Immunoelectrophoretic studies on pig intestinal brush border proteins

A note on the interpretation of identity reactions in immunoelectrophoresis

Immunoelectrophoretic methods, used in studies of the tissue specificity of two membrane-bound peptides obtained from pig, were observed to produce new types of reactions resembling those of partial immunochemical identity. In addition, the results obtained by fused rocket and rocket-line immunoelectrophoresis suggested that the unshared antigenic determinants were situated on the heterologous antigens. When analyzed in the immunodiffusion system, the antigens showed a reaction of total immunochemical identity. Zone electrophoresis of the antigens demonstrated large differences in electrophoretic mobility. Neuraminidase treatment of the antigens almost abolished these differences, and the antigens now exhibited almost complete antigenic identity, suggesting that the precipitate patterns observed arose from the mobility differences.

Purification and properties of neutral maltase from human granulocytes

A procedure for the purification of neutral maltase from human polymorphonuclear leukocytes is described, involving solubilization with Triton X-100, proteolytic attack and three chromatographic steps: DEAE ion exchange, AcA 22 gel filtration and a second DEAE chromatography. The enzyme was obtained with a final specific activity of 30 units/mg of protein, comparable with that of other neutral maltases previously purified. The Mr of the enzyme was 550,000 as determined by gel filtration. SDS/polyacrylamide-gel electrophoresis, under non-denaturing conditions, led to a major band of 500,000 and a minor one of 260,000, both active, suggesting a polymeric or aggregated form of the protein. The catalytic properties of the human granulocytic neutral maltase were investigated. The pH optimum was around 6. The enzyme exhibited a broad range of substrate specificity, hydrolysing di- and oligosaccharides with alpha (1----2), alpha (1----3) and alpha (1----4) glucosidic linkages. The highest activities were observed for alpha (1----4) glucose oligomers of three to five residues. It was also found to hydrolyse polysaccharides such as starch and glycogen. The results of the inhibition studies are interpreted in terms of the existence of a large site including several subsites. The enzyme properties are broadly similar to those observed for other purified neutral alpha-glucosidases, in particular that of human kidney origin.

Complete amino acid sequence of human intestinal aminopeptidase N as deduced from cloned cDNA

The complete primary structure (967 amino acids) of an intestinal human aminopeptidase N (EC 3.4.11.2) was deduced from the sequence of a cDNA clone. Aminopeptidase N is anchored to the microvillar membrane via an uncleaved signal for membrane insertion. A domain constituting amino acid 250-555 positioned within the catalytic domain shows very clear homology to E. coli aminopeptidase N and contains Zn2+ ligands. Therefore these residues are part of the active site. However, no homology of the anchor/junctional peptide domain is found suggesting that the juxta- and intra-membraneous parts of the molecule have been added/preserved during development. It is speculated that this part carries the apical address.

Tyrosine sulphation is not required for microvillar expression of intestinal aminopeptidase N

The effect of 2,6-dichloro-4-nitrophenol (DCNP), an inhibitor of phenol sulphotransferases (EC 2.8.2.-), on the biosynthesis of aminopeptidase N (EC 3.4.11.2) was studied in organ-cultured pig intestinal mucosal explants. At 50 microM DCNP did not affect protein synthesis but it decreased incorporation of [35S]sulphate into aminopeptidase N and other major microvillar hydrolases by 70-85% compared with controls, indicating an inhibition of their post-translational tyrosine sulphation. In labelling experiments with [35S]methionine from 0.5 to 5 h, DCNP was tested for its possible influence on synthesis, processing and microvillar expression of aminopeptidase N, but no effect on any of these parameters could be detected. It can therefore be concluded that tyrosine sulphation is not required (for instance as a sorting signal) for the targeting of newly synthesized enzymes to the microvillar membrane.

Improved transfer of high molecular weight proteins using fabric-reinforced soft polyacrylamide gels

Rapid transfer of electrophoretically separated, high molecular weight proteins from fabric-reinforced, soft, low-concentration polyacrylamide gels (3.5%T, 2.6%C) is described. Polyester fabrics proved useful for gel reinforcement during vertical slab gel electrophoresis. The fabric-reinforced gel retains its dimension during transfer, equilibration and drying, facilitating comparison of protein bands in the separation gel and transfer matrix.

Identification and characterization of brush-border membrane-bound neutral metalloendopeptidases from rat small intestine

Neutral metalloendopeptidase enzymes were identified and partially characterized in the brush-border membranes of rat small intestinal mucosal cells using insulin B chain and glutaryl-trialanine-4-methoxy-beta-naphthylamide as substrates. Three different molecular species of endopeptidase were identified by disc gel electrophoresis. These enzymes were shown to be distinct from pancreatic endopeptidases on the basis of the following: enrichment in the brush-border membrane fraction, site of hydrolysis of peptide substrates, sensitivity to specific proteinase inhibitors, and the presence of brush-border membrane-associated endopeptidase activity in mucosal cells of Thirty-Vella loops. Hydrolysis of the substrates was shown to be a two-step process involving initial cleavage by endopeptidase with secondary hydrolysis of the peptide products by brush-border membrane aminopeptidase N. Hydrolysis of both substrates was maximum at a neutral pH and was strongly inhibited by metal chelating agents, phosphoramidone, and amastatin. Intestinal perfusion studies using glutaryl-trialanine-4-methoxy-beta-naphthylamide suggest that these enzymes play a physiologic role in protein digestion. It was concluded that neutral endopeptidases are integral components of the intestinal brush-border membrane and work in concert with aminopeptidase N to hydrolyze dietary protein. This process may be of nutritional importance in normal subjects and those with diminished exocrine pancreatic function.

Immunoelectrophoretic analysis of vascular, membrane-bound angiotensin I converting enzyme, aminopeptidase M, and dipeptidyl(amino)peptidase IV

Antisera raised against specific renal brush border peptidases have been used to characterize vascular surface membrane angiotensin I converting enzyme (ACE; EC 3.4.15.1), aminopeptidase M (AmM; EC 3.4.11.2), and dipeptidyl(amino)peptidase IV (DAP IV; EC 3.4.14.5) by techniques of differential solubilization, fused-rocket immunoelectrophoresis and crossed immunoelectrophoresis. The vascular membrane-bound enzymes are immunologically indistinguishable from their brush border counterparts and can be solubilized by treatment with detergent and/or papain. The electrophoretic mobilities of the papain-treated forms of each enzyme were greater than those of the detergent-treated forms. This increased mobility is associated with the removal of small, hydrophobic, non-antigenic components of the enzymes. Regardless of the method of solubilization, the electrophoretic mobilities of the vascular enzymes were greater than those of the brush border enzymes. However, after treatment with neuraminidase to remove sialic acid, their respective mobilities were similar. The mobilities of serum AmM and DAP IV were identical to the respective papain-solubilized vascular enzymes both before and after neuraminidase. Thus, like the brush border enzymes, the data presented are consistent with the model that vascular ACE, AmM and DAP IV are intrinsic membrane peptidases bound to their surface membranes by small, non-antigenic, hydrophobic anchors associated with the lipid bilayer. In addition, these vascular surface membrane peptidases are similar to and may be a source of the circulating enzymes.

Identification of cell surface dipeptidylpeptidase IV in human fibroblasts

An antigen with dipeptidylpeptidase IV activity was identified at the surface of normal human fibroblasts. Hydrophobic interaction electrophoresis in phenyl-Sepharose revealed that the enzyme contained a hydrophobic domain, while lactoperoxidase-catalysed iodination with 125I of living cells indicated that the protein was located at the cell surface. Crossed immunoelectrophoresis with specific antibodies of acid-extracted or papain-treated cells showed a shift of the dipeptidylpeptidase IV peak to a faster mobility. The molecular properties of the fibroblast enzyme were clearly different from those described for dipeptidylpeptidase IV from other tissues and species. Fibroblast dipeptidylpeptidase IV contained two different disulphide-linked subunits, of apparent Mr values 125000 and 135000 (denatured and reduced). In gel filtration, an Mr of about 400000 was observed for the unreduced molecule. The enzymic properties of fibroblast dipeptidylpeptidase IV were very similar to those of the well-characterized pig kidney enzyme. Activity towards glycyl-L-prolyl-beta-naphthylamide was inhibited 50% by 0.023 mM-di-isopropylphosphorofluoridate. L-Alanyl-L-alanyl-beta-naphthylamide was hydrolysed ten times more slowly than glycyl-L-prolyl-beta-naphthylamide.

Immunoelectrophoretic analysis of renal and intestinal brush border converting enzyme

Antibodies raised against purified hog renal or intestinal brush border protein or against purified hog kidney angiotensin I converting enzyme (ACE) were used to characterize renal and intestinal brush border ACE by techniques of differential solubilization, fused-rocket, line absorption and crossed-immunoelectrophoresis. Renal ACE is immunologically identical to intestinal ACE. ACE is present as a major intrinsic protein of renal brush border and a minor intrinsic protein of intestinal brush border. Renal and intestinal brush border ACE could be solubilized by detergent and/or papain. The electrophoretic mobilities of the papain-treated forms of ACE were greater than the detergent-treated forms. This increased mobility was associated with the removal of a small, non-antigenic component of the enzyme. Thus, like several other intrinsic brush border peptidases, ACE is bound to renal and intestinal brush border by a small hydrophobic anchor.

Purification and partial characterization of chick intestinal sucrase

The sucrase was purified from the small intestinal mucosa of the adult chick. Purification procedure involved solubilization with papain, ethanol precipitation, chromatography on Sephadex G-200 and DEAE-Sephadex. Several characters of the chick intestinal sucrase resembled those of the intestinal sucrase-isomaltase complex of some mammals (rabbit, rat and human).

Further characterization of intestinal lactase/phlorizin hydrolase

Pig intestinal lactase/phlorizin hydrolase (EC 3.2.1.23/62) was purified in its amphiphilic form by immunoadsorbent chromatography. The purified enzyme was free of other known brush border enzymes and appeared homogeneous in immunoelectrophoresis and polyacrylamide gel electrophoresis in the presence of SDS. Pig lactase/phlorizin hydrolase was shown to have the same quaternary structure as the human enzyme, i.e., built up of two polypeptides of the same molecular weight (160000). In addition to hydrolyzing lactose, phlorizin and a number of synthetic substrates, both the human and the pig enzyme were shown to have a considerable activity against cellotriose and cellotetraose, and a low but significant activity against cellulose. The lactase/phlorizin hydrolase isolated from pigs in which the pancreatic ducts had been disconnected 3 days before death and from Ca2+-precipitated enterocyte membranes (basolateral and intracellular membranes) exhibited in SDS-polyacrylamide gel electrophoresis the same size of constituent polypeptides and the same catalytic and immunological properties as a normal brush border lactase/phlorizin hydrolase.

Immunochemical analysis of high molecular mass urinary proteins

Urinary high molecular mass proteins (fraction P) solubilized in Triton X-100 and by papain have been compared with the solubilized human renal brush border membrane proteins. Crossed immunoelectrophoresis of Triton X-100 fraction P extract, by means of two polyspecific antisera directed against either renal membrane or fraction P, revealed eleven immunoprecipitates antigenically identical with detergent renal membrane antigens. Among them, five hydrolases were identified by zymogram staining: microvillus aminopeptidase, maltase, trehalase, gamma-glutamyltransferase and alkaline phosphatase. Eight papain-solubilized fraction P proteins and Triton X-100-solubilized membrane extract presented 'identity' patterns in tandem crossed immunoelectrophoresis, but differed in their amphiphilicity, as demonstrated by the change of precipitation pattern on charge-shift caused immunoelectrophoresis. Among the eleven detergent-solubilized fraction P antigens, nine were proved to be amphiphilic proteins and six presented bidirectional charge shifting properties similar to those of renal membrane antigens. Quantitatively, five detergent fraction P proteins were found in the same amounts as in renal membrane extract, two in lesser amounts and four in greater. Moreover, the same two plasma proteins were identified in fraction P as in the renal membrane. Thus important similarities exist between the urinary fraction P and the native renal membrane.

Hypotonic elution, a new desorption principle in immunoadsorbent chromatography

A largely unrecognized immunoadsorbent desorption technique, hypotonic elution, has been successfully used in the immunoadsorbent purification of the microvillar enzymes aminopeptidase N (EC 3.4.11.2), dipeptidyl peptidase IV (EC 3.4.14.5), sucrase-isomaltase (EC 3.2.1.48-10), lactase-phlorizin hydrolase (EC 3.2.1.23-62) and maltase-glucoamylase (EC 3.2.1.20). This elution method proved capable of achieving an acceptable yield (30-70%) while at the same time preserving the purified enzymes in an enzymically active state. It hereby offers a solution to the problem in immunoadsorbent chromatography of finding an efficient means of elution which is not denaturing to neither the purified enzyme nor the immunoadsorbent column. Common properties of the microvillar enzymes with regard to amphiphilicity, glycosylation or subunit composition could hypothetically account for the similar elution properties of the enzymes but were considered unlikely on several grounds. Hypotonic elution in immunoadsorbent chromatography, therefore, may have a much broader range of applicability, and the method is recommended to be tried out by workers in other areas of protein chemistry.

Biosynthesis of intestinal microvillar proteins. Characterization of intestinal explants in organ culture and evidence for the existence of pro-forms of the microvillar enzymes

Explants of pig small intestine were maintained at 37 degrees C in organ culture for periods up to 24 h in a system using Trowell T-8 medium supplemented with 10% foetal-calf serum. The mucosal morphology was well preserved during culture, as judged by light and electron microscopy. The explant contents of protein and two brush-border enzymes, microvillus aminopeptidase (EC 3.4.11.2) and dipeptidyl peptidase IV (EC 3.4.14.5), were not significantly modified during culture compared with controls, but a moderate, continuous release of both protein and enzyme activities into the medium was observed. Continuous labelling with [35S]methionine resulted in an even incorporation of radioactivity in the protein components, and the rate of labelling only moderately decreased over the 24 h period. The polypeptide compositions of sucrase (EC 3.2.1.48)--isomaltase (EC 3.2.1.10), maltase--glucoamylase (EC 3.2.1.20) lactase (EC 3.2.1.23)--phlorizin hydrolase (EC 3.2.1.62), microvillus aminopeptidase and aspartate aminopeptidase (EC 3.4.11.7) synthesized during culture were studied, and some were found to be similar to those of the pro-forms of the enzymes isolated from animals that had had their pancreatic duct disconnected 3 days before being killed. These results confirmed earlier findings of the existence of pro-forms of some of the microvillar enzymes and thus indicate a low activity of pancreatic proteinases in the culture system.

Crossed-immunoelectrophoretic study on human renal brush border membrane vesicles

The human kidney brush border membrane proteins were studied by crossed-immunoelectrophoresis. An antiserum against membrane vesicles was raised in rabbits and used in establishing a reference immunoelectrophoregram with the antigens released by Triton X-100. Among the precipitates observed, the following hydrolases were identified by zymogram staining: Microvillus aminopeptidase (EC 3..4.11.2), gamma-glutamyltransferase (EC 2.3.2.2), maltase (EC3.2.1.20) and trehalase (EC 3.2.1.28). Depletion of the antiserum with sealed, right-side-out vesicles was performed. No precipitates could be seen when the Triton X-100 extract was electrophoresed in a gel containing the depleted antibody. It is therefore suggested that the precipitation of membrane components by the complete antibody is mainly due to externally-located determinants and that the precipitates of the reference pattern correspond to membrane components pointing, at least in part, towards the tubular lumen. Evidence was also noted for a differential removal of antibodies directed against the different antigens. Such an observation could not be explained by the antigen accessibility nor by its amount in the membrane. Parallel crossed-immunoelectrophoresis of Triton X-100 and papain extracts gave rise to an "identity" pattern for only some antigens, particularly for microvillus aminopeptidase and maltase. It is thus strongly suggested that the papain-released form of these enzymes bears nearly all the antigenicity of the whole molecule.

Proteins of the kidney microvillar membrane. Aspartate aminopeptidase: purification by immunoadsorbent chromatography and properties of the detergent- and proteinase-solubilized forms

Aminopeptidase A (aspartate aminopeptidase, EC 3.4.11.7) was purified 2000-fold from pig kidney cortex. The essential step in the purification was chromatography on an immunoadsorbent column prepared from a rabbit antiserum raised against pig intestinal aminopeptidase A. Glutamyl and aspartyl substrate were attacked most rapidly and their hydrolyses were stimulated by Ca2+. The 2-naphthylamide derivatives of neutral and basic amino acids were also hydrolysed by aminopeptidase A, but at rates about two orders of magnitude lower, and Ca2+ was inhibitory. The possibility that these atypical substrates were hydrolysed by traces of aminopeptidase M (EC 3.4.11.2) contaminating the preparation could be excluded on several grounds. Aminopeptidase A was sensitive to inhibition by chelating agents and the inactive enzyme could be reactivated by Ca2+ or Mn2+. Atomic absorption spectrophotometry revealed 1 g-atom of Ca/143000 g of protein. Two forms of the enzyme were purified: an amphipathic form solubilized from the membrane by Triton X-100 (detergent form) and a hydrophilic form released by incubation with trypsin (proteinase form). The detergent form exhibited charge-shift in crossed immunoelectrophoresis when anionic or cationic detergents were present. On gel filtration, mol.wts. of 350000--400000 and 270000 were calculated for the detergent and proteinase forms. Electron microscopy after negative staining of the proteinase form revealed a dimeric structure. Electrophoresis of either form in the presence of sodium dodecyl sulphate revealed four polypeptides with mobilities corresponding to apparent mol.wts. of 155000, 110000, 90000 and 45000. All four bands stained positively for carbohydrate. Pig serum possesses weak aminopeptidase A activity; immunological experiments showed it to be a similar protein.

Simultaneous isolation of brush border and basolateral membrane from rabbit enterocytes. Presence of brush border hydrolases in the basolateral membrane of rabbit enterocytes

By a slight modification of the procedure described by Gratecos et al. (Gratecos, D., Knibiehler, M., Benoit, V. and Sémériva, M. (1978) Biochim. Biophpys. Acta 512, 508-524), the basolateral and brush border membranes of rabbit enterocytes have been purified concomitantly from the same aliquot of mucosa. The two types of membrane have been obtained with the same yield (15%) and enrichment of specific markers (18-fold). The presence in the basolateral membrane of hydrolases known to be specific of the brush border membrane has been confirmed by using immunological techniques.

Rabbit intestinal aminopeptidase N. Purification and molecular properties

The detergent and protease forms of rabbit intestinal aminopeptidase N were purfied for chemical investigations and future specific immunological labeling of the enzyme in situ. The purification of the detergent form required a special technique called 'reverse immunoabsorbant chromatography'. The specific activity of the detergent form finally obtained was identical to that of the protease form. A significant charge micro heterogeneity persisted in the most purified preparations, due probably to a certain level of variability in the sugar moiety. The major proteolytic cleavage which occurred at the hydrophilic-hydrophobic junction of the detergent form during its conversion into the protease form was well defined. But additional splittings probably in C-terminal region of the molecules led to several protease forms differing by their size. The molecular weight assigned to the peptide liberated during the above conversion was overestimated due to preferential detergent binding to hydrophobic structures. The correct value, estimated by a new isotopic dilution method, was 3800 (36-38 residues) for the peptide originating from the rabbit enzyme. The real anchor plunging into the membrane core is possibly still shorter. Comparative N-terminal residue determinations in the detergent form, the protease form and the peptide difinitely confirmed that the enzyme is anchored to the bursh border membrane by its N-terminal region.

Purification and characterization of an aminopeptidase A from hog intestinal brush-border membrane

The aminopeptidase A of the porcine intestinal brush-border membrane has been purified following solubilization by trypsin (p-form) or Emulphogen (d-form). Full purification of d-amino-peptidase A required the use of anti-impurities immunoabsorbant chromatography. The d-amino-peptidase A constitutes about 4% of the total proteins of the membrane, compared to 8-12% for another, already characterized, brush-border aminopeptidase N. Both d-form and p-form of aminopeptidase A have been clearly shown to be dimeric. Experimental evidence is presented favoring the view that they are symmetrical dimers, with the consequence that each of the two subunits of the d-form possesses an hydrophobic anchor holding them at the membrane surface. As already demonstrated for several other brush border hydrolases, the hydrophobic anchor is N-terminal in porcine intestinal aminopeptidase A. The molecular weight of the peptide including the anchor liberated by trypsin during the conversion of the d-form into the p-form has been estimated by an isotopic dilution method to be about 4500 (42 residues). This value which compares well with those recently obtained in the case of rabbit aminopeptidase N (3700-3800; 36-38 residues), indicates that the anchor is much shorter than believed earlier. A preliminary survey of the specificity of both aminopeptidases A and N towards four synthetic amino acid p-nitroanilides confirms that aminopeptidase A mostly cleaves acidic residues. Its activity towards neutral residues is much lower, but probably significant in certain cases.

Proteins of the kidney microvillar membrane. Asymmetric labelling of the membrane by lactoperoxidase-catalysed radioiodination and by photolysis of 3,5-di[125I]iodo-4-azidobenzenesulphonate

Two methods were used to label pig kidney microvillar membrane proteins from the luminal and cytoplasmic surfaces of closed membrane vesicles. The first method was lactoperoxidase-catalysed radioiodination. The enzyme reagents, lactoperoxidase and glucose oxidase, were positioned inside the vesicles before sealing or externally after sealing, iodination being initiated by the subsequent addition of glucose and 125I-. After resolution of the labelled proteins by electrophoresis in the presence of dodecyl sulphate, asymmetric labelling patterns on radioautographs were observed. However, the major disadvantage of this method is the high degree of intramembrane labelling of the fatty acid chains of membrane lipids, a reaction that undermines any conclusions about the location of the label in that region of the protein supposedly exposed at the surface of the membrane. The second method overcame this disadvantage. A new hydrophilic photoreagent, 3,5-di[125I]iodo-4-azidobenzesulphonate, was synthesized via the intermediate, diazotized 3,5-di[125I]iodosulphanilic acid. It was transported by a Na+-dependent system into microvillar vesicles, thus permitting labelling from either side of the membrane when the vesicles were photolysed. The labelling of membrane lipids was less than with the first method and was essentially confined to the polar headgroups. The activity of several microvillar peptidases survived the labelling reaction and they could be identified in the immunoprecipitates after resolution of the detergent-solubilized membrane proteins by crossed-immunoelectrophoresis. Treatment with papain converted the detergent-solubilized form of susceptible enzymes into the proteinase-solubilized form, which lacked the intramembrane domain and any portion exposed at the cytoplasmic surface. Radioautography established that aminopeptidases M and A, dipeptidyl peptidase IV and neutral endopeptidase were transmembrane proteins. This novel approach to the investigation of membrane topology may be applicable to other complex membranes.

Peptide hydrolases of the human small intestinal mucosa: identification of six distinct enzymes in the brush border membrane

For this investigation highly purified brush border membranes from human small intestine, prepared according to the method described in the preceding paper [1], have been used. Solubilisation of brush border membrane proteins by sodium dodecyl sulphate, Triton X-100 and papain followed by electrophoresis in polyacrylamide gels revealed six distinct peptide hydrolases. These included the enzymes aminopeptidase A (EC 3.4.11.7), dipeptidylpeptidase IV (EC 3.4.14.-), gamma-glutamultranspeptidase (EC 2.3.2.2) and aminopeptidase M (EC 3.4.11.2), which were clearly separable on polyacrylamide gels after solubilisation with Triton X-100 or papain. Activity recovered in the aminopeptidase M peak in the above gel system could be resolved into two distinct peptidases in addition to aminopeptidase M, by SDS-gel-electrophoresis. One of these peptidases was most active towards aliphatic tripeptide (aminopeptidase 1) while the other appeared to be specific for dipeptides (aminopeptidase 2).

The insertion of pig microvillus aminopeptidase into the membrane as probed by [125I]iodonaphthylazide

The insertion of pig intestinal microvillus aminopeptidase (EC 3.4.11.2) into the membrane was studied by the hydrophobic photolabel [125I]iodonaphthylazide. The aminopeptidase was either labelled in the microvillus membrane and purified, or labelled after detergent solubilization and purification in a buffer containing Triton X-100, and then isolated from the reaction mixture. Of the three subunits A (Mr 162000), B (Mr 123000) and C (Mr 62000) of the aminopeptidase, A and B, but not C contained radioactivity, indicating that both subunit A and B carry anchoring peptide. The radioactivity when released from the subunits by trypsin treatment was connected to low-molecular-weight material.

Immunoelectrophoretic studies on human small intestinal brush border proteins. A qualitative study of the protein composition

The human small intestinal brush border proteins were studied qualitatively by crossed immunoelectrophoresis. Brush border membranes were purified from human jejunum and the proteins released by Triton X-100. Rabbits were immunized with the released proteins and by using a double layer immunofluorescence technique the obtained antisera were shown to be specific against the brush border proteins. The precipitates obtained in crossed immunoelectrophoresis were identified by enzymatic staining techniques. Sucrase (EC 3.2.1.48), isomaltase EC 3.2.1.10), maltase (EC 3.2.1.20), phloretin-glucosidase (EC 3.2.1.62), lactase (EC3.2.1.23), microvillus aminopeptidase (aminopeptidase (microsomal), EC 3.4.11.2), dipeptidyl peptidase IV (EC 3.4.14.X), and alkaline phosphatase (EC 3.1.3.1) were identified while asparate aminopeptidase (EC 3.4.11.7), gamma-glutamyl transferase (EC 2.3.2.2) and trehalase (EC 3.2.1.28) could not be visualized. This work demonstrates that cross immunoelectrophoresis can be used in the study of human small intestinal brush border proteins.