Release of peptide hydrolases during incubation of intact intestinal segments in vitro

Intestinal dipeptidases and the dipeptide transport in the monkey and in man

Among the powerful dipeptidases of the cytosol fraction the well-characterized 'master' dipeptidase of broad substrate specificity is an example of a 'true' dipeptidase. There are only a limited number of peptidases which together can hydrolyse the total range of the theoretically possible dipeptides. In the lumen, the dipeptides of dietary origin apparently enter the cell through a single transport system of broad specificity in the monkey and possibly also in man. Perfusion studies indicated impared absorption of glycine and Gly-Gly in Indian compared to English subjects, but the kinetic advantage of dipeptide uptake over that of the free amino acid was maintained. In patients with tropical sprue, apart from decreased peptide absorption, there was an increased 'backflow' of the constituent amino acids. The back-flow assumes nutritional significance since dipeptide uptake is competitively inhibited by amino acids of a certain specificity. Inhibition of brush border glycylglycine hydrolase (glycylglycine dipeptidase, EC 3.4.13.1) by L- leucine is non-competitive while that of cytosol enzyme is competitive. Certain other amino acids have no effect on glycylleucine hydrolase (glycylleucine dipeptidase, EC 3.4.13.2) from either fraction but inhibit Gly-Leu uptake. Thus, the inhibition of dipeptide uptake appears to be a consequence of interaction of amino acids directly with the dipeptide transport system and not with dipeptidases.

Dipeptide transport in the intestinal mucosa of developing rabbits

Influxes of glycyl-L-proline (a dipeptide which is not hydrolysed in the membrane and is transported intact across the brush border) and of glycyl-L phenylalanine (a dipeptide which has affinity for the peptide transport system and is hydrolysed at the brush border membrane) have been studied in the small intestine of fetal, newborn and suckling rabbits. For glycyl-L-phenylalanine, transport as the intact dipeptide and 'membrane hydrolysis + amino acid transport' have been measured separately by using glycyl-L-proline and L-leucine as selective inhibitors of each pathway. For comparison, uptake of free glycine and of free phenylalanine has also been studied. The intestine of newborn rabbits is shown to have a translocation process for intact dipeptides which is saturable with a low Kt and stimulated by sodium ions, and which is not shared by free amino acids. This process resembles that described in adult animals, except that the maximal velocity is much higher in newborns. The developmental pattern of this uptake process for dipeptides differs markedly from that of free glycine, thus providing a new type of evidence for the distinction between amino acid and dipeptide transport processes. The developmental pattern of the free phenylalanine uptake process also differs from the development of the 'superficial hydrolysis + amino acid transport' component of glycl-L-phenylalanine uptake. These data suggest that the advantage of mucosal uptake of peptides, compared to the uptake of free amino acids, is much greater in the early stages of postnatal life than in the adult.

Enteral nutrition: background, indications and management

Enteral nutrition is only part of the wider field of clinical nutrition in which great advances in both theory and practice have been made over the last decade. We have attempted to summarize what we consider to be the advances that have most relevance to the clinical practice of enteral nutrition. This chapter reviews our present understanding of the processes of digestion and absorption of protein, carbohydrate and fats, and examines how this theoretical understanding can be applied to patients in the clinical situation. A broad classification of the different enteral diets is undertaken, and the reasons for the development of particular diets are discussed. The clinical value of these diets is assessed. The wide variety of indications for enteral (as opposed to parenteral) nutrition are discussed and the specific benefits of enteral nutrition for the patient are highlighted. Techniques of administration of enteral nutrition are reviewed in detail, and the methods by which enteral nutrition should be monitored are outlined. Finally, complications of enteral nutrition are summarized and advice given on how to prevent or treat them.

Release of enzymes from rat jejunal mucosa by bile salts

The technique of segmental perfusion was applied in vivo to study the release of enzymes from the mucosa cells under the influence of bile salts. Five brush border membrane enzymes, five cytosolic, one mitochondrial and two lysosomal enzymes were measured. Spontaneous release (Ringer solution), due to their superficial localization, was greatest for enteropeptidase and alpha-glucosidase. 3 and 10 mmol/l sodium taurocholate, and 0.5 and 3.0 mmol/l chenodeoxycholate were used. Surprisingly, the majority of cytosolic enzymes was released to a greater extent than membrane enzymes. On the other hand, lysosomal or mitochondrial enzymes were low or absent in the perfusion medium, a finding that excludes a serious injury to the mucosa cells. It must be inferred, therefore, that cytosolic enzymes are regularly externalized during the digestive process, the glycocalyx perhaps being a common matrix for pancreatic, membrane-associated and cytosolic enzymes.

Different pathways for lysine transport across neonatal pig intestine

1. Undirectional fluxes of lysine, presented free and in the form of a dipeptide, have been measured in vitro using the small intestines of pigs taken during the first 2 weeks of post-natal life. 2. The concentration dependence of free lysine transfer to the serosal surface of new-born pig intestine (Jms) can be described as the sum of a saturable (Km 0.2 mM; Jm 3 n- mole cm-2 in-1) and a non-saturable (1.0 n-mole cm-2 min-1 mM-1) component. 3. The dipeptide isoleucyl-lysine was hydrolysed completely during transfer to the serosal surface of intestines taken at all stages of development. The concentration dependence of peptide-derived lysine transfer can be described as if it took place through a single saturable process (Km 5 mM; Jm 27 n-mole cm-2 min-1). 4. The mucosa to serosa transfer of free lysine fell to a half and that of peptide-derived lysine fell to a quarter, during the first 8 days of post-natal life. The decline in peptide-derived lysine transport correlated with changes in peptide hydrolase activity, measured in homogenates of intestinal mucosa using isoleucyl-lysine as substrate. 5. The new-born pig intestine transported lysine derived from isoleucyl-lysine twice as quickly as from lysyl-isoleucine. Lysine and isoleucine did not inhibit the transfer of isoleucine and lysine derived from isoleucyl-lysine. The transport of lysine, but not isoleucine, from isoleucyl-lysine took place more quickly than from equimolar solutions of the free amino acids. 6. It is suggested from these results that lysine and isoleucyl-lysine use different pathways when crossing the intestinal mucosa. Attempts are made to assess the probable contributions made by these pathways to the total absorption of lysine taking place during neonatal development.

[Spontaneous solubilization of mcuosal peptide hydrolases in rat small intestine]

In vitro incubations of small intestine preparation lead to the spontaneous release of mucosal enzymes into the incubation medium, due to superficial dissolution and membrane lesions. One-hour anaerobic incubation of everted small intestine segments produces the liberation of from 13% (leucine arylamidase) to 30% (Gly-L-Pro-splitting peptide hydrolase) of the mucosal enzyme activities. Under the conditions of in vivo perfusion, the solubilization rates are from 50 to 80% lower than these values. The tendency to solubilization is only in part reduced to a significant extent by the saturation of the incubation medium with oxygen, but it does not reach the in vivo level. The detectability of membrane-bound and intracellular marker enzymes in the incubation medium permits no conclusions concerning the physiological localization of the released peptidases.

Amino acid and peptide absorption from partial digests of proteins in isolated rat small intestine

1. Absorption of each of sixteen amino acids, free and peptide-bound, has been measured in isolated rat small intestine perfused with five partial digests of proteins. 2. At low concentrations net absorption of each amino acid was proportional to its luminal concentration and independent of the nature of the amino acid. 3. A series of first-order multiple regressions was found to describe well the characteristics of absorption. 4. Rate constants for disappearance of free and peptide-bound amino acids from the lumen were closely similar. However, substantial back-flux occurred of amino acids derived from peptide hydrolysis. Hence 60-70% of the amino-N entering the serosal tissue fluid probably had left the lumen as free amino acids. 5. Intact peptides crossed the mucosa during absorption from a soy bean hydrolysate and in substantial quantities during absorption from one casein digest but not from another. With other hydrolysates there was no evidence for passage of peptides to the serosa. 6. In several cases there was a serious discrepancy between the amount of amino-N absorbed from the lumen and the amount accounted for as peptide or free amino acid in the serosal secretion. 7. The characteristics of absorption were similar (apart from the exceptions in 5 above) for all the digests studied except for soy bean hydrolysate.

Structure, function, and long-term maintenance of the isolated turtle colon

We describe the 5-day maintenance of sacs of turtle colonic mucosa in enriched bathing solutions. The mean maximum transepithelial potential difference (PD) developed by the sacs in Ringer solution enriched with tissue-culture medium and gassed with 95% air-5% CO2 was 126 mV at 24 hours. Lower values were observed in other solutions. The PD of 24-hour sacs was partially or totally inhibited by ouabain, replacement of Na by choline in mucosal bathing fluids, or removal of Ca from serosal bathing fluids. The sacs transported Na in excess of H2O forming a dilute mucosal solution. The response of four different sac preparations (normally oriented or everted, and stripped normally oriented or everted) to long incubation were compared. Stripped normally oriented tissue developed the highest PD and maintained the lowest water content. The morphology of fresh and long-incubated tissue was examined. This investigation demonstrates that the turtle colon can be maintained in vitro for long periods, and it provides information on the morphology and physiology of this tissue.

Relationships between mucosal hydrolysis and transport of two phenylalanine dipeptides

In order to investigate the source of free amino acids found in the gut lumen during absorption of dipeptides, as well as evaluating the role of brush border peptidases in the mucosal hydrolysis of dipeptides during absorption, rates of dipeptide disappearance and appearance of hydrolytic products were measured during perfusion of rat jejunum and ileum in vivo with buffered and unbuffered 10 mM solutions of glycl-L-phenylalanine (Gly-Phe) and L-phenylalanyl-glycine (Phe-Gly). Mucosal brush border peptidase activity was then measured in the perfused segments in vitro at luminal pH and at two substrate concentrations. In addition cytosol peptidase activity in the perfused segments was measured at pH 7-4 and at 10 mM substrate concentrations. In the jejunum, there was a relationship between rates of free phenylalanine appearance in vivo (Phe-Gly greater than Gly-Phe) and rates of brush border (Phe-Gly greater than Gly-Phe) rather than cytosol (Gly-Phe greater than Phe-Gly) peptidase activities. No constant relationship between free phenylalanine appearance and hydrolysis of the dipeptides by either brush border or cytosol peptidases was observed in the ileal studies. These findings suggest that, in the jejunum, hydrolytic products originate from the surface of the cell whereas, in the ileum, hydrolytic products originate from both the intracellular compartment as well as from the surface of the mucosal cell. In the jejunum, in vitro rates of brush border hydrolysis of Gly-Phe were always less than in vivo disappearance rates, whereas rates of Phe-Gly brush border hydrolysis always exceeded luminal disappearance rates. These data imply that Gly-Phe is predominantly transported intact and hydrolysed by cytosol peptidases, In contrast, brush border peptidases play an importnat role in the mucosal hydrolysis of Phe-Gly.