Absorption Of Free Amino Acids Research Articles

Digestion of milk proteins: Comparing static and dynamic in vitro digestion systems with in vivo data

In the frame of the COST action INFOGEST, a static in vitro digestion protocol has been elaborated aiming at the improvement of data comparability by harmonizing the experimental conditions. The success in harmonization was confirmed with inter-laboratory trials using skim milk powder as a standardized model food. Moreover, the physiological relevance of the gastric and intestinal endpoints of the static digestion protocol was demonstrated in a pig in vivo trial, with the same skim milk powder and samples collected from different sections of the digestive tract, as well as in a human study with from jejunal effluents. In vivo, digestion is a dynamic process influenced by peristalsis and by the gradual secretion of enzymes and juices and the dwell time of the food. To mimic these physiological mechanisms, dynamic in vitro digestion protocols are widely used. Until now, the differences of protein hydrolysis taking place during dynamic and static in vitro digestion have not been investigated. In this study, the gradual hydrolysis of the main milk proteins present in skim milk powder was digested with the dynamic DIDGI®-system using adult digestion protocol and the static harmonized INFOGEST method. Protein hydrolysis was analyzed by gel electrophoresis, peptide patterns were measured with mass spectrometry, and free amino acids with high pressure liquid chromatography. The peptide patterns at the gastric and intestinal endpoints of in vitro digestion showed a good approximation to the in vivo results from pigs. Moreover, gradual peptide generation was comparable in both in vitro digestion conditions. However, the dynamic protocol reflected the physiological situation better at the level of free amino acid release. Nonetheless, in both in vitro digestion protocols, absorption of free amino acids is not simulated, and they are therefore limited in reflecting the in vivo situation at this level.

Evaluation of role of the peptide transport system in absorption of dipeptides in the rat small intestine in chronic experiments in vivo

Hydrolysis and absorption of glycylgycine and glycyl-L-leucine as well as absorption of glycine and leucine were studied in chronic experiments on rats with their isolated small intestine loop. Values of the true kinetic constants (with taking into account effect of the preepithelial layer) were determined to be as follows: 1) Kt = 46.7 +/- 4.0 and 2.15 +/- 0.59 mM; Jmax = 0.74 +/- 0.15 and 0.16 +/- 0.03 micromol x min(-1) x cm(-1) (for the transport of free glycine and leucine, respectively); 2) Kt = 4.4 +/- 0.6 and 4.8 +/- 0.9 mM, Jmax = 0.24 +/- 0.02 and 0.23 +/- 0.02 micromol x min(-1) x cm(-1) (for the transport of glycylgycine and glycyl-L-leucine, respectively); 3) Km = 5.4 +/- 1.0 and 38.2 +/- 4.4 mM, Vmax = 0.09 +/- 0.02 and 0.24 +/- 0.07 micromol x min(-1) x cm(-1) (for membrane hydrolysis of these dipeptides, respectively). According to our calculations, in the wide range of the initial glycylgycine concentrations (2.5-40 mM) the part of the peptide component in its total absorption accounts for 0.77-0.80. In the case of glycyl-L-leucine the part of the peptide component in the total glycine absorption decreases from 0.89 to 0.84, while in the total leucine absorption--from 0.86 to 0.71, the initial dipeptide concentration rising from 5 to 40 mM. The obtained results show that the peptide component prevails in absorption of the studied dipeptides in the rat small intestine, but its role is much lesser that what many authors believe. In the case of glycyl-L-leucine, the peptide component can achieve saturation in the range of high substrate concentrations, its part decreasing essentially to become compared with absorption of free amino acids formed as a result of the dipeptide membrane hydrolysis.

Nutritional physiology during development of Senegalese sole ( Solea senegalensis)

The Senegalese sole, a species with a complex metamorphosis, difficulties in weaning and with occasional problems of malpigmentation and skeletal deformities, is a good model species to study larval nutritional physiology. In addition, the early metamorphosis and acquisition of a peculiar non-proactive bottom-feeding behaviour make early weaning an important issue in sole hatcheries. The present work reviews recent findings in different aspects of nutritional physiology during the development of Senegalese sole, in an attempt to optimize the composition of sole diets and to understand what are the limiting factors for weaning sole. Both digestive enzymes activity and tracer studies using 14C- Artemia show that sole larvae, even at young stages, have a high capacity for digesting live preys. This is reflected in a high growth potential and low mortality rates for this species during the larval stage compared to other marine fish species. Based on the observation of the digestive enzymes profile, early introduction of inert microdiets in co-feeding with Artemia does not seem to affect intestinal function. However, when co-feeding is not provided, intestinal activity may be depressed. Furthermore, early introduction of microdiets in co-feeding with Artemia may have a positive effect on survival rates, but at the expense of lower growth rates and higher size dispersal. This may reflect variation in the adaptation capacity of individual larvae to inert diets. High dietary neutral lipid (soybean oil) content results in reduced growth and accumulation of lipid droplets in the enterocytes and affects the capacity of Senegalese sole larvae to absorb and metabolise dietary fatty acids (FA) and amino acids (AA). Through tube feeding of different 14C-lipids and free FA it has been shown that FA absorption efficiency increases with unsaturation and that sole larvae spare DHA from catabolism. In addition, it was demonstrated that absorption efficiency varies according to molecular form, being highest for free FA, lowest for triacylglycerols and intermediate for phospholipids. Live preys commonly used in larviculture do not seem to have a balanced AA profile for sole larvae. Furthermore, the ideal dietary AA composition probably changes during development. Rotifers and Artemia metanauplii are apparently deficient in one or more of the following AA depending on the larval development stage: histidine, sulphur AA, lysine, aromatic AA, threonine and arginine. It has also been demonstrated that balancing the dietary AA profile with dipeptides in Artemia-fed larvae increases AA retention and reduces AA catabolism. When supplementing larval diets with limiting AA it should also be considered that sole larvae have different absorption, and retention efficiencies for individual AA, and that they have the capacity to spare indispensable AA. In addition, the absorption of free AA is faster and more efficient than that of complex proteins. Improvements in biochemical composition of inert microdiets for sole are likely to contribute to the reproducible weaning success of Senegalese sole.

Absorption, assimilation and catabolism of individual free amino acids by larval Atlantic halibut ( Hippoglossus hippoglossus)

Following the completion of the lecithotrophic phase, most marine larvae rely on an incompletely developed digestive tract to absorb amino acids (AA) and other nutrients needed for rapid growth. Despite their undeveloped state, larvae must absorb nutrients in sufficient amounts to fuel exceptionally high rates of growth. This study examined the ability of larval Atlantic halibut to absorb, assimilate and catabolise dispensable (alanine, glutamate) and indispensable (arginine, lysine) dietary free amino acids (FAA) using tube-fed AA solutions with 14C tracers. Absorption of FAA was rapid with an average of 71% absorbed from the gut within 30 min after tube feeding. Evacuation of FAA by larvae was low, averaging only 6% of tube-fed dose. Dispensable amino acids (DAA) were catabolised in greater proportion (17%) than indispensable FAA (54%). Saturation of FAA transporters was not attained under the present conditions (20 mM, 20% midgut filling). Absorption rates did not differ significantly between the four FAA.

Influence of the gut flora and of some growth-promoting feed additives on nitrogen metabolism in pigs. II. Studies in vivo

Analysing the gastrointestinal contents of 22 slaughtered pigs, it was found that about 10% and 25% of the non-protein nitrogen (NPN) in the small and large intestine respectively, consisted of ammonia derived from deamination of amino acids and from urea degradation. Also the decarboxylation of amino acids was important. Both deamination and decarboxylation processes were greatly reduced by nutritional levels (20 p.p.m.) of Virginiamycin and Spiramycin. From the analysis of the N and urea content of the urine it could be deduced that Virginiamycin and Spiramycin can enhance N retention in pigs. Digestibility experiments with re-entrant cannulated pigs on a semi-purified diet revealed that Virginiamycin (50 p.p.m.) increases ( P < 0.15–0.30) the apparent ileal digestibility of nitrogen (+2.1%), lysine (+1.4%), glycine (+4.8%), valine (+2.2%) and methionine (+3.3%). Absorption experiments based on perfusion of a temporarily isolated loop (2 m) of the terminal small intestine of the pig, demonstrated that Virginiamycin enhances the net absorption of free amino acids by about 9% ( P < 0.05–0.001). Growth promotion by nutritional antibacterials can therefore partly be explained by an altered digestion, absorption and retention of N in the pig.

Habitat selection in two species of aquatic hyphomycetes

Two species of aquatic hyphomycetes,Tetracladium marchalianum andTricladium splendens, were isolated from decaying leaves in a stream.T. marchalianum was abundant on alder leaves but absent on beech leaves, which were dominated byT. splendens. It was hypothesized that differences in some chemical key factors in the leaves would account for differences in the distribution of the 2 species. In the experiment designed to test the hypothesis, combinations of sterilized leaves and isolated fungi were used. Differences in growth of FDA active mycelium were related to differences in leaf weight loss;T. splendens decomposed beech leaves andT. marchalianum decomposed alder leaves. Extracellular protease activity corresponded with these trends but there was no detectable protein loss in the leaves. Both fungi showed a nitrogen demand, and hydrolysis of leaf proteins was complemented with absorption of free amino acids and ammonium. High concentrations of free amino acids modified the pattern for habitat selection so thatT. splendens grew substantially on alder leaves andT. marchalianum colonized beech leaves. No protease activity was, however, found fromT. marchalianum on beech leaves, and it is concluded that a more general metabolic inhibition prevents extensive growth of this species on beech leaves. The low natural abundance ofT. splendens on alder leaves, where it may grow well, may be a consequence of a specific protease inhibition and competition from other species.

Characterization and nutritional significance of peptide transport in man.

An in vivo intestinal perfusion technique has been used to investigate the characteristics of di- and tripeptides absorption in man. The results suggest that the model peptides studied were absorbed by a transport system separate to those responsible for mediating absorption of free amino acids. Two modes of intestinal uptake have been identified, namely transport of unhydrolysed peptides and transport of free amino acid residues liberated from the peptides by the action of brush border peptidases. A consistent finding was that amino acid residues were absorbed more rapidly from di- and tripeptides than from free amino acids. When amino acid uptake from four partial enzymic hydrolysates of protein was investigated, the 'kinetic advantage' conferred by peptides on amino acid uptake was found to be a generalized and concentration-dependent phenomenon. As later test meal studies showed that orally administered peptide mixtures were assimilated at least as rapidly as free amino acid mixtures, serious consideration could be given to using the less hypertonic and more palatable oligopeptide-based nitrogen sources in chemically defined elemental diets.

Effect of oral amino acid supplementation on liver disease after jejunoileal bypass for morbid obesity

Previous work in our laboratory and others suggests that protein malnutrition plays an important role in the pathogenesis of hepatic steatosis and dysfunction which characteristically appears after jejunoileal bypass for morbid obesity. Postoperative protein-calorie malnutrition is at least in part a consequence of diminished intestinal absorption of free amino acids. In an attempt to prevent liver disease, six morbidly obese patients were orally supplemented with essential amino acids for 4 months after surgery. Oral amino acid supplementation only partially influenced protein malnutrition and had no effect on deterioration of hepatic morphology and dysfunction. Although this mode of therapy appears to be ineffective in preventing postoperative liver abnormalities, other studies suggest that oral oligopeptide supplementation and parenteral administration of amino acids are beneficial. In addition to protein deificiency, other factors which may contribute to the development of liver disease are reviewed.

Absorption from a mixture of seventeen free amino acids by the isolated small intestine of the rat.

Absorption and secretion from a mixture of seventeen free amino acids has been measured in isolated perfused rat small intestine. 2. The absorption rate of an amino acid from this mixture is proportional to its concentration in the perfusate and independent of its chemical constitution. The constant of proportionality is the same as that previously observed when the perfusate contained peptides as well as amino acids. 3. Amino acids are concentrated, on average, sixfold during passage across the mucosa, and the free amino acid composition of the secretion into the tissue fluid is very similar to that of the luminal perfusate. 4. Peptides do not appear to be added to the tissue fluid during absorption of free amino acids. 5. It is concluded that the mechanisms for absorption of free amino acids are in general independent of those for absorption of peptides.

The Intestinal Absorption of Amino Acids and Peptides with Special Reference to the Domestic Fowl: A Literature Review

SummaryThe transport of amino acids via the chick small intestine has been shown to be an active process, obeying Michaelis-Menten type kinetics and having preference for L amino acids. Different transport systems can be discriminated for methionine and related aliphatic amino acids, proline and related amino acids, glycine and the basic amino acids. Maximum transport of the amino acids usually occurs in the second half of the small intestine. The amino acid transfer is sodium-dependent and is inhibited by sugars, although the exact nature of these interrelations is still a matter of debate. Metabolic inhibitors as well as vitamin deficiencies reduce amino acid transport. The age and pyhsical condition of the animal may have a marked effect on amino acid transfer. From the nutrition viewpoint the uptake of protein digestion products in the form of small peptides may be more important than the absorption of free amino acids.

1Absorption of amino acids and peptides from the intestine

Until recently it was generally believed that proteins underwent complete hydrolysis within the lumen of the gut, and that only free amino acids were taken up by the mucosa of the small intestine. It has now been shown that this picture of protein absorption was incomplete, and that though, with a few exceptions, intact peptides do not enter the blood, there is another important mode of protein absorption involving mucosal uptake of small peptides followed by cellular hydrolysis. A great deal is known about the mechanisms of absorption of free amino acids, and this subject has beeia covered in a number of recent reviews (Wiseman, 1968; Gray and Cooper, 1971; Matthews, 1971). This review will deal mainly with the differences between the characteristics of peptide absorption and those of amino acid absorption, the mechanisms of peptide absorption, and the clinical and nutritional implications of absorption of peptides. Where references are not given, they will be found in more detailed accounts (Matthews, 1971, 1972, 1974). Standard three-letter abbreviations for free and peptide-bound amino acids will be used throughout. Amino acids should be taken to be the Lforms unless otherwise indicated.