Isolation and characterization of brush border membrane vesicles from pig small intestine

Extensive in vitro gastrointestinal digestion markedly reduces the immune-toxicity of Triticum monococcum wheat: implication for celiac disease

SCOPE

The ancient diploid Triticum monococcum is of special interest as a candidate low-toxic wheat species for celiac disease patients. Here, we investigated how an in vitro gastro-intestinal digestion, affected the immune toxic properties of gliadin from diploid compared to hexaploid wheat.

METHODS AND RESULTS

Gliadins from Triticum monococcum, and Triticum aestivum cultivars were digested using either a partial proteolysis with pepsin-chymotrypsin, or an extensive degradation that used gastrointestinal enzymes including the brush border membrane enzymes. The immune stimulatory properties of the digested samples were investigated on T-cell lines and jejunal biopsies from celiac disease patients. The T-cell response profile to the Triticum monococcum gliadin was comparable to that obtained with Triticum aestivum gliadin after the partial pepsin-chymotrypsin digestion. In contrast, the extensive gastrointestinal hydrolysis drastically reduced the immune stimulatory properties of Triticum monococcum gliadin. MS-based analysis showed that several Triticum monococcum peptides, including known T-cell epitopes, were degraded during the gastrointestinal treatment, whereas many of Triticum aestivum gliadin survived the gastrointestinal digestion.

CONCLUSION

The pattern of Triticum monococcum gliadin proteins is sufficiently different from those of common hexaploid wheat to determine a lower toxicity in celiac disease patients following in vitro simulation of human digestion.

Characteristics of (+)-catechin and (-)-epicatechin transport across pig intestinal brush border membranes

BACKGROUND/AIMS

(+)-Catechin and (-)-epicatechin are considered as disease preventive flavan-3-ols of foods like fruits, beverages and chocolate. We investigated mechanisms and kinetics of (+)-catechin and (-)-epicatechin uptake employing a validated in vitro model with isolated pig brush border membrane vesicles.

METHODS

Vesicles were isolated from pig small intestine employing the divalent cation method. Characterization (marker enzymes, electron microscopy) confirmed their purity and function. Transport studies with (+)-catechin and (-)-epicatechin under predefined conditions [presence/absence of sodium, pH gradient, temperature (8-37 degrees C), various initial substrate concentrations (2-20 mmol/l)] revealed a measurable transport (HPLC analyses) across the brush border membrane for both substrates.

RESULTS

Catechin transport was stimulated by an outwardly directed H(+) gradient (pH(i) 5.5/pH(o) 7.5). The presence of an inwardly directed Na(+) gradient did not result in a transient overshoot in (+)-catechin and (-)-epicatechin uptake. At 37 degrees C, subtraction of diffusion from the total transport rate showed saturation kinetics.

CONCLUSION

Our in vitro study indicate that both (+)-catechin and (-)-epicatechin are transported across the basolateral membrane using a dual transport system consisting of free diffusion (dominant at low concentrations) and carrier-mediated facilitated diffusion.

Na(+)-dependent and Na(+)-independent transport of L-arginine and L-alanine across dog intestinal brush border membrane vesicles

We prepared intestinal brush border membrane vesicles (BBMVs) from beagle dogs fed a commercial diet (protein content: 24-26%), and investigated the characteristics of transport for basic and neutral amino acids across the intestinal BBMVs. To determined the kinetic parameters for L-arginine and L-alanine uptake, their total uptake was resolved into three routes: (1) Na(+)-dependent carrier-mediated transport; (2) Na(+)-independent carrier-mediated transport; and (3) simple diffusion. We could observe subtle, but clear-cut, Na(+)-dependent basic amino acid transport for the first time among studies with intestinal BBMVs prepared from mammals fed a normal diet. The Na(+)-dependent system for L-arginine transport can be best characterized as 'low affinity, low capacity', in contrast to that for L-alanine transport, which is 'low affinity, high capacity'. Maximal velocities of the Na(+)-dependent carrier-mediated transport are estimated to be higher for both L-arginine and L-alanine in dog intestinal BBMVs than in rabbit intestinal BBMVs reported previously. These results suggest that food habit of mammals is an important factor to decide the characteristic of system B0,+, a Na(+)-dependent carrier-mediated transport system common to basic and neutral amino acids across intestinal brush border membranes, as is protein content of the diet.

The effect of surgical bowel manipulation and anesthesia on intestinal glucose absorption in rats

The effects of surgical bowel manipulation and anesthesia on intestinal glucose absorption were determined in chronically catheterized rats. Total and passive rates of glucose absorption were measured using 3-O-methyl-glucose (3OMG) and L-glucose, metabolically inert analogues of D-glucose. The rates of 3OMG absorption immediately postoperative and 4 h later were 86 and 62% less than the absorption rate 6 d postoperative. The absorption rates of 3OMG 1 and 2 d postoperative were not different from 6 d postoperative. Absorption of L-glucose was not altered by bowel manipulation and anesthesia. Even after correction for the increased resistance of the unstirred water layer (UWL) after bowel manipulation, the rates of total and active intestinal glucose absorption immediately postoperative were only 11 and 15% of predicted rates of absorption. In chronically catheterized rats, > 75% of luminal 3OMG at a concentration of 400 mM was absorbed by active transport. The Km and Vmax of 3OMG active transport corrected for the resistance of the UWL were 11.3 mM and 15.6 mumoles/min, respectively. We conclude that measurements of intestinal glucose absorption performed within 24 h of surgical bowel manipulation greatly underestimate active absorption even if corrections are made to account for the increased resistance of the UWL.

Active transport of 3-O-methyl-glucose by the small intestine in chronically catheterized rats

A method is described for determining the fraction of intestinal 3-O-methyl-glucose (3OMG) absorption that occurs by active transport in chronically catheterized rats without the influence of anesthesia or surgical bowel manipulation. That fraction was determined by simultaneously measuring portal venous-aortic blood concentration gradients (delta C) of 3-O-methyl-glucose (3OMG) and L-glucose, metabolically inert analogues of D-glucose. 3OMG is actively and passively absorbed by the same mechanisms as D-glucose, L-glucose is only passively absorbed. The fraction of 3OMG that is actively transported was calculated from the difference between 3OMG and L-glucose absorption, divided by total 3OMG absorption. We found that more than 94% of 3-O-methyl-glucose is absorbed by active transport when luminal concentrations range from 50 to 400 mM. We conclude that in unrestrained, unanesthetized chronically catheterized rats, most 3OMG is actively absorbed by the intestine even at high luminal concentrations.

Anionic amino acid uptake by microvillous membrane vesicles from human placenta

The transport mechanisms for anionic amino acids in trophoblast microvillous (maternal facing) membrane were investigated by characterization of L-[3H]aspartate and L-[3H]glutamate uptake in membrane vesicles. Uptake of the anionic amino acids was by a single high-affinity Na+-dependent K+-stimulated cotransporter that is pH sensitive and electrogenic. A second Na+-dependent transporter could not be discriminated, and there was no observable Na+-independent uptake. An outwardly directed K+ gradient (100 mM KCl inside) resulted in a 5- to 10-fold stimulation in glutamate uptake in the presence of Na+. Intravesicular KCl had no effect on transporter affinity but increased transporter velocity in a concentration-dependent manner. Inhibition of Na+-K+-dependent uptake of L-aspartate and L-glutamate (20 mM, 30 s) by 2 mM unlabeled amino acids demonstrated stereoselectivity for L-glutamate but not for L-aspartate. The neutral amino acids (L-alanine, L-threonine, L-serine, L-cysteine, L-phenylalanine) were not effective inhibitors. These data are consistent with an anionic amino acid transporter in the microvillous membrane of the trophoblast, which has characteristics qualitatively similar to the X-AG system found in other epithelia. This system may mediate the concentrative placental uptake of anionic amino acids from maternal blood in utero.

Activation of chloride conductance in pig jejunal brush border vesicles

Requirements for the activation of Cl- conductance have been investigated in pig jejunal brush border vesicles. The stability of ATP as a substrate for protein kinase activity, the stability of the phosphoprotein product of protein kinase action, and the choice of buffer system used for vesicle preparation were studied as variables which affected the outcome of in vitro activation attempts. Arsenate was selected as the most effective agent in protecting ATP from hydrolysis by the phosphatase activity in this vesicle system. Brush border vesicle protein appeared to prevent the accumulation of phosphoprotein in a cAMP-dependent protein kinase reaction, and vesicle protein only had phosphate acceptor activity when KF was added as a presumptive inhibitor of phosphoprotein phosphatase. A Cl- conductance response to a potassium gradient and valinomycin was present in vesicles prepared in buffers containing tetramethylammonium. Cl- conductance activity was not increased in this system by the addition of ATP, dibutyryl cyclic AMP, and cyclic AMP-dependent protein kinase. There was no Cl conductance response to a potassium gradient in vesicles buffered with imidazolium-acetate. Incorporation of ATP, AsO4(3-), and F- into these nonconductive vesicles by homogenization, followed by addition of dibutyryl cAMP, produced substantial conductance activity. Maximal activation of Cl- conductance was obtained with vesicles prepared in imidazolium-acetate buffering, using precautions to stabilize ATP and phosphoprotein prior to conductance measurements.

Proton efflux from rat intestinal basal-lateral membrane vesicles is stimulated by ATP and Na+

1. ATP-stimulated 22Na uptake and 14C-methylamine efflux were studied in inside-out rat intestinal basal-lateral membrane vesicles (BLMV). 2. Uptake of 22Na by basal-lateral membrane vesicles was stimulated by addition of ATP and by an acidic vesicle interior. 3. Efflux of 14C-methylamine was stimulated by ATP and Na+. 4. 14C-methylamine efflux was not influenced by vanadate or amiloride by themselves but was inhibited by the presence of both agents. 5. These data are consistent with a basal-lateral proton translocation mechanism which may be responsible for alkalinization of the lateral intercellular space and implicates the Na+-pump in this mechanism.

Effect of Na+ on intestinal succinate transport and metabolism in vitro

The effect of Na+ on 14CO2 production from [14C]succinate was studied in isolated rat enterocytes, and Na+-dependent succinate transport was characterized in pig intestinal brush-border membrane vesicles. The production of 14CO2 from [14C]succinate by enterocytes was decreased 12-fold when Na+ was replaced by N-methyl-D-glucamine in the absence of glutamine and 20-fold in the presence of 0.2 or 0.5 mM glutamine. The ratio of 14CO2 produced from [1,4-14C]succinate to that produced by [2,3-14C]succinate was not affected by Na+ replacement, indicating that the pattern of tricarboxylic acid cycle metabolism was not altered. The uptake of [14C]succinate by brush-border membrane vesicles was stimulated 10-fold in the presence of 100 mM NaCl compared with 100 mM KCl. When succinate uptake was corrected to transport into an osmotically sensitive space, the magnitude of the Na+ stimulation was 20-fold. Succinate transport into brush-border membrane vesicles was Na+ dependent, electroneutral, nonconcentrative, with an apparent Na+-succinate coupling ratio of 2:1. Results of this study indicate that Na+-stimulated CO2 production by enterocytes can be explained by the effect of Na+ on succinate transport across the brush-border membrane.