Covalent cross-linking of porcine small-intestine microvillar aminopeptidase. Subunit structure of the membrane-bound and the solubilized enzyme

Aminopeptidases and proteolipids of intestinal brush border

The presence of human blood group A determinants has been shown on the A+ rabbit intestinal brush border glycoproteins, particularly hydrolases. Sugar compositions of aminopeptidases N from A+ and A- rabbits were compatible with the presence in these molecules of eight N-linked glycans and of two O-linked glycans bearing the A determinants in A+ animals. The exact relative molecular masses of hydrophobic domain(s) of aminopeptidases N and A from pig and rabbit intestinal brush border have been determined by an isotopic dilution technique. The values obtained were compatible with the anchorage in the membrane of the monomeric rabbit enzymes, or of each subunit of the dimeric pig enzymes, by their N-terminal sequences, composed of 20-25 hydrophobic amino acids. This N-terminal hydrophobic sequence (14 residues) has been determined for rabbit aminopeptidase N. Short peptides containing approximately 60% hydrophobic amino acids have been extracted by chloroform-methanol from purified brush border and basolateral membranes of pig enterocytes. Their molecular properties were very similar to those of the aminopeptidase anchors released by trypsin treatment of detergent-extracted enzymes. However, several lines of evidence failed to support the assumption that these free hydrophobic peptides can be identified with anchors left inside the bilayer after proteolytic cleavage of surface hydrolases.

Dietary effects of faba-bean (Vicia fabaL.) tannins on the morphology and function of the small-intestinal mucosa of weaned pigs

The objective of the present study was to evaluate effects of condensed tannins in faba beans (Vicia fabaL.) on morphological and functional variables of the small-intestinal mucosa of piglets. In an experiment with young piglets (8–17 kg body weight), fed on either a control diet or a diet containing 200 g/kg of low- or high-tannin faba bean hulls (with < 0·10 and 3·3% catechin equivalents of condensed tannins respectively), morphological and functional characteristics of the jejunal mucosa were determined. Results of the study showed that the morphological variables of the mucosa of the three groups of piglets were similar. Also, no changes due to dietary tannins were observed in sucrase (EC3.2.1.48)–isomaltase (EC3.2.1.10) activity in homogenates of mucosa plus submucosa. However, aminopeptidase (EC3.4.11.2) activity in these homogenates in the proximal part of the small intestine of the animals of the group fed on the high-tannin diet was significantly lower than that in the animals fed on the control diet or the diet with low-tannin hulls (P< 0·05).

The oligomeric structure of renal aminopeptidase N from bovine brush-border membrane vesicles

Bovine renal brush-border membrane vesicle aminopeptidase N at various stages of purity was treated with two bifunctional cross-linking agents. A pattern of emergence of higher molecular weight forms was observed. By using a cleavable cross-linker, aminopeptidase N was shown to cross-link both to itself and to its breakdown products as well as to dipeptidyl peptidase IV. Using this technique it was possible to identify three of the breakdown products as 45 kDa, 66 kDa and 95 kDa peptides. N-terminal amino acid sequence analysis was used to define the precise cleavage points for the bovine renal aminopeptidase N breakdown products. The short amino acid sequences obtained show strong sequence similarity with the human intestinal and rat kidney aminopeptidase N.

Biosynthesis of intestinal microvillar proteins. Dimerization of aminopeptidase N and lactase-phlorizin hydrolase

The pig intestinal brush border enzymes aminopeptidase N (EC 3.4.11.2) and lactase-phlorizin hydrolase (EC 3.2.1.23-62) are present in the microvillar membrane as homodimers. Dimethyl adipimidate was used to cross-link the two [35S]methionine-labeled brush border enzymes from cultured mucosal explants. For aminopeptidase N, dimerization did not begin until 5-10 min after synthesis, and maximal dimerization by cross-linking of the transient form of the enzyme required 1 h, whereas the mature form of aminopeptidase N cross-linked with unchanged efficiency from 45 min to 3 h of labeling. Formation of dimers of this enzyme therefore occurs prior to the Golgi-associated processing, and the slow rate of dimerization may be the rate-limiting step in the transport from the endoplasmic reticulum to the Golgi complex. For lactase-phlorizin hydrolase, the posttranslational processing includes a proteolytic cleavage of its high molecular weight precursor. Since only the mature form and not the precursor of this enzyme could be cross-linked, formation of tightly associated dimers only takes place after transport out of the endoplasmic reticulum. Dimerization of the two brush border enzymes therefore seems to occur in different organelles of the enterocyte.

Synthesis and intracellular processing of aminooligopeptidase by human intestine

Aminooligopeptidase is an intrinsic glycoprotein of the brush border membrane important for hydrolysis of the oligopeptide products of intraluminal protein digestion. To study its synthesis and intracellular processing, we performed pulse-chase experiments using [35S]methionine to label proteins of cultured human intestinal explants obtained by endoscopic biopsy. Aminooligopeptidase was isolated by immune precipitation with a monoclonal antibody and its molecular size was assessed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and fluorography. A precursor of relative molecular weight (Mr) 127,000 appeared within 10 min of chase and appeared to begin conversion to an Mr 150,000 form (the size of brush border membrane aminooligopeptidase) within 60 min. To determine if the change in molecular size was the consequence of alterations in glycosylation, we studied the susceptibility of the two forms to endo-beta-N-acetylglucosaminidase H, which cleaves immature high-mannose N-linked carbohydrate chains, and to peptide: N4-(N-acetyl-beta-glucosaminyl)asparagine amidase, which cleaves both the high-mannose and complex N-linked carbohydrate chains. Only the early Mr 127,000 aminooligopeptidase was sensitive to endo-beta-N-acetylglucosaminidase H, suggesting that the larger form results from trimming of high-mannose cores and adding terminal sugars in the Golgi complex. Both forms were sensitive to peptide: N4-(N-acetyl-beta-glucosaminyl)asparagine amidase, generating an Mr 114,000 species. The kinetics of the synthesis and processing of aminooligopeptidase and sucrase-isomaltase were compared by immunoprecipitation of both proteins from the same tissue after separating the microvillous membrane from the remainder of the cellular membranes. Labeled aminooligopeptidase was present intracellularly in its mature form within 60 min and was detected exclusively in the brush border membrane by 90 min. Most of the labeled sucrase-isomaltase pool had not yet undergone complex glycosylation during the same period. These data demonstrate that although human intestinal aminooligopeptidase undergoes N-linked glycosylation like sucrase-isomaltase, the synthesis of aminooligopeptidase differs from that of sucrase-isomaltase in respect to the absence of a high-molecular-weight precursor and more rapid pre-Golgi processing.

Structural studies on microvillus aminopeptidase from pig small intestine

Two amphiphilic forms of intestinal microvillus aminopeptidase were obtained by purification of the enzyme in the absence and in the presence of aprotinin respectively. By subsequent treatment with trypsin in vitro a hydrophilic form was obtained. These three enzyme preparations all contained three subunits. A, B and C (molecular weight 168000, 11800 and 54000 respectively). The individual subunits were purified by gel filtration on Ultrogel AcA 22 in the presence of sodium dodecyl sulphate. Subunits A and B from aminopeptidase prepared in the presence of aprotinin contained both N-terminal lysine and alanine, whereas N-terminal alanine and glycine were found for both of these subunits from the other amphiphilic form. Conversion of amphiphilic aminopeptidase to the hydrophilic form in vitro led only to a change in the N terminus of the B subunit to a valine. N-terminal serine was found in subunit C from all three forms of the aminopeptidase. The amino acid composition and peptide maps of the isolated subunits from one of the amphiphilic forms are reported. It is suggested that subunit B and C arise by cleavage of subunit A, whereas it seemed unlikely that C originates from B. Brush border membrane vesicles were cross-linked by dimethyl-3,3-dithiobispropionimidate, and the aminopeptidase subsequently isolated. Analysis of molecular weights and subunit composition of its constituent polypeptide complexes by two-dimensional polyacrylamide slab gel electrophoresis in the presence of sodium dodecyl sulphate showed that AB, BBC, BCC, BB, BC and CC complexes had been formed. The subunit structures ABC and BBCC were thus proposed in agreement with the maximum molecular weight of the membrane-bound enzyme previously determined as 330000.U

Changes of the quaternary structure of microvillus aminopeptidase in the membrane

Microvillus aminopeptidase (EC 3.4.11.2) is an enzyme with a molecular weight around 300 000. Normal preparations contain three different subunits (subunit A, Mr 162 000; subunit B, Mr 123 000; subunit C, Mr 61 000). The relationship between the three subunits was studied by immunoelectrophoresis using specific antibodies against individual denatured subunits and by densitometric scanning of polyacrylamide gels after separation of the three subunits. The results suggest that microvillus aminopeptidase initially appears in the membrane as a symmetric molecule built up to two identical A subunits. These subunits are then split into equimolar amounts of subunit B and subunit C by trypsin. Subunit B cannot generate subunit C but may be further degraded. The reaction sequence described is one which occurs in vivo. Treatment of purified aminopeptidase with trypsin increases the specific activity twofold. This phenomenon does not seem to be correlated to the generation of subunit B and subunit C or to the transformation of amphiphilic form into hydrophilic form.