How many Na+-dependent carriers for L-alanine and L-proline in the eel intestine? Studies with brush-border membrane vesicles

The role of environmental salinity on Na+-dependent intestinal amino acid uptake in rainbow trout (Oncorhynchus mykiss)

Na⁺/K⁺-ATPases (NKA) in the basolateral membrane of the intestinal enterocytes create a Na⁺-gradient that drives both ion-coupled fluid uptake and nutrient transport. Being dependent on the same gradient as well as on the environmental salinity, these processes have the potential to affect each other. In salmonids, L-lysine absorption has been shown to be higher in freshwater (FW) than in seawater (SW) acclimated fish. Using electrophysiology (Ussing chamber technique), the aim was to explore if the decrease in L-lysine transport was due to allocation of the Na⁺-gradient towards ion-driven fluid uptake in SW, at the cost of amino acid transport. Intestinal NKA activity was higher in SW compared to FW fish. Exposure to ouabain, an inhibitor of NKA, decreased L-lysine transport. However, exposure to bumetanide and hydrochlorothiazide, inhibitors of Na⁺, K⁺, 2Cl^--co-transporter (NKCC) and Na⁺, Cl^--co-transporter (NCC) respectively, did not affect the rate of intestinal L-lysine transport. In conclusion, L-lysine transport is Na⁺-dependent in rainbow trout and the NKA activity and thus the available Na⁺-gradient increases after SW acclimation. This increased Na⁺-gradient is most likely directed towards osmoregulation, as amino acid transport is not compromised in SW acclimated fish.

Physical characterization of high-affinity gastrointestinal Cu transport in vitro in freshwater rainbow trout Oncorhynchus mykiss

This study investigated the transport of copper (Cu) in the gut of trout. Examination of the spatial distribution of Cu along the digestive tract and a physical characterization of the uptake process was carried out using an in vitro gut sac technique and (64)Cu as a tracer. Unidirectional Cu uptake was highest in the anterior intestine followed in decreasing order by the posterior intestine, mid intestine and the stomach. Cu uptake was resistant to hypoxia and appeared to be fueled equally well by Cu(II) or Cu (I) at Cu concentrations typically found in the fluid phase of the chyme in vivo in the trout intestine. Transport demonstrated saturation kinetics (e.g. K (m) = 31.6 microM, J (max) = 17 pmol cm(-2) h(-1), in mid intestine) at low Cu levels representative of those measured in the chyme in vivo, with a diffusive component at higher Cu concentrations. Q (10) analysis indicated Cu uptake is via diffusion across the apical membrane and biologically mediated across the basolateral membranes of enterocytes. The presence of L-histidine but not D-histidine stimulated both Cu and Na uptake suggesting a common pathway for the transport of Cu/Na with L-histidine.

Transport of L-proline, L-proline-containing peptides and related drugs at mammalian epithelial cell membranes

Membrane transport of L-proline has received considerable attention in basic and pharmaceutical research recently. Of the most recently cloned members of the solute carrier family, two are "proline transporters". The amino acid transporter PAT1, expressed in intestine, kidney, brain and other organs, mediates the uptake of proline and derivatives in a pH gradient-dependent manner. The Na(+)-dependent proline transporter SIT1, cloned in 2005, exhibits the properties of the long-sought classical IMINO system. Proline-containing peptides are of interest for several reasons. Many biologically important peptide sequences contain highly conserved proline residues. Xaa-Pro peptides are very often resistant to enzymatic hydrolysis and display, in contrast to Pro-Xaa peptides, a high affinity to the H(+)/peptide cotransporter PEPT1 which is expressed in intestinal, renal, lung and biliary duct epithelial cells. Furthermore, several orally available drugs are recognized by PEPT1 as Xaa-Pro analogues due to their sterical resemblance to small peptides.

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

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

An L-proline-dependent proton flux is located at the apical membrane level of the eel enterocytes

This study has demonstrated the existence of an L-proline-dependent (Na independent) proton flux at the apical membrane level of the eel intestinal absorbing cells. Using isolated eel enterocytes and the pH-sensitive fluorescent dye 2', 7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein, acetoxymethyl ester (BCECF), it was shown that a 20 mM concentration of the imino acid L-proline in the extracellular medium determined an intracellular acidification of approximately 0.28 pH units. However, neither sucrose nor other amino acids were able to significantly acidify the resting intracellular pH. A hyperbolic relationship between extracellular proline concentration and intracellular proton accumulation was observed. Using both isolated brush-border and basolateral membrane vesicles, it was demonstrated that this proline-proton cotransport mechanism was located at the apical membrane level only. In addition, the existence of a coupling mechanism between proline and proton fluxes was demonstrated by the observation that, in brush-border membrane vesicles, the presence of a pH gradient (pH(in) > pH(out)) stimulated the uptake of L-proline.

Effects of zinc on L-[3H]proline uptake by lobster (Homarus americanus) hepatopancreatic brush-border membrane vesicles

Epithelial brush-border membrane vesicles (BBMVs) from the hepatopancreas of the lobster Homarus americanus were prepared using a magnesium precipitation technique and employed in transport experiments designed to demonstrate the effects of external and internal divalent cationic heavy metals on the uptake of l-[(3)H]proline. When BBMVs were exposed to a high external concentration (2.5 mmol l(−)(1)) of Cd(2+), Cu(2+), Fe(2+), Mn(2+) or Zn(2+), l-[(3)H]proline (0.5 mmol l(−)(1)) uptake was significantly (P<0.05) decreased by each metal. However, if a 30 min pre-incubation period with each metal was used before incubation of the vesicles with amino acid and metal, a significant (P<0.05) enhancement of l-[(3)H]proline transport occurred. Zinc was the most stimulatory metal of those tested. Proline influxes (1.0 and 2.5 mmol l(−)(1)) were hyperbolic functions of bilateral [Zn(2+)], with a lower apparent zinc half-saturation constant (K(m)) at the higher amino acid concentration. l-[(3)H]proline influx was a hyperbolic function of external [l-proline] (K(m)=2.10+/−0.26 mmol l(−)(1); J(max)=2290+/−600 pmol mg(−)(1)protein 10 s(−)(1)) (means +/− s.e.m., N=3), and bilateral exposure to zinc significantly (P<0.05) increased the maximal rate of influx, J(max), of proline (J(max)=4890+/−250 pmol mg(−)(1)protein 10 s(−)(1)), but had no effect (P>0.05) on apparent l-[(3)H]proline binding to the membranes (K(m)=1.66+/−0.23 mmol l(−)(1)) (means +/− s.e.m., N=3). In the presence of 0.5 mmol l(−)(1)l-pipecolate, bilateral zinc-stimulated, carrier-mediated, l-[(3)H]proline influx was abolished. At low external concentrations of zinc alone (e.g. below 1.0 mmol l(−)(1)), l-[(3)H]proline influx was enhanced by the metal. Enhanced amino acid uptake in the presence of external zinc alone was abolished by l-pipecolate. A model accounting for external and internal zinc enhancements of l-[(3)H]proline influx by the Na(+)-dependent l-pipecolate-sensitive IMINO transport system in these membranes is proposed.

Solubilization and reconstitution of high- and low-affinity Na(+)-dependent neutral L-alpha-amino acid transporters from rabbit small intestine

High- and low-affinity Na(+)-dependent neutral L-alpha-amino acid transporters were solubilized with 0.25% octaethylene glycol dodecyl ether (C12E8) after removal of the proteins from the brush-border membrane vesicles with 2% CHAPS and 4 M urea. When the CHAPS-insoluble protein was treated with papain before its solubilization with C12E8, a substantial amount of protein was removed without any decrease of the transport activities. The solubilized transporters were reconstituted into proteoliposomes after removal of C12E8 with Bio-Beads SM2. Several parameters proved to be important for optimal reconstitution efficiency: (a) the type of detergent, and (b) the phospholipid/protein and detergent/protein ratio during reconstitution, and (c) the salt concentration during reconstitution. Reconstituted proteoliposomes showed rapid uptake of neutral L-alpha-amino acids but not imino acid, basic or acidic amino acids driven by an electrochemical potential of Na+ (out > in). The uptakes under low- and high-substrate condition were further augmented by an artificial membrane potential introduced by K+ diffusion via valinomycin (negative interior). Kinetic analysis revealed that both the brush-border membranes and the solubilized fraction involved two carrier-mediated pathways for alanine transport. The kinetic parameters were determined by curve fitting with a computer to be Kt1 = 0.28 mM (0.21 mM) and Kt2 = 43.2 mM (28.4 mM), respectively (those with brush-border membrane vesicles in parentheses). Studies on the specific activities for transport of individual amino acids under low or high substrate concentration and the cross-inhibitory effects of various amino acids on alanine uptake (low concentration) revealed that these transporters possess broad specificity for neutral L-alpha-amino acids.

The Na(+)-dependent proline carrier, of eel intestinal brush-border membrane, sequentially binds proline and then Na+

The mechanism of Na+/L-proline cotransport, present on brush-border membrane (BBM) vesicles of the European eel intestine, was studied. Initial cotransport rates, depending on increasing proline and Na+ concentrations in the extravesicular medium (zero-trans conditions), were measured by monitoring the decay of an inside-negative membrane potential, i.e. the fluorescence quenching of the voltage-sensitive cyanine dye 3,3'-diethylthiacarbocyanine iodide (DiS-C2(5)). By simultaneously estimating the substrate-dependent Na(+)-influx (with the fluorescent dye) and the Na(+)-dependent [3H]substrate influx, it was concluded that proline was cotransported with 1 Na+ ion and glucose with 2 Na+ ions. The kinetics of proline/Na+ cotransport were then investigated. Graphical analysis excluded a ping-pong mechanism. Under rapid equilibrium assumptions, by fitting model equations to rate values it was possible to exclude the random and the ordered Na+/proline mechanisms. Therefore, in eel intestinal BBM vesicles, the mechanism of proline/Na+ cotransport is ordered and prolineout binds to the carrier prior to Na+out.