Interaction of purified brush-border membrane aminopeptidase N and dipeptidyl peptidase IV with lectin-sepharose derivatives

The glycoprotein nature of two peptidases purified from the rat intestinal brush-border membrane was examined by their interaction with several lectin-Sepharose derivatives. Aminopeptidase N (EC 3.4.11.2), which contains 20% carbohydrate by weight, was bound minimally (less than 30%) by columns of Con A-, RCAI- and WGA-Sepharose. Alternatively, a greater proportion of dipeptidyl peptidase IV (EC 3.4.14.-) was bound by these immobilized lectins with 50% of the enzyme binding to Con A-Sepharose. Treatment of both enzymes with neuraminidase enhanced the binding of aminopeptidase to RCAI-Sepharose by 4-fold but did not alter the binding patterns of dipeptidyl peptidase IV. A sequential fractionation of the two peptidases with columns of Con A- and RCAI-Sepharose gave four fractions of each enzyme with differing lectin-binding specificities. Approximately 60% of the dipeptidyl peptidase IV interacted with either one or both of the lectins while only 30% of the aminopeptidase N did so. Kinetic analysis of the four isolated fractions revealed some differences, possibly related to variations in the carbohydrate moiety. The findings confirm that these two purified rat intestinal brush-border membrane peptidases are glycoproteins and, while they share a common physiologic function and source, they apparently have very different and possibly unique asparagine-linked oligosaccharide side-chains. In addition, a considerable degree of microheterogeneity exists in the carbohydrate structure of these two enzymes.     

Biosynthesis and degradation of altered immature forms of intestinal dipeptidyl peptidase IV in a rat strain lacking the enzyme.

We have used a strain of rat (Fischer 344) lacking brush border membrane dipeptidyl peptidase IV activity to examine its effect on the intestinal assimilation of prolyl peptides. In addition, we have examined the biochemical basis for the enzyme deficiency. An analysis of several brush border membrane hydrolases in different regions of the small intestine demonstrates that these rats lack only dipeptidyl peptidase IV. They also have a greatly reduced ability to hydrolyze and absorb in vivo peptides of the NH2-X-Pro-Y type which are known substrates for the enzyme. Immunoblot analysis with polyclonal and monoclonal antibody indicates that the animals lack an identifiable dipeptidyl peptidase IV protein in intestinal epithelial cells. Levels and types of dipeptidyl peptidase IV mRNA were analyzed in several tissues and found to be similar to that of control animals. Biosynthetic labeling of intestinal explants revealed that two distinct forms (102 and 108 kDa) of dipeptidyl peptidase IV are initially synthesized by deficient rats, in contrast to the single protein (106 kDa) observed in normal animals. Pulse-chase labeling experiments (+/- endoglycosidase H) show that these two altered forms of dipeptidyl peptidase IV, although initially glycosylated with N-linked high mannose carbohydrate, fail to be processed to the mature complex glycosylated form and undergo intracellular degradation.     

Equilibrium and kinetic analysis of the unusual binding behavior of a highly immunogenic gluten peptide to HLA-DQ2

HLA-DQ2 predisposes an individual to celiac sprue by presenting peptides from dietary gluten to intestinal CD4(+) T cells. A selectively deamidated multivalent peptide from gluten (LQLQPFPQPELPYPQPELPYPQPELPYPQPQPF; underlined residues correspond to posttranslational Q --> E alterations) is a potent trigger of DQ2 restricted T cell proliferation. Here we report equilibrium and kinetic measurements of interactions between DQ2 and (i) this highly immunogenic multivalent peptide, (ii) its individual constituent epitopes, (iii) its nondeamidated precursor, and (iv) a reference high-affinity ligand of HLA-DQ2 that is not recognized by gluten-responsive T cells from celiac sprue patients. The deamidated 33-mer peptide efficiently exchanges with a preloaded peptide in the DQ2 ligand-binding groove at pH 5.5 as well as pH 7.3, suggesting that the peptide can be presented to T cells comparably well through the endocytic pathway or via direct loading onto extracellular HLA-DQ2. In contrast, the monovalent peptides, and the nondeamidated precursor, as well as the tight-binding reference peptide show a much poorer ability to exchange with a preloaded peptide in the DQ2 binding pocket, especially at pH 7.3, suggesting that endocytosis of these peptides is a prerequisite for T cell presentation. At pH 5.5 and 7.3, dissociation of the deamidated 33-mer peptide from DQ2 is much slower than dissociation of its constituent monovalent epitopes or the nondeamidated precursor but faster than dissociation of the reference high-affinity peptide. Oligomeric states involving multiple copies of the DQ2 heterodimer bound to a single copy of the multivalent 33-mer peptide are not observed. Together, these results suggest that the remarkable antigenicity of the 33-mer gluten peptide is primarily due to its unusually efficient ability to displace existing ligands in the HLA-DQ2 binding pocket, rather than an extremely low rate of dissociation.     

Hydrolysis and transport of proline-containing peptides in renal brush-border membrane vesicles from dipeptidyl peptidase IV-positive and dipeptidyl peptidase IV-negative rat strains

In this investigation, we have demonstrated that the renal brush-border membrane of Fischer 344 rats from the Japanese Charles River Inc. specifically lacks dipeptidyl peptidase IV (DPP IV) activity, whereas the renal brush-border membrane of Fischer 344 rats from three different sources within the United States possesses normal levels of DPP IV activity. Comparison of the brush-border proteins between Charles River (U.S.A.) Fischer 344 rats (DPP IV positive) and Japanese Charles River Fischer 344 rats (DPP IV negative) revealed that a protein band (Mr = 100,000), apparently identical with DPP IV, was absent in the membranes from Japanese Charles River Fischer 344 rats. We examined the handling of radiolabeled beta-casomorphin fragment 1-5 (Tyr-Pro-[3H]Phe-Pro-Gly), a specific substrate for DPP IV, in renal brush-border membrane vesicles isolated from DPP IV-positive and DPP IV-negative rats. Although the membrane vesicles from DPP IV-positive rats were able to hydrolyze the pentapeptide to di- and tripeptides with the subsequent active transport of these products via the H+ gradient-dependent peptide transport system, the membrane vesicles from DPP IV-negative rats failed to hydrolyze the pentapeptide and hence lacked the ability to transport the radiolabel actively from the parent peptide. The H+ gradient-dependent glycyl-sarcosine uptake and the Na+ gradient-dependent proline uptake, however, were normal in DPP IV-negative rats. Urine analysis revealed that the DPP IV-negative rats excreted proline- and hydroxyproline-containing peptides in significantly increased amounts in their urine compared with control rats. Furthermore, following intravenous administration of Tyr-Pro-Phe-Pro-NH2, a peptide that is exclusively hydrolyzed by DPP IV, urinary excretion of the peptide in the intact form was many-fold greater in DPP IV-negative rats than in control rats. These data provide conclusive evidence for the obligatory role of DPP IV in the renal handling of proline (and hydroxyproline)-containing peptides.     

The preferred substrates for transglutaminase 2 in a complex wheat gluten digest are Peptide fragments harboring celiac disease T-cell epitopes

Background

Celiac disease is a T-cell mediated chronic inflammatory disorder of the gut that is induced by dietary exposure to gluten proteins. CD4+ T cells of the intestinal lesion recognize gluten peptides in the context of HLA-DQ2.5 or HLA-DQ8 and the gluten derived peptides become better T-cell antigens after deamidation catalyzed by the enzyme transglutaminase 2 (TG2). In this study we aimed to identify the preferred peptide substrates of TG2 in a heterogeneous proteolytic digest of whole wheat gluten.

Methods

A method was established to enrich for preferred TG2 substrates in a complex gluten peptide mixture by tagging with 5-biotinamido-pentylamine. Tagged peptides were isolated and then identified by nano-liquid chromatography online-coupled to tandem mass spectrometry, database searching and final manual data validation.

Results

We identified 31 different peptides as preferred substrates of TG2. Strikingly, the majority of these peptides were harboring known gluten T-cell epitopes. Five TG2 peptide substrates that were predicted to bind to HLA-DQ2.5 did not contain previously characterized sequences of T-cell epitopes. Two of these peptides elicited T-cell responses when tested for recognition by intestinal T-cell lines of celiac disease patients, and thus they contain novel candidate T-cell epitopes. We also found that the intact 9mer core sequences of the respective epitopes were not present in all peptide substrates. Interestingly, those epitopes that were represented by intact forms were frequently recognized by T cells in celiac disease patients, whereas those that were present in truncated versions were infrequently recognized.

Conclusion

TG2 as well as gastrointestinal proteolysis play important roles in the selection of gluten T-cell epitopes in celiac disease.     

Celiac sprue: correlation with murine T cell responses to wheat gliadin components

Celiac sprue is a disease in humans that is characterized by small intestinal mucosal injury and malabsorption. Dietary exposure to gliadin and similar proteins in rye and barley activates the disease in susceptible individuals. Celiac sprue appears to be the only disease with a marked HLA-association in which the proteins that activate the disease currently are well known. However, bread wheat gliadins are a complex mixture of proteins that contain at least 40 different components. In the present study we have purified the major gliadin components of Scout 66 wheat and used these proteins to examine murine T cell proliferative responses to gliadin. Differences in T cell proliferation stimulated by alpha-, beta-, gamma-, and omega-gliadins paralleled the known structural differences among these proteins. After priming with whole gliadin, the components that stimulated T cell proliferation were the same as those recognized to activate celiac sprue in humans. Studies with reduced and alkylated A-gliadin (i.e., S-methyl A-gliadin) suggested that epitopes determined by the native conformation of A-gliadin may be important in its interaction with T cells. By using three different A-gliadin peptides that span the entire molecule, T cell proliferative responses were shown to be stimulated predominantly by antigenic determinants on the NH2-terminal peptide.     

A Food-Grade Enzyme Preparation with Modest Gluten Detoxification Properties

Background and Aims

Celiac sprue is a life-long disease characterized by an intestinal inflammatory response to dietary gluten. A gluten-free diet is an effective treatment for most patients, but accidental ingestion of gluten is common, leading to incomplete recovery or relapse. Food-grade proteases capable of detoxifying moderate quantities of dietary gluten could mitigate this problem.

Methods

We evaluated the gluten detoxification properties of two food-grade enzymes, aspergillopepsin (ASP) from Aspergillus niger and dipeptidyl peptidase IV (DPPIV) from Aspergillus oryzae. The ability of each enzyme to hydrolyze gluten was tested against synthetic gluten peptides, a recombinant gluten protein, and simulated gastric digests of whole gluten and whole-wheat bread. Reaction products were analyzed by mass spectrometry, HPLC, ELISA with a monoclonal antibody that recognizes an immunodominant gluten epitope, and a T cell proliferation assay.

Results

ASP markedly enhanced gluten digestion relative to pepsin, and cleaved recombinant α2-gliadin at multiple sites in a non-specific manner. When used alone, neither ASP nor DPPIV efficiently cleaved synthetic immunotoxic gluten peptides. This lack of specificity for gluten was especially evident in the presence of casein, a competing dietary protein. However, supplementation of ASP with DPPIV enabled detoxification of moderate amounts of gluten in the presence of excess casein and in whole-wheat bread. ASP was also effective at enhancing the gluten-detoxifying efficacy of cysteine endoprotease EP-B2 under simulated gastric conditions.

Conclusions

Clinical studies are warranted to evaluate whether a fixed dose ratio combination of ASP and DPPIV can provide near-term relief for celiac patients suffering from inadvertent gluten exposure. Due to its markedly greater hydrolytic activity against gluten than endogenous pepsin, food-grade ASP may also augment the activity of therapeutically relevant doses of glutenases such as EP-B2 and certain prolyl endopeptidases.     

Evidence for tripeptide-proton symport in renal brush border membrane vesicles. Studies in a novel rat strain with a genetic absence of dipeptidyl peptidase IV

We have investigated the transport characteristics of L-phenylalanyl-L-prolyl-L-alanine in renal brush-border membrane vesicles isolated from Japan Fisher 344 rats. This particular rat strain genetically lacks dipeptidyl peptidase IV. Owing to the absence of this enzyme, the tripeptide was found to be completely resistant to hydrolysis by the renal brush-border membrane vesicles. Uptake of the tripeptide into these membrane vesicles in the presence of an inwardly directed Na+ gradient was slightly greater than in the presence of a K+ gradient, but there was no evidence for active transport. On the contrary, uptake was very rapid in the presence of an inside-alkaline transmembrane pH gradient, and accumulation of the tripeptide inside the vesicles against a concentration gradient could be demonstrated under these conditions. The uptake was drastically reduced by dissipation of the pH gradient. The uptake was stimulated by an inside-negative membrane potential and inhibited by an inside-positive membrane potential. Moreover, the uptake was greater in voltage-clamped membrane vesicles than in control vesicles. Many di- and tripeptides inhibited this pH gradient-stimulated uptake of Phe-Pro-Ala. The apparent dissociation constant for the tripeptide was 48 microM. High performance liquid chromatography analysis of the intravesicular content at the peak of the overshoot revealed that the tripeptide was transported across the membrane almost entirely in the intact form. These data provide the first direct evidence for the presence of an electrogenic tripeptide-proton symport in renal brush-border membranes.     

A dipeptidyl carboxypeptidase in brain synaptic membranes active in the metabolism of enkephalin containing peptides

A dipeptidyl carboxypeptidase activity has been localized in synaptic plasma membranes which have been prepared from isolated rat brain cortical synaptosomes. The specificity of this proteolytic activity towards various synthetic and biological active peptides is compared to the peptidase activities of intact synaptosomes. In contrast to the synaptosomal peptidases which are capable of cleaving all peptide bonds of Met-enkephalin-Arg6-Phe7 the peptidase activity associated with the synaptic plasma membrane exclusively hydrolyses a dipeptide from the carboxyl terminus of all hepta- and hexapeptides tested. The fact that this dipeptidyl carboxypeptidase does not cleave the Gly3-Phe4 peptide bond of Met-enkephalin suggests that this enzyme is different from "enkephalinase". The synaptic membrane dipeptidyl carboxypeptidase is inhibited by metal chelating agents and thiols but is not affected by compounds known to inhibit serine proteases, thermolysin and "enkephalinase".     

Characterization of the transport system for beta-lactam antibiotics and dipeptides in rat renal brush-border membrane vesicles by photoaffinity labeling

The uptake of the alpha-aminocephalosporin cephalexin into brush-border membrane vesicles from rat renal cortex was independent on an inward H+-gradient in contrast to the intestinal transport system. The transport system could be irreversibly inhibited by photoaffinity labeling. Two binding polypeptides for beta-lactam antibiotics and dipeptides with apparent molecular weights 130,000 and 95,000 were identified by photoaffinity labeling with [3H]benzylpenicillin and N-(4-azido[3,5-3H]benzoyl) derivatives of cephalexin and glycyl-L-proline. The uptake of cephalexin and the labeling of the respective binding proteins was inhibited by beta-lactam antibiotics and dipeptides as with intestinal brush-border membranes. These data indicate that the transport systems for beta-lactam antibiotics and dipeptides in the brush-border membrane from rat kidney and small intestine are similar but not identical.     

Role of hepatocyte nuclear factor 1alpha and 1beta in the transcriptional regulation of human dipeptidyl peptidase IV during differentiation of Caco-2 cells

Caco-2 cells undergo differentiation to an enterocytic-like cell when maintained in a post-confluent state for 1-2 weeks. During this period Caco-2 cells begin to express high levels brush border membrane associated enzymes such as dipeptidyl peptidase IV. Using the dipeptidyl peptidase IV gene promoter in electrophoretic mobility shift assays, we have shown for the first time that levels of hepatocyte nuclear factor 1alpha increase three- to fourfold during Caco-2 cell differentiation. Transient cotransfection experiments with 3T3 cells using dipeptidyl peptidase IV promoter constructs and expression vectors containing hepatocyte nuclear factor 1alpha and beta show that the ratio of alpha and beta modulates reporter gene expression. These results suggest that the increase in levels of hepatocyte nuclear factor 1alpha that occur during intestinal cell differentiation, are important for expression of dipeptidyl peptidase IV and other intestinal proteins.     

Purification of Two Dipeptidyl Aminopeptidases II from Rat Brain and Their Action on Proline-Containing Neuropeptides

From the soluble and membrane fractions of rat brain homogenate, two enzymes that liberate dipeptides of the type Xaa-Pro from chromogenic substrates were purified to homogeneity. The two isolated dipeptidyl peptidases had similar molecular and catalytic properties: For the native proteins, molecular weights of 110,000 were estimated; for the denatured proteins, the estimate was 52,500. Whereas the soluble peptidase yielded one band of pI 4.2 after analytical isoelectric focusing, two additional enzymatic active bands were detected between pI 4.2 and 4.3 for the membrane-associated form. As judged from identical patterns after neuraminidase treatment, both peptidases contained no sialic acid. A pH optimum of 5.5 was estimated for the hydrolysis of Gly-Pro- and Arg-Pro-nitroanilide. Substrates with alanine instead of proline in the penultimate position were hydrolyzed at comparable rates. Acidic amino acids in the ultimate N-terminal position of the substrates reduced the activities of the peptidases 100-fold as compared with corresponding substrates with unblocked neutral or, especially, basic termini. The action of the dipeptidyl peptidase on several peptides with N-terminal Xaa-Pro sequences was investigated. Tripeptides were rapidly hydrolyzed, but the activities considerably decreased with increasing chain length of the peptides. Although the tetrapeptide substance P 1-4 was still a good substrate, the activities detected for the sequential liberation of Xaa-Pro dipeptides from substance P itself or casomorphin were considerably lower. Longer peptides were not cleaved. The peptidases hydrolyzed Pro-Pro bonds, e.g., in bradykinin 1-3 or 1-5 fragments, but bradykinin itself was resistant. The enzymes were inhibited by serine protease inhibitors, like diisopropyl fluorophosphate or phenylmethylsulfonyl fluoride, and by high salt concentrations but not by the aminopeptidase inhibitors bacitracin and bestatin. Based on the molecular and catalytic properties, both enzymes can be classified as species of dipeptidyl peptidase II (EC 3.4.14.2) rather than IV (EC 3.4.14.5). However, some catalytic properties differentiate the brain enzyme from forms of dipeptidyl peptidase II of other sources.     

Comparison of two pig intestinal brush border peptidases with the corresponding renal enzymes.

Intestinal dipeptidyl peptidase IV and gamma-glutamyltransferase were compared to the corresponding kidney enzymes with respect to immunological and electrophoretic properties. The influences of selected effectors on the two enzymes were also studied. The two kidney peptidases exhibited the reaction of total identity with the corresponding intestinal enzymes in immunodiffusion. Furthermore, the intestinal dipeptidyl peptidase IV and gamma-glutamyl transferase showed the same inhibition patterns as the corresponding kidney enzymes and the acceptor specificity of the intestinal gamma-glutamyl-transferase was found to be identical to that of the kidney enzyme. The electrophoretic mobilities of dipeptidyl peptidase IV from the two organs differed greatly. The difference was almost abolished by treatment with neuraminidase, suggesting that the variation in mobility was due to different contents of sialic acid. It is suggested that the intestinal brush border peptidases, dipeptidyl peptidase IV and gamma-glutamyltransferase, are closely related to the corresponding enzymes obtained from the kidney.     

Variable Processing and Cross-presentation of HIV by Dendritic Cells and Macrophages Shapes CTL Immunodominance and Immune Escape

Dendritic cells (DCs) and macrophages (Møs) internalize and process exogenous HIV-derived antigens for cross-presentation by MHC-I to cytotoxic CD8⁺ T cells (CTL). However, how degradation patterns of HIV antigens in the cross-presentation pathways affect immunodominance and immune escape is poorly defined. Here, we studied the processing and cross-presentation of dominant and subdominant HIV-1 Gag-derived epitopes and HLA-restricted mutants by monocyte-derived DCs and Møs. The cross-presentation of HIV proteins by both DCs and Møs led to higher CTL responses specific for immunodominant epitopes. The low CTL responses to subdominant epitopes were increased by pretreatment of target cells with peptidase inhibitors, suggestive of higher intracellular degradation of the corresponding peptides. Using DC and Mø cell extracts as a source of cytosolic, endosomal or lysosomal proteases to degrade long HIV peptides, we identified by mass spectrometry cell-specific and compartment-specific degradation patterns, which favored the production of peptides containing immunodominant epitopes in all compartments. The intracellular stability of optimal HIV-1 epitopes prior to loading onto MHC was highly variable and sequence-dependent in all compartments, and followed CTL hierarchy with immunodominant epitopes presenting higher stability rates. Common HLA-associated mutations in a dominant epitope appearing during acute HIV infection modified the degradation patterns of long HIV peptides, reduced intracellular stability and epitope production in cross-presentation-competent cell compartments, showing that impaired epitope production in the cross-presentation pathway contributes to immune escape. These findings highlight the contribution of degradation patterns in the cross-presentation pathway to HIV immunodominance and provide the first demonstration of immune escape affecting epitope cross-presentation.     

Specific localization of membrane dipeptidase and dipeptidyl peptidase IV in secretion granules of two different pancreatic islet cells.

Endocrine cells require several protein convertases to process the precursors of hormonal peptides that they secrete. In addition to the convertases, which have a crucial role in the maturation of prohormones, many other proteases are present in endocrine cells, the roles of which are less well established. Two of these proteases, dipeptidyl peptidase IV (EC 3.4.14.5) and membrane dipeptidase (EC 3.4.13.19), have been immunocytochemically localized in the endocrine pancreas of the pig. Membrane dipeptidase was present exclusively in cells of the islet of Langerhans that were positive for the pancreatic polypeptide, whereas dipeptidyl peptidase IV was restricted to cells positive for glucagon. Both enzymes were observed in the content of secretory granules and therefore would be released into the interstitial space as the granules undergo exocytosis. At this location they could act on secretions of other islet cells. The relative concentration of dipeptidyl peptidase IV was lower in dense glucagon granules, where the immunoreactivity to glucagon was higher, and vice versa for light granules. This suggests that, in A-cells, dipeptidyl peptidase IV could be sent for degradation in the endosomal/lysosomal compartment during the process of granule maturation or could be removed from granules for continuous release into the interstitial space. The intense proteolytic activity that takes place in the endocrine pancreas could produce many potential dipeptide substrates for membrane dipeptidase. (J Histochem Cytochem 47:489-497, 1999)     

Proline-specific dipeptidyl peptidase from the blue blowfly Calliphora vicina hydrolyzes in vitro the ecdysiostatic peptide trypsin-modulating oostatic factor (Neb-TMOF)

To elucidate the mechanisms of inactivation of the ecdysiostatic peptide trypsin-modulating oostatic factor (Neb-TMOF) in the blue blowfly Calliphora vicina, we investigated its proteolytic degradation. In homogenates and membrane and soluble fractions, this hexapeptide (sequence: NPTNLH) was hydrolyzed into two fragments, NP and TNLH, suggesting the involvement of a proline-specific dipeptidyl peptidase. The dipeptidyl peptidase activity was highest in the late larval stage. It was purified 240-fold from soluble fractions of pupae of mixed age and classified on the basis of several catalytic properties as an invertebrate homologue of mammalian dipeptidyl peptidase IV (EC 3.4.14.5). Fly dipeptidyl peptidase IV has a molecular mass of 200 kDa, showed a pH optimum of 7.5–8.0 with the chromogenic substrate Gly-Pro-4-nitroanilide, and cleaved other chromogenic substrates with penultimate Pro or, with lower activity, Ala. It liberated Xaa-Pro dipeptides from the N-terminus of several bioactive peptides including substance P, neuropeptide Y, and peptide YY but not from bradykinin, indicating that the peptide bond between the two proline residues was resistant to cleavage. Fly dipeptidyl peptidase belongs to the serine class of proteases as the mammalian enzyme does; the fly enzyme, however, is not inhibited by several selective or nonselective inhibitors of its mammalian counterpart. It is suggested that dipeptidyl peptidases exert a regulatory role for the clearance not only of TMOF in flies but for other bioactive peptides in various invertebrates. Arch. Insect Biochem. Physiol. 37:146–157, 1998. © 1998 Wiley-Liss, Inc.     

Biosynthesis of intestinal microvillar proteins. Pulse-chase labelling studies on maltase-glucoamylase, aminopeptidase A and dipeptidyl peptidase IV

The biogenesis of three intestinal microvillar enzymes, maltase-glucoamylase (EC 3.2.1.20), aminopeptidase A (aspartate aminopeptidase, EC 3.4.11.7) and dipeptidyl peptidase IV (EC 3.4.14.5), was studied by pulse-chase labelling of pig small-intestinal explants kept in organ culture. The earliest detectable forms of the enzymes were polypeptides of Mr 225000, 140000 and 115000 respectively. These were found to represent the enzymes in a 'high-mannose' state of glycosylation, as judged by their susceptibility to treatment with endo-beta-N-acetylglucosaminidase H (EC 3.2.1.96). After about 40-60 min of chase, maltase-glucoamylase, aminopeptidase A and dipeptidyl peptidase IV were further modified to yield the mature polypeptides of Mr 245000, 170000 and 137000 respectively, which were expressed at the microvillar membrane after 60-90 min of chase. The fact that the enzymes before reaching the microvillar membrane were found in a Ca2+-precipitated membrane fraction (intracellular and basolateral membranes), but not in soluble form, indicates that during biogenesis maltase-glucoamylase, aminopeptidase A and dipeptidyl peptidase IV are transported and assembled in a membrane-bound state.     

Jejunal brush-border folate hydrolase. A novel enzyme.

Dietary folate, a vitamin required for DNA synthesis and cell regeneration, occurs as pteroylpolyglutamates that are hydrolyzed to pteroylglutamate during the process of intestinal absorption. Studies from our laboratory over the past 15 years have shown that jejunal brush-border folate hydrolase is essential and rate-limiting in folate absorption. Brush-border folate hydrolase activity and pteroylpolyglutamate hydrolysis are inhibited in disease and conditions associated with folate deficiency, including celiac and tropical sprue, the use of sulfasalazine to treat inflammatory bowel disease, and chronic alcoholism. Brush-border folate hydrolase is an exopeptidase located on the jejunal brush-border surface that liberates hydrolytic products of pteroylpolyglutamates in a progressive fashion, with a final release of pteroylglutamate. Subsequent steps in folate absorption include uptake by a brush-border folate-binding-protein receptor and transport across the brush-border membrane into the enterocyte. These steps are probably followed by an intracellular synthesis of pteroylglutamates for folate-dependent reactions and intracellular hydrolysis to pteroylglutamate for transport across the basolateral membrane to the portal circulation. In pigs, the active form of jejunal brush-border folate hydrolase has a molecular weight of 240 kd and is probably a homodimer of the 120-kd protein found after immunoprecipitation with specific antibody. Regulating the synthesis and expression of brush-border folate hydrolase may be critical to the availability of dietary folate.     

Bifidobacterium bifidum monoassociation of gnotobiotic mice: Effect on enterocyte brush-border enzymes

The effect of intestinal colonization withBifidobacterium bifidum (Gram-positive anaerobic bacterium colonizing the intestine of healthy new-born mammals, exhibiting a probiotic effect, protecting the intestinal mucosa against colonization by pathogenic microflora) on enterocyte brush-border enzymes was examined in weaned 23-d- and in 2-month-old gnotobiotic inbred mice and compared with that in corresponding germ-free (GF) and conventional (CV) controls. The two groups of GF mice were associated with humanB. bifidum 11 d before the end of the experiment. Specific activity of enterocyte brush-border enzymes—lactase, alkaline phosphatase and γ-glutamyltranspeptidase was significantly higher in both age groups of GF mice in comparison with CV ones; on the other hand, sucrase and glucoamylase activities were higher in CV mice. Monoassociation withB. bifidum accelerates biochemical maturation of enterocytes resulting in a shift of specific activities of brush-border enzymes between the values found for GF and CV mice. This effect ofB. bifidum supplementation was less pronounced for alkaline phosphatase, sucrase, glucoamylase and dipeptidyl peptidase IV in immature gut of weaned mice than of 2-month-old ones.     

Human genome search in celiac disease: mutated gliadin T-cell-like epitope in two human proteins promotes T-cell activation

Discovery of a number of novel and known human genes whose protein products bear striking similarity to two or more wheat gliadin domains raised the possibility that human intestinal non-HLA peptides homologous to celiac T-cell epitopes could play a role in non-HLA gene specification in celiac disease. Database searching of the entire human genome identified only 11 gut-expressed proteins with high T-cell epitope homology, particularly to the DQ2-gamma-I-gliadin epitope (i.e. TFIIA, FOXJ2 and IgD; mean BestFit quality score=40 versus random value of 24). Others were similar to DQ2-alpha-I-gliadin (i.e. PAX9; BestFit quality 46 versus 20 for random), or DQ2-alpha-II-gliadin (PHLDA1, known in mice as the T-cell death-associated gene; BestFit quality 43 versus 30 for random) epitopes. Among proteins previously screened for gliadin homology, noteworthy was achaete scute homologous protein (DQ2-alpha-I-gliadin; BestFit quality 41 versus 22 for random). With the exception of IgD, all are nuclear factors. Paying particular attention to the position of potential major histocompatibility complex (MHC) anchor residues, several were selected for testing in a DQ2-gamma-I-gliadin-restricted T-cell system. All native 10-mer peptides were inactive, even when deamidated, but V96F substitution of deamidated TFIIA amino acid residues 91-100 stimulated IL-2 release at levels exceeding the wheat gliadin positive control. Also active, but only slightly, was L1009F substitution of AIB3 amino acid residues 1004-1013. PlotSimilarity alignment of TFIIAs from eight species revealed subthreshold similarity score in the peptide region, in contrast to the highly conserved amino and carboxy termini. Molecular modeling of TFIIA[V96F] peptide points to an important juxtaposition of an upwardly projecting phenylalanine residue at peptide position 6 that likely contacts a receptor complementarity-determining region, and a downwardly projecting glutamic acid residue that fits into the shallow MHC P7 pocket. These observations tentatively point to a new multi-gene hypothesis for the initiation of celiac disease in which deamidated free human peptides with T-cell epitope homology (particularly those made more homologous by mutation) escape negative selection, as per deamidation of the HEL(48-62) peptide in the hen egg lysozyme model of autoimmunity. Deamidation following peptide release due to injury triggers inflammation, thereafter repeatedly provoked by dietary gliadin immunodominant peptides concentrated in the proximal small intestine.