Small intestinal sugar and amino acid transport in semistarvation.

The effect of semistarvation on small intestinal transport of D-glucose, L-valine, and NaCl was studied in an in vitro system of isolated rat brush border membrane vesicles. Whereas semistarvation enhanced the transport rate for L-valine by 19-29%, there was no change in D-glucose transport. When energy in the form of a NaSCN gradient was supplied to the membrane vesicles prepared from semistarved animals, L-valine was concentrated to a greater extent than those from well-fed animals. Strain differences were observed in the manner semistarvation affected NaCl transport across the brush border membrane. Semistarvation increased the NaCl transport rate by a factor of 3.5 in one rat strain and not at all in another. These results provide a partial explanation for the cellular basis of elevated neutral amino acid absorption by the small intestine in semistarvation.     

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.     

Sodium-coupled amino acid and sugar transport byNecturus small intestine

Summary Necturus small intestine actively absorbs sugars and amino acids by Na-coupled mechanisms that result in increases in the transepithelial electrical potential difference ( ^ms ) and the short-circuit current (I _sc) which can be attributed entirely to an increase in the rate of active Na absorption. Studies employing conventional microelectrodes indicate that the addition of alanine or galactose to the mucosal solution is followed by a biphasic response. Initially, there is a rapid depolarization of the electrical potential difference across the apical membrane ( ^ms ) which reverses polarity (i.e. cell interior becomes positive with respect to the mucosal solution) and a marked decrease in the ratio of the effective resistance of the mucosal membrane to that of the serosal membrane (R ^m /R ^s ); these events do not appear to be dependent on the availability of metabolic energy. These initial, rapid events are followed by a slow increase in (R ^m /R ^s ) toward control values which is paralleled by a repolarization of ^ms and increases in ^ms andI _sc; this slow series of events is dependent upon the availability of metabolic energy.The results of these studies indicate that: (i) the Na-coupled mechanisms that mediate the entry of sugars and amino acids across the apical membrane are rheogenic (conductive) and result in a decrease inR ^m and a depolarization of ^ms ; and (ii) the subsequent increase in (R ^m /R ^s ) and repolarization of ^ms are the results of a decrease inR ^s which is associated with an increase in the activity of the Na pump at the basolateral membrane.The physiologic implications of these findings are discussed and an equivalent electrical circuit model for rheogenic Na-coupled solute transport processes is analyzed.     

Effect of phenethylbiguanide on sugar and amino acid transport in rabbit ileum

Phenethylbiguanide has been shown to be an inhibitor of sugar and amino acid uptake in both $\text{in vivo}$ and $\text{in vitro}$ conditions. This action could be due to a competition for sodium sites on the sugar and amino acid carrier molecules. The effects of phenethylbiguanide on $\text{in vitro}$ intestinal preparations indicate that this compound has a time-dependent effect, it is most effective when placed on the mucosal surface but is also effective on the serosal surface. Furthermore, competition studies indicate that it is a competitive inhibitor of sugar uptake and a non-competitive inhibitor of amino acid uptake. These results are consistent with the differences in the mechanism of coupled transport between sugars and amino acids, but, do not substantiate the idea that phenethylbiguanide competes for the sodium site on the ternary carrier.     

On the mechanism of sugar and amino acid interaction in intestinal transport.

The influence of amino acids on D-glucose transport was studied in isolated vesicles of brush border membrane from rat small intestine. It is demonstrated that: (a) Uptake of D-glucose by the membranes is inhibited by simultaneous flow of L- and D-alanine into the vesicles. (b) Addition of L-alanine to membranes pre-equilibrated with D-glucose causes efflux of this sugar. (c) The influence of amino acids on D-glucose is dependent on the presence of Na+. (d) The ionophorous agents monactin and valinomycin are able to prevent the transport interaction of D-glucose and amino acids. Monactin is effective in the presence of Na+ without further addition of other cations, while valinomycin is effective only with added K+, in accordance with the known specificity of these antibiotics. (e) The inhibitory effect increases with L-alanine concentration up to about 50 mM after which it levels off. The experiments provide evident that the Na+-dependent sugar and amino acid fluxes across the brush border membrane are coupled electrically.     

Calcium movements accompanying the transport of sugar or amino acid by rabbit enterocytes

Isolated rabbit enterocytes can be loaded with radioactive calcium most of which is presumably in intracellular stores. In the presence of sugar or amino acid there is a transient loss of calcium, followed by replenishment. It is suggested that this movement might be related in the signalling leading to increased potassium permeability observed in enterocytes transporting sugar and amino acids.     

Regulation of sugar transport and metabolism in lactic acid bacteria

Abstract The phosphoenolpyruvate (PEP)-dependent lactose: phosphotransferase system (PTS), P-β-galactosidase, and enzymes of the d -tagatose-6P pathway, are prerequisite for rapid homolactic fermentation of lactose by Group N ('starter') streptococci. Moreover, the reactions of transport and catabolism constitute an open cycle in which ATP and lactic acid are metabolic products. The efficient and controlled operation of this cycle requires 'fine-control' mechanisms to ensure: (i) tight coupling between transport and energy-yielding reactions, (ii) co-metabolism of both glucose and galactose moieties of the disaccharide, and (iii) coordination of the rate of sugar transport to the rate of sugar catabolism. The elucidation of these fine-control mechanisms in intact cells of Streptococcus lactis has required the isolation of glucokinase (GK) and mannose-PTS defective mutants, the synthesis of novel lactose analogs, and the use of high resolution [³¹P]NMR spectroscopy. It has been established that PEP provides the crucial link between transport and energy-yielding reactions of the PTS: glycolysis cycle, and that both ATP-dependent glucokinase and PEP-dependent mannose-PTS can participate in the phosphorylation of intracellular glucose. Finally, evidence has been obtained in vivo, for modulation of pyruvate kinase activity in response to fluctuation in, concentrations of positive (FDP), and negative (P_i) effectors of the allosteric enzyme. Fine-control of pyruvate kinase activity may in turn regulate: (i) the distribution of PEP to either the PTS or ATP synthesis, (ii) overall activity of the PTS: glycolysis cycle, and (iii) the formation of the endogenous PEP-potential in starved organisms. The accumulation of non-metabolizable PTS sugars (e.g., 2-deoxy- d -glucose) by growing cells can perturb these fine-control mechanisms and, by establishment of a PEP-dissipating futile cycle, may result in bacteriostasis.     

Role of transport systems in amino acid metabolism: leucine toxicity and the branched-chain amino acid transport systems.

The livR locus, which leads to a trans-recessive derepression of branched-chain amino acid transport and periplasmic branched-chain amino acid-binding proteins, is responsible for greatly increased sensitivity toward growth inhibition by leucine, valine, and serine and, as shown previously, for increased sensitivity toward toxicity by branched-chain amino acid analogues, such as 4-azaleucine or 5',5',5'-trifluoroleucine. These phenotypes are similar to those of relA mutants; however, the livR mutants retain the stringent response of ribonucleic acid synthesis. However, an increase in the rate of transport or in the steady-state intracellular level of amino acids in the livR strain cannot completely account for this sensitivity. The ability of the LIV-I transport system to carry out exchange of pool amino acids for extracellular leucine is a major factor in leucine sensitivity. The previous finding that inhibition of threonine deaminase by leucine contributes to growth inhibition is confirmed by simulating the in vivo conditions using a toluene-treated cell preparation with added amino acids at levels corresponding to the internal pool. The relationship between transport systems and corresponding biosynthetic pathways is discussed and the general principle of a coordination in the regulation of transport and biosynthetic pathways is forwarded. The finding that the LIV-I transport system functions well for amino acid exchange in contrast to the LIV-II system provides another feature that distinguishes these systems in addition to previously described differences in regulation and energetics.     

Relation between sodium-coupled amino acid and sugar transport and sodium/potassium pump activity in isolated intestinal epithelial cells

Cells isolated from the epithelium of the small intestine are used to study the relationship between amino acid or sugar-coupled sodium transport and potassium uptake through the sodium/potassium pump. Potassium influx is a saturable function of the external potassium concentration. Uptake in the presence of ouabain, a specific pump inhibitor, is greatly reduced. This remaining influx is linearly related to the concentration up to 6 mM potassium. Sugars and amino acids are actively accumulated by the intestinal cells. Their transport is accompanied by an initial extra influx of sodium. Although cells seem to regulate their internal sodium concentrations, this is not accompanied with a concomitant increase in potassium uptake through the pump. Thus L-alanine, 3-0-methyl-D-glucoside, and alpha-methyl-D-glucoside all fail to increase the rate of ouabain-sensitive potassium uptake. A very high coupling ratio of sodium efflux to potassium influx through the pump would be a likely explanation of the present results though they cannot be regarded as conclusive.     

Peptide and amino acid transport in Streptococcus bovis

Summary In amino acid transport studies with Streptococcus bovis using ¹⁴C-labelled amino acids, it has been shown that between 87% and 95% of cell-associated radioactivity was located in the cytosol. In similar studies with unlabelled peptides, most test peptide associated with S. bovis was truly intracellular. Using sodium dodecyl sulphate-polyacrylamide gel electrophoresis, the proteolytic activity in S. bovis was found to be largely cell-associated and of the serine-protease type, but stimulated by dithiothreitol. A wide range of extracellular peptide hydrolysing activities was demonstrated against the pentapeptide Leu-Trp-Met-Arg-Phe, which was completely hydrolysed to eight products after 10 min incubation. Some of this pentapeptide was transported intact, indicating the existence of mechanisms for the transport of peptides up to 751 Da. In studies with Arg-Phe-Ala, only Phe (F) and Ala (A), and to a much lesser extent Phe-Ala (FA) were transported after extracellular hydrolysis to FA, Arg (R), F and A. In this case, amino acid transport was much more predominant than peptide transport. The extent and nature of peptide transport was affected by the addition of protease inhibitors. Offprint requests to: R. I. Mackie