Rat enterokinase: the effect of ions and the localization in the intestine

The Global Status and Trends of Enteropeptidase: A Bibliometric Study

Background

Enteropeptidase (EP) is a type II transmembrane serine protease and a physiological activator of trypsinogen. Extensive studies related to EP have been conducted to date. However, no bibliometric analysis has systematically investigated this theme. Our study aimed to visualize the current landscape and frontier trends of scientific achievements on EP, provide an overview of the past 120 years and insights for researchers and clinicians to facilitate future collaborative research and clinical intervention.

Methods

Quantitative analysis of publications relating to EP from 1900 to 2020 was interpreted and graphed through the Science Citation Index Expanded of Web of Science Core Collection (limited to SCIE). Microsoft office 2019, GraphPad Prism 8, VOSviewer, and R-bibliometrix were used to conduct the bibliometric analysis.

Results

From 1900 to 2020, a total of 1,034 publications were retrieved. The USA had the largest number of publications, making the greatest contribution to the topic (n = 260, 25.15%). Active collaborations between countries/regions were also enrolled. Grant and Hermontaylor were perhaps the most impactful researchers in the landscape of EP. Protein Expression and Purification and the Journal of Biological Chemistry were the most prevalent (79/1,034, 7.64%) and cited journals (n = 2,626), respectively. Using the top 15 citations and co-citations achievements clarified the theoretical basis of the EP research field. Important topics mainly include the structure of EP, the affective factors for activating substrates by EP, EP-related disorders, and inhibitors of EP.

Conclusion

Based on the bibliometric analysis, we have gained a comprehensive analysis of the global status and research frontiers of studies investigating EP, which provides some guidance and reference for researchers and clinicians engaged in EP research.

Activation of Human Pancreatic Proteolytic Enzymes: The Role of Enteropeptidase and Trypsin

The role of enteropeptidase and trypsin in the process by which pancreatic proteolytic zymogens are converted into active enzymes has been investigated in the past, using purified enzymes and proenzymes of animal origin. In the present study, we wanted to study this process under conditions which come near to the physiological situation, which prevails in the human duodenum and upper small intestine.

Patients and Methods

Duodenal contents were collected from 2 patients with intestinal enteropeptidase deficiency. The samples expressed no tryptic activity and were used as the source of zymogens. Enteropeptidase or trypsin was added to these samples and the process of zymogen activation was followed by measuring trypsin and chymotrypsin activities.

Results

When exogenous trypsin was added to the duodenal contents of patients with enteropeptidase deficiency, having no tryptic activity, activation of intrinsic trypsinogen was not observed. When purified porcine or human enteropeptidase was added to the same samples of duodenal contents, this resulted in a rapid, dose-dependent activation of trypsinogen followed by the activation of chymotrypsinogen.

Conclusion

The study underlines the key role of enteropeptidase in the cascade process, which leads to the presence of active proteolytic enzymes in the human small intestine. The results also explain why patients with congenital deficiency of enteropeptidase are unable to activate trypsinogen by alternative pathways and therefore suffer from a severe disturbance of protein digestion with failure to thrive at young age, hypoproteinemia, and anemia.

Compensatory changes in villus morphology of lactating Mus musculus in response to insufficient dietary protein

Energetic challenges match intestinal size to dietary intake but less is known about how the intestine responds to specific macronutrient challenges. We examined how intestinal size responds to insufficient dietary protein at the microscopic level. Villi, enterocytes and surface area were measured across the length of the small intestine in non-reproductive and lactating Mus musculus fed isocaloric control or protein-deficient diets. Lactating mice on the protein-deficient diet exhibited a 24% increase in villus height and a 30% increase in enterocyte width in the proximal small intestine and an overall similar increase in surface area; on the control diet, changes in villus height were localized in the mid region. Flexibility localized to the proximal small intestine suggests that enterokinase, a localized enzyme, may be a candidate enzyme that promotes compensation for a protein-deficient diet. Such flexibility could allow species to persist in the face of anthropogenically induced changing dietary profiles.

Structure of murine enterokinase (enteropeptidase) and expression in small intestine during development

Enterokinase (enteropeptidase) is expressed only in proximal small intestine, where it initiates digestive enzyme activation by converting trypsinogen into trypsin. To investigate this restricted expression pattern, mouse enterokinase cDNA was cloned, and the distribution of enterokinase mRNA and enzymatic activity were determined in adult mice and during gestation. Analysis of enterokinase sequences showed that a mucinlike domain near the NH2 terminus is composed of repeated approximately 15-amino acid Ser/Thr-rich motifs. By Northern blotting and trypsinogen activation assays, enterokinase mRNA and enzymatic activity were undetectable in stomach, abundant in duodenum, and decreased distally until they were undetectable in midjejunum, ileum, and colon. By in situ mRNA hybridization, enterokinase mRNA was localized to the enterocytes throughout the villus. Expression was not observed in goblet cells, Paneth cells, or Brunner's glands. Enterokinase mRNA and enzymatic activity were not detected in the duodenum of fetal mice but were easily detected in the duodenum on postnatal days 2-6. Both enterokinase mRNA and enzymatic activity decreased to very low levels after day 7 but increased after weaning and reached a high level characteristic of adult life by day 60. Therefore, in mice, duodenal enterocytes are the major type of cells expressing enterokinase, which appears to be regulated at the level of mRNA abundance.

Posthatch development of mucosal function in the broiler small intestine

Changes in small intestinal morphology, in the indices of cellular development, and in the activities of some mucosal enzymes were examined in broilers from hatch through 14 d. In addition the effects of holding birds without access to feed for 36 h and of deutectomy were examined. Development of the intestine was rapid from 2 d after hatch although rates of development were different in duodenum, jejunum, and ileum. Villus volume in the duodenum reached a plateau after 7 d although it continued to increase in the jejunum and ileum. Enterocyte density changed little with age. Indices of tissue activity, ribosomal capacity, and cell size decreased with age but at differing rates at the three intestinal sites. Sucrase-maltase activity was low in the duodenum and highest in jejunum and ileum and increased in the jejunum to a maximum 2 d after hatch, then decreased. Delayed access to feed after hatch depressed mucosal development for several days, with the duodenum generally reaching control values earlier than the jejunum. Morphological changes following delayed access to feed included some clumping of microvilli on Day 1 after hatching and abnormal crypt structure between Days 7 and 9. Deutectomy resulted in initial decreased intestinal growth, but this was compensated for by 6 to 8 d in the duodenum, after which development was parallel to that of normal chicks. It appears that nutrient supply from yolk is less crucial for mucosal development then withholding feed for 36 h, which delays normal intestinal development.

Effect of chronic ethanol consumption on the activities of residual small bowel brush-border enzymes after proximal jejunum resection in the rat

Ethanol consumption has a toxic effect on the epithelium of the small bowel, but enterocyte maturity is very difficult to measure under these circumstances. However, when ethanol intake is combined with enterectomy, enterocyte immaturity is greater, permitting an easier separation of these two effects. In a group of rats (13 male Wistar rats weighing approximately 220 g) fed a liquid diet containing 35% ethanol for 4 weeks after resection of the proximal jejunum, the residual small intestine brush border maltase, sucrase, and lactase activities were similar to those of a pair-fed control group (13 animals). However, alkaline phosphatase activity was decreased in the mucosa and in the enterocyte brush border, probably because of the lower activity of this enzyme in the jejunum-ileum remnant of the alcoholic group.

Observation of duodenal bile shuttle in the sheep

Relation between duodenal anatomy and function was studied in cannulated sheep. It was concluded that during the interperistaltic periods, gravitational backflow of bile and admixed pancreatic juice towards the pylorus may provide a means for desorption of enterokinase from the mucosal surface of the cranial duodenum. The biliary back and forth ("bile shuttle") may contribute to the utilization of duodenal enterokinase.

Interaction of epidermal growth factor with specific binding sites of enterocytes isolated from rat small intestine during development

Isolated enterocytes from rat small intestine were characterized for their specific binding of epidermal growth factor (EGF). Intestinal epithelial cells were isolated at 4 degrees C to minimize the loss of receptor sites during the isolation procedure. 125I-labelled EGF binding to enterocytes from adult rats was found to be specific, saturable, temperature dependent and trypsin sensitive. Binding performed in the presence of a lysosomotropic agent (NH4Cl) increased the time required to reach maximal binding at 25 degrees C. NH4Cl had no significant effect on the time-course of EGF binding at 4 degrees C and 37 degrees C. A Scatchard plot showed a curvilinear relationship indicating that EGF binds to enterocytes with more than one binding site. Developmentally, enterocytes from fetuses and pups showed characteristic temperature dependence and trypsin sensitivity, but with different levels of binding to EGF. Specific EGF binding was demonstrably higher in enterocytes from small intestine of term fetuses. EGF binding to isolated enterocytes declined rapidly after birth, and the level stayed fairly constant thereafter. Pretreatment of enterocytes from fetal intestine with mature rat milk led to a dose-dependent decrease in EGF binding. These results suggest the presence of endogenous milk factors that modify EGF binding and account for, at least partly, the observed rapid decrease of EGF binding after birth.

Physiological factors controlling release of enterokinase from rat enterocytes

The quantitative release of enterokinase from isolated rat enterocytes following treatment with taurocholate-taurodeoxycholate, papain, chymotrypsin, elastase, carbamylcholine, and cholecystokinin-octapeptide was examined. Alkaline phosphatase and lactate dehydrogenase activities were evaluated simultaneously to check for specificity. Bile salts promoted a concentration-dependent release of all enzymes. Concomitantly, bile salts also led to cell destruction in proportion to the amount of enzymes released. Proteases caused the release of enterokinase and alkaline phosphatase with no concomitant increase of lactate dehydrogenase or cell lysis. At equal concentrations, papain released more enzymes than chymotrypsin and elastase. Chymotrypsin and elastase, however, led to higher ratios of enterokinase to alkaline phosphatase found in the media and suggested a selective release of enterokinase (EK) over that of alkaline phosphatase. Bile salts and pancreatic proteases together seem to have an additive effect of the release of EK. Carbamylcholine and cholecystokinin-octapeptide had no effect on enzyme release. These results suggested that pancreatic proteases are involved in the release of enterokinase by a selective action. Bile salts may also play a role through a nonselective detergent effect.

Investigations about influence of the content of plant crude protein in the ration on the utilization of urea in dairy cattle. 6. Digestive enzymes and their interactions in contents of proximal small intestine

Forty-two samples were taken from the contents of the proximal small intestine of two lactating dairy cows fitted with re-entrant duodenal cannula. Most samples were free of detectable amylase activity. The (chymo)trypsinogen present was only partially activated to (chymo)trypsin. The activation was continued in vitro: slowly at the original pH of the samples (between pH 2.8 and 4.2), and faster after neutralization or a slight alkalinization. The effect of Ca, EDTA and soybean inhibitor on the activation of trypsinogen was also studied. The pancreatic enzymes were inactive in the acid pH range of the samples, but pepsin was markedly active. At pH 3.8 casein was digested rapidly by purified pepsin and slowly by the samples (agar-plate experiments). In model experiments performed with purified enzymes, pepsin digested (chymo)trypsin rapidly at pH 1-2 and slowly at pH 3.8. In the intestinal juice (chymo)trypsin and their zymogens seemed to be unaffected by pepsin under the conditions of the samples. It is concluded that the conditions prevailing in the duodenum/upper jejunum of the experimental cows account for a gastric-type digestion, despite the presence of pancreatic enzymes. In vivo the intestinal contents pass in distal direction. Meanwhile the pH of the chyme gradually increases and gives rise to first an increase of enterokinase activity accounting for a faster activation of the zymogens; second a start of function of activated pancreatic proteases and third a gradual decrease of pepsin activity and finally to its irreversible denaturation. Thus the development of intestinal type digestion is delayed in ruminants.

Electrophoretic analysis of pancreatic proteases and zymogen-activating factors in the mouse

Mouse pancreatic proteases were analyzed by one- and two-dimensional electrophoresis. Active proteases that existed in the luminal fluid were separated into at least eight bands in 8% polyacrylamide gel. Pancreatic proteases activated by intestinal extract were separated into at least seven bands. The mobilities of these bands were exactly the same as those of proteases in the luminal fluid except for those of the most cathodal band. Two kinds of trypsin (Try-I group and Try-II) and one kind of chymotrypsin (Chy-I) were determined by specific and nonspecific protease staining. Try-I group and Try-II were derived from different trypsinogens (Try G-I group and Try G-II), whereas Chy-I was derived from a single chymotrypsinogen (Chy G). Although Try G-II was activated by both intestinal extract and by bovine trypsin, Try G-I group activated only by intestinal extract. Intestinal-activating factors were analyzed by two-dimensional electrophoresis. Mouse enterokinase (enteropeptidase EC 3.4.4.8), which can activate bovine trypsinogen, had a slow mobility. In the intestine of the mouse there are several activating factors in addition to enterokinase. Although it is unclear what intestinal-activating factors can activate Chy G, there is a factor that can convert chymotrypsinogen into chymotrypsin directly. These data suggest that intestinal-activating factors play an important role in the activating mechanisms of mouse pancreatic zymogens.

Effect of lectins from leguminous seeds on rat duodenal enterokinase activity

Enterokinase activity from rat duodenal brush borders was assayed in vitro in the presence of purified lectins from 3 leguminous seeds. Noncompetitive inhibition of the enzyme was observed in each case. Phaseolus hemagglutinin was the most potent inhibitor among the 3 lectins tested.

The application of a new synthetic substrate to the determination of enteropeptidase in rat small intestine and human intestinal biopsies

The application of a new synthetic substrate to the direct determination of enteropeptidase is described. The substrate Gly-(L-Asp)4-L-Lys-2-naphthylamide contains the amino acid sequence of the activation peptides of trypsinogen linked via an amide bond to the fluorophore 2-naphthylamine. The sequence of amino acids is responsible for the specificity and substrate recognition of the enteropeptidase-catalyzed activation of trypsinogen. Interference in the assay by trypsin is prevented by the addition of soybean trypsin inhibitor to the substrate solution. The fluorimetric determination of the liberated 2-naphthylamine allows the direct observation of the reaction kinetics. For the hyrolysis of the synthetic substrate by purified enteropeptidase the pH optimum was 8.2 and the Km 0.17 mmol/l. The new substrate was used to determine the distribution of enteropeptidase along the rat small intestine and also to measure enteropeptidase activity in human intestinal biopsies.

An automated continuous flow procedure for the determination of enterokinase

A continuous flow method has been developed for the automatic determination of enterokinase in rat small intesstine mucosa and/or luminal content. Trypsinogen was first hydrolysed by enterokinase under conditions which minimize autocatalytic activation. L-benzoyl-arginine paranitroanilide was then added and split to paranitroaniline by the trypsin so formed. Liberated paranitroalinine was diazotized and converted by the Bratton-Marshall reagent (N-naphthyl ethylene diamine) to an azodye, with maximum absorption at 550 nm. This method of determination was found to be six times more sensitive than the direct p-nitroaniline determination method. 36 determinations can be made hourly.

Regulation of enterokinase synthesis in animal and human small intestine by luminal signals: its implication in upper gastrointestinal surgery

Enterokinase is an enzyme produced by the mucosa of the small intestine. Its sole function is to activate trypsinogen to trypsin. In animals and man the duodenum and proximal jejunum have high levels of activity whereas the remaining small bowel has minimal levels. A reproducible assay was developed for measuring mucosal enterokinase activity applicable to operative and endoscopic biopsies. Anaesthetic and operative techniques were developed for small intestinal resections in guinea-pigs to ensure their long term survival. Transposition of high-enterokinase-secreting segments of guinea-pig small intestine to low-enterokinase regions and vice versa showed no alteration of enterokinase activity in the transposed segments. Similarly, resection of the enterokinase region in five proximal pancreaticoduodenectomy operations in man revealed no induction of enterokinase in the remaining jejunum at endoscopy 6 months later. Isolation of high-enterokinase-secreting segments of small bowel from their luminal continuity by fashioning of Thiry--Vella fistulas led to a decay of enterokinase activity to minimal levels within 12--16 h. Perfusion of these fistulas with trypsin and sodium, or chymotrypsin and sodium, prevented this decay. If the enterokinase was allowed to decay over 24 h its activity could be restored to 80 per cent of its normal level by perfusion for 24 h with trypsin and sodium. Trypsin and sodium acti in combination on an enterocyte membrane receptor to stimulate enterokinase synthesis.

Intestinal brush border enzymes and chronic alcohol ingestion

The effect of graded (5, 10, 20, and 50%) chronic ethanol administration on the intestinal brush border enzymic activities has been investigated in the rat at three levels of the intestinal tract (duodenum, jejunum, ileum). Ethanol has been administered for 8, 15, 30, and 90 days. A 30% to 50% decrease of sucrase and alkaline phosphatase results, showing that the effect of alcohol appears in the first 8 days of intoxication is not reversible after 8 days of an alcohol-free diet. The effect of ethanol is not limited to disaccharidases. Impairment of alkaline phosphatase, peptidases and also enterokinases is observed. The decrease is more marked in the duodenum and jujunum than the ileum. The decrease of enzymic activity is generally maximal after 30 days of intoxication. There is then little further deterioration or even significant improvement. At the 30th day of ethanol administration, a clearcut dose-response relationship has been established. The results obtained suggest that ethanol exerts an effect on the intestinal mucosa which is not directly correlated to morphological villus changes.

The role of enteropeptidase in the digestion of protein and its development in human fetal small intestine

Enteropeptidase converts trypsinogen into active trypsin, which not only hydrolyses some peptide bonds of food proteins but also activates a number of pancreatic zymogens. For this reason enteropeptidase is a key enzyme in the digestion of dietary proteins and its absence may result in gross protein malabsorption.

Effect of bile acids and pH on the release of enteropeptidase in man

Bile acids increase the release of human enteropeptidase as well as other brush-border enzymes (alkaline phosphatase, leucine aminopeptidase) from duodenal mucosa, as had been shown earlier in experimental animals. The action of bile acids is independent of their known enhancing effect on enteropeptidase activity. The pH of duodenal juice is an important, hitherto unrecognized, factor in the release mechanism of brush-border enzymes. All of the above enzymes tested were released to a markedly greater extent at pH 8.2 than 6.3, regardless of the presence or absence of bile acid. Contrary to some results obtained with animal tissue, by other investigators, our experiments with human duodenal mucosa indicate that enteropeptidase, under all conditions tested, is released at a rate considerably greater than that for alkaline phosphatase or leucine aminopeptidase. The looser association of enteropeptidase with cellular components relative to that of other brush-border enzymes, as indicated by our observations, may be related to the unique function of enteropeptidase as the trigger enzyme of protein digestion.

Subcellular localization of enterokinase (enteropeptidase EC 3.4.21.9) in rat small intestine

The subcellular localization of enterokinase is controversial. In this study, enterokinase was extracted from a soluble fraction and a brush border fraction of rat small intestine by differential centrifugation. The soluble fraction contained 41% of the initial enterokinase activity while the brush border fraction contained only 4.6% of the initial activity. In contrast, alkaline phosphatase monitored as a brush border marker, yielded 26.3% in the brush border fraction and only 6% in the soluble fraction. Further separation of the soluble fraction on a Sepharose 4B column revealed three peaks of enterokinase activity. One small peak (3%) of a bound enzyme (Mr, 2 - 10(6)) and two larger peaks of free enzyme (Mr, 3 - 10(5) and 9 -10). In contrast, alkaline phosphatase major fraction was in a high molecular weight peak of bound enzyme. When the brush border fraction was chromatographed only a single peak of bound enterokinase and alkaline phosphatase were found. In the lower part of the small intestine, no brush border-bound enterokinase was found, while the peak of alkaline phosphatase was the same as in the upper intestine. These data suggest that enterokinase activity in the rat intestine is mainly in a free form localized in the mucin and soluble fraction and to a negligible extent in the brush border.

Immunofluorescent localisation of enterokinase in human small intestine

The distribution of enterokinase in human intestine was studied in operative mucosal biopsies using specific antiserum to human enterokinase, previously purified to apparent homogeneity by affinity chromatography and immunoabsorption. Fluorescence was observed in the brush-border and glycocalyx of the duodenum and proximal 15 cm of jejunum distal to the D/J flexure. Distal jejunum and ileum as well as stomach and colon were consistently negative. Brunner's glands and goblet cells were never stained by specific antibody. Preliminary evidence was obtained that the human enterokinase molecule contains a specific antigenic determinant in its polypeptide component and a second determinant in the oligosaccharide moiety which cross-reacts with blood group A. Preliminary evidence was also obtained that mucosal synthesis of enterokinase may be impaired in jaundice due to carcinoma of the pancreas and induced in the small intestine distal to the normal limit of synthesis after pancreatico-duodenectomy.

Identification of a mucosal form of enteropeptidase in triton X-100 extracts of porcine duodenal mucosa

Porcine enteropeptidase (EC 3.4.21.9) purified from acetone powders of fresh duodenal fluid shows a molecular weight, as determined on Ultragel AcA-34, of 190000. Enteropeptidase has been solubilised from pig intestinal mucosa using 1% (v/v) Triton X-100. When Triton X-100 extracts of freeze-dried mucosa after partial fractionation on DEAE-cellulose were chromatographed on Sephadex G-200, the bulk of the activity eluted in the void volume rather than with an expected Ve/V0 ratio of about 1.24 corresponding to a molecular weight of around 200000. Gel filtration of aqueous mucosal extracts obtained in the absence of Triton X-100 showed two regions of enzymic activity in approximately equal proportions, one in the void volume, and the other with the expected Ve/V0 ratio of 1.24, whereas the Triton X-100 extracts of the residue from the above extract showed the presence of only the macromolecular species of enteropeptidase. This species was excluded from Sepharose 4B. It was confirmed that aminopeptidase was also extracted by Triton X-100 in a molecular form which was excluded from Sepharose 4B. The results suggest that Triton X-100 extracts enteropeptidase with a membrane component attached and in agreement with this it was found that proteolysis rapidly converted the macromolecular form to a stable smaller molecular species corresponding in size to that found in solution in the duodenal fluid. There was full recovery of the enzymic activity following this conversion. Papain and trypsin brought about an almost complete conversion to the smaller form of enteropeptidase whereas chymotrypsin, pancreatin and an intestinal peptidase preparation were only partially effective. It is concluded that membrane bound enzymes such as enteropeptidase and aminopeptidase are bound to the intestinal brush border membrane in a similar manner and are not actively secreted into the lumen but rather are largely released or solubilised by the combined action of the bile and pancreatic secretions.

The effect of lanthanide ions on enteropeptidase-catalyzed activation of trypsinogen

Rare earths were found to be powerful inhibitors of enteropeptidase-catalyzed (enterokinase, EC 3.4.21.9) activation of trypsinogen. Inhibition was complete at a La3+ concentration of 12.5-10(-6) M in the assay system used and still detectable at a concentration of 1.25-10(-6) M. Inhibition was observed with all lanthanides tested. No significant differences between individual metals could be established under the conditions of the inhibition assay. Increasing ionic strength decreased enzyme activity and progressively diminished the inhibitory effect of rare earth ions suggesting an electrostatic basis for the mechanism of this inhibition. La3+ did not significantly affect enteropeptidase-mediated hydrolysis of N-benzoyl-L-arginine ethyl ester. Its inhibitory effect on activation of trypsinogen by enteropeptidase, therefore, must be attributed to interaction with the zymogen rather than the enzyme. Kinetic measurements show that inhibition by rare earths is noncompetitive in nature. Binding of lanthanides to the tetraaspartyl sequence near the aminoterminus of trypsinogen may prevent this group from interacting with a critical specificity subsite on the enzyme.

The assay of exocrinous peptidases in clinical chemistry

The various peptidases secreted by such exocrine tissues as gastric mucosa, pancreas and prostate are usually determined by catalytic methods. Another approach utilizes immunoassay. Endopeptidases were formerly assayed with protein substrates such as hemoglobin and albumin. These techniques are increasingly replaced by more specific ones using artificial peptide derivatives as substrates, some of which allow an increase in absorbance or fluorescence to be continuously recorded. The presently available methods of assaying pepsin, pancreatic trypsin, trypsinogen and carboxypeptidase A, enterokinase and several peptidases of human sperm are reviewed.

Modification of an assay for trypsin and its application for the estimation of enteropeptidase

A sensitive method for the estimation of trypsin and enteropeptidase is described. The use of alpha-N-benzoyl-DL-arginine-p-nitroanilide as substrate in combination with the Bratton-Marshall reaction increased the sensitivity of an established method to a degree which permits the determination of enteropeptidase at considerably lower activity levels than heretofore such as those expected in brush border membrane preparations from peroral biopsies of human small intestine.

[Digestion and resorption of proteins]

A survey is given of the ultrastructure of the intestinal epithelial cell, especially of its lumenward membrane. Special attention is paid to the peptidases which are located in the ciliated border and within the cell. The authors deal with the purification of the membrane-bound aminopeptidase which is of importance in splitting dietary peptides and illustrate its specificity by the cleavage of casein. The amino acids which are liberated by peptide splitting have in part aminopeptidase-inhibiting properties. The possible digestion physiological consequences are discussed. In vitro experiments were performed to investigate the composition of the content of the distal part of the small intestine of the rat with regard to its possible dependence on the composition of various dietary proteins. The composition of the peptides of the intestinal content is essentially undependent of the amino-acid composition of the diet. There is no enrichment of certain amino acids. The importance of the resorption of the peptides is also evidenced by resorption studies in which enzymatic hydrolysates of proteins (i.e. peptide mixtures) were confronted with a free amino-acid mixture of the same over-all composition. Taking glycyl-glycine-glycine as a model, the authors demonstrate that, in determined ranges of concentration, tripeptides may in part be resorbed without degradation. Finally, the importance of peptide resorption is evaluated and conclusions are drawn as to further studies on the physiology and physiopathology of digestion.

Changes in sucrase, enterokinase, and peptide hydrolase after intestinal resection. The association of cellular hyperplasia and adaptation

In a study of changes in digestive enzymes after massive intestinal resection and the mechanisms by which such changes occur, rats were sacrified 4 wk after removal of the proximal two-thirds of the small intestine. Alterations in the mucosal levels of sucrase, enterokinase, and dipeptide hydrolase (L-leucyl-L-alanine substrate) were examined in the light of associated changes in protein. DNA and wet mucosal weight, measured in standardized gut segments from various regions of intestine. Metabolic studies showed that normal growth patterns were reestablished after the operation but significant elevations in stool weight and fecal nitrogen occurred in the second postoperative week, falling towards normal by the 4th wk. In standard gut segments wet weight of mucosa, protein, and DNA rose, especially in distal segments, DNA increasing disproportionately. Mucosal levels of the proximally distributed and membrane-bound enzymes, sucrase and enterokinase, showed similar patterns of change: when enzyme activity was expressed in terms of the total per segment, proximally there were considerable increases in both enzymes, but, expressed in terms of specific activity, that of sucrase fell and that of enterokinase was unaltered. By contrast, the largely soluble and more distally distributed dipeptide hydrolase increased more in distal segments and the increases in total activity were accompanied by lesser increases in specific activity. However, in spite of increases in total activity, enzyme activity per milligram DNA fell by over 50% in postanastomotic segments. Subcellular distribution studies showed no change in the percentage of the total activity which was membrane-bound and zymograms confirmed that no new dipeptide hydrolase had appeared after resection. It is concluded that increases in the segmental totals of various enzymes seen after resection are achieved by disproportinate increases in the number of mucosal cells per segment and that the greatest change in a particular enzyme occurs in the region where the enzyme is normally found in highest concentration.

Enterokinase in normal intestinal biopsies and those from patients with untreated coeliac disease

Enterokinase was measured in peroral intestinal biopsies showing normal histology and in those from untreated coeliac patients which showed gross villous atrophy. There was no significant difference in the specific activity of enterokinase between these two groups. These results do not support the recent hypothesis that enterokinase is a brush border enzyme, but would be consistent with the idea that it is adsorbed to the cell membrane following secretion.