following rank of preference: Leu, His, Val, Ile, Phe, Ser > Lys, Arg, Gln > Pro, 2-amino-2-norbornane-carboxylic acid, Ala, Gly. The kinetic analysis performed in BBMV prepared from the posterior midgut portion indicates that the low-affinity, high-capacity uniporter is present along the entire length of the silkworm larval midgut with similar expression and functional properties.

Cloning and characterization of a potassium-coupled amino acid transporter

Active solute uptake in bacteria, fungi, plants, and animals is known to be mediated by cotransporters that are driven by Na+ or H+ gradients. The present work extends the Na+ and H+ dogma by including the H+ and K+ paradigm. Lepidopteran insect larvae have a high K+ and a low Na+ content, and their midgut cells lack Na+/K+ ATPase. Instead, an H+ translocating, vacuolar-type ATPase generates a voltage of approximately -240 mV across the apical plasma membrane of so-called goblet cells, which drives H+ back into the cells in exchange for K+, resulting in net K+ secretion into the lumen. The resulting inwardly directed K+ electrochemical gradient serves as a driving force for active amino acid uptake into adjacent columnar cells. By using expression cloning with Xenopus laevis oocytes, we have isolated a cDNA that encodes a K+-coupled amino acid transporter (KAAT1). We have cloned this protein from a larval lepidopteran midgut (Manduca sexta) cDNA library. KAAT1 is expressed in absorptive columnar cells of the midgut and in labial glands. When expressed in Xenopus oocytes, KAAT1 induced electrogenic transport of neutral amino acids but excludes alpha-(methylamino)isobutyric acid and charged amino acids resembling the mammalian system B. K+, Na+, and to a lesser extent Li+ were accepted as cotransported ions, but K+ is the principal cation, by far, in living caterpillars. Moreover, uptake was Cl(-)-dependent, and the K+/Na+ selectivity increased with hyperpolarization of oocytes, reflecting the increased K+/Na+ selectivity with hyperpolarization observed in midgut tissue. KAAT1 has 634 amino acid residues with 12 putative membrane spanning domains and shows a low level of identity with members of the Na+ and Cl(-)-coupled neurotransmitter transporter family.

A new prenylated flavone from Artocarpus champeden inhibits the K(+)-dependent amino acid transport in Bombyx mori midgut

The effect of some flavonoids on the K(+)-dependent and K(+)-independent leucine uptake into brush border membrane vesicles from Bombyx mori larval midgut was investigated. Among the compounds tested, cyclochampedol, recently purified from Artocarpus champeden, was able to inhibit in micromolar range the leucine transport. The inhibition occurred both in the absence and in the presence of potassium and was not affected by leucine concentration. The apparent Ki was 0.25 mM. Cyclochampedol represents the first non-competitive inhibitor of an amino acid transport system in Lepidoptera. The relevance of this result is discussed.

Effect of arginine modification on K(+)-dependent leucine uptake in brush-border membrane vesicles from the midgut of Philosamia cynthia larvae

The effect of phenylglyoxylation on the midgut K(+)-dependent leucine transport was studied using lepidopteran brush-border membrane vesicles. The inhibition of leucine uptake by phenylglyoxal (PGO) showed a biphasic inactivation pattern. The second-order rate constant for the slow and fast phases were 0.0020 mM-1 min-1 and 0.0091 mM-1 min-1, respectively. However, substitution of borate buffer for Hepes-Tris buffer produced a mono-exponential inactivation pattern, suggesting modification of a single arginine group. The effect of PGO was dose-dependent and the concentration causing half-maximal inhibition of leucine uptake was 5.1 +/- 0.3 mM. Leucine transport was significantly inhibited also in the absence of a potassium electrochemical gradient (i.e., [K+]in = [K+]out = 100 mM), suggesting that inhibition was not related to a decrease in the driving force. Moreover, intravesicular volume remained unchanged after preincubation with PGO. Kinetic analysis of the interaction of PGO with the leucine cotransporter revealed that (i) inhibition was related to a decrease in the Vmax value and (ii) neither leucine nor K+ were able to prevent the inhibition. Our results suggest an important role for arginine residues in the molecular mechanism of K+/leucine cotransport in lepidopteran larvae midgut.

Proline transport into brush border membrane vesicles from the midgut of Manduca sexta larvae

The unidirectional transport of proline by midgut epithelium cells of the tobacco hornworm, Manduca sexta, was investigated in brush border membrane vesicles. Both K.(+)-stimulated and K(+)-insensitive transport pathways were identified. Analyses of K(+)-dependent proline transport revealed 1:1 ratio of K+ to proline, a Km of 13 mM for K+ and a decrease in both Km (from 18 mM to 3 mM) and Vmax (from 37 nmol/mg protein/min to 10 nmol/mg protein/min) for proline in the presence of a K+ gradient. The profiles of cis-inhibition by other amino acids demonstrated that proline is transported into midgut cells by a transport system that is shared by other neutral amino acids.

Cation-dependent leucine, alanine, and phenylalanine uptake at pH 10 in brush-border membrane vesicles from larval Manduca sexta midgut

Using the rapid filtration technique, cation gradient driven leucine, alanine and phenylalanine uptake by brush-border membrane vesicles (BBMV) from the highly studied model insect, Manduca sexta, is characterized at the physiological pH of 10. The vesicles are sealed and nonspecific binding is small. Almost identical initial time courses of leucine uptake are obtained whether the vesicles are osmotically balanced initially or at equilibrium. The maximum accumulation values are also similar and the equilibrium values are identical with either treatment. Equilibrium is reached by 60 min. Amino acid accumulation is cation gradient dependent and is abolished by 18 microM valinomycin. Uptake of all three amino acids occurs over a broad pH range with maximum rates at approximately pH 10 and lower rates at pH 7.5. The cation selectivity of phenylalanine and alanine uptake changes with pH; the sequence is K+ > Na+ > Cs+ >> Rb+ = Li+ at pH 10.0, whereas K+ = Na+ at pH 8.0; the selectivity of leucine uptake is K+ = Na+ > Cs+ >> Rb+ = Li+ at pH 10. Maximum K+ driven accumulation of all three amino acids decreases with anions in the order: SCN- > NO3- > Cl- = CO(3)2- = So(4)2- = HPO(4)2- > gluconate-.Vmax values are similar for all three amino acids. There are large differences in initial uptake rates (leucine > phenylalanine = alanine), and maximum accumulation values (leucine > phenylalanine > alanine).

Preparation and partial characterization of amino acid transporting brush border membrane vesicles from the larval midgut of the gypsy moth (Lymantria dispar)

Brush border membrane vesicles were prepared from both fresh and frozen midguts of Lymantria dispar larvae by Mg/EGTA precipitation and differential centrifugation. The vesicles were enriched 10 to 13-fold, relative to the homogenate, in aminopeptidase and gamma-glutamyltransferase activity. No significant difference was found in enzyme enrichment of vesicles prepared from fresh and frozen midguts. Inwardly directed potassium salt gradients resulted in transient accumulation of leucine and lysine but not glutamic acid by the vesicles.