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

Novel Carbohydrate-binding Activity of Pancreatic Trypsins to N-Linked Glycans of Glycoproteins

How glycosylation affects the reactivity of proteins to trypsin is not well understood. Bovine and porcine pancreatic trypsins were discovered to bind to alpha-Man, Neu5Acalpha2,6Galbeta1,4Glc, and alpha-galactose sequences by binding studies with biotinylated sugar-polymers. Quantitative kinetic studies supported that phenylmethylsulfonyl fluoride (PMSF)-treated trypsin binds to glycolipid analogues possessing alpha-Man or alpha-NeuAc but not to those possessing beta-galactose or beta-GlcNAc residue. Enzyme-linked immunosorbent assay (ELISA) showed that trypsin binds to six kinds of biotinylated glycoproteins possessing high mannose-type and complex-type N-glycans but not to bovine submaxillary mucin, which possesses only O-glycans. Further, the binding of trypsin to glycoproteins was differentially changed by treatments with sequential exoglycosidases, endoglycosidase H, or N-glycosidase F. Quantitative kinetic studies indicated that PMSF-treated trypsin binds with bovine thyroglobulin with the affinity constant of 10(10) m(-1), which was the highest among the glycoproteins examined, and that alpha-galactosidase treatment decreased it to 10(5) m(-1). PMSF-treated trypsin bound to other glycoproteins, including ovomucoid, a trypsin inhibitor, with the affinity constants of 10(8)-10(5) mol(-1) and were markedly changed by glycosidase treatments in manners consistent with the sugar-binding specificities suggested by ELISA. Thus, the binding site for glycans was shown to be distinct from the catalytic site, allowing trypsin to function as an uncompetitive activator in the hydrolysis of a synthetic peptide substrate. Correspondingly the carbohydrate-binding activities of trypsin were unaffected by treatment with PMSF or soybean trypsin inhibitor. The results indicate the presence of an allosteric regulatory site on trypsin that sugar-specifically interacts with glycoproteins in addition to the proteolytic catalytic site.

Identification of proline-specific carboxypeptidase localized to brush border membrane of rat small intestine and its possible role in protein digestion

A proline-specific carboxypeptidase (carboxypeptidase P, EC 3.4.12.-) was identified and partially characterized in the brush border membrane fraction of rat intestinal enterocytes and shown to be distinct from pancreatic proteases. The carboxypeptidase activity of isolated brush border membranes, with Z-Gly-Pro-Leu as substrate, was 43 nmol/min/mg protein representing a 16-fold purification when compared with mucosal cell homogenates. Activity was maximal in the middle region of the small intestine, and villus cells had twice the activity of crypt cells. Carboxypeptidase activity was maximal at pH 7.0, was stimulated by divalent cations, and was inhibited by metal chelating agents, suggesting that it is a metalloenzyme. The enzyme had the highest activity with synthetic peptides containing proline penultimate to the carboxy terminus. In vivo patterns of hydrolysis and absorption of amino acids from Z-Pro-Trp were examined using an intestinal perfusion technique. These studies indicate that brush border membrane carboxypeptidase may play an important role in the digestion of proline-containing peptides and proteins.

Purification and characterization of amphiphilic trehalase from rabbit small intestine

Rabbit intestinal trehalase (alpha,alpha-trehalose glucohydrolase, EC 3.2.1.28) was solubilized with Triton X-100 and purified in the presence of EDTA. The purified enzyme was homogeneous on polyacrylamide gel electrophoresis in the presence of Triton X-100 or SDS. It showed amphiphilic properties on gel filtration. polyacrylamide gel electrophoresis, charge-shift electrophoresis and phenyl-Sepharose chromatography. Its molecular weight was estimated to be about 330 000 by gel filtration under nondenaturing conditions and in the presence of Triton X-100, the value being in satisfactory agreement with the sum of the weight of one Triton X-100 micelle and twice the molecular weight (105 000) of purified hydrophilic trehalase which had been deprived of the anchor segment. The two purified trehalases gave almost the same molecular weights (about 75 000) on SDS-polyacrylamide gel electrophoresis. These results suggest that intestinal trehalase consists of two subunits with a molecular weight of 75 000 and that its anchor segment is small (less than 5000). Triton X-100 extracts freshly prepared from intestinal microvilli essentially showed one form of trehalase, which behaved on phenyl-Sepharose and Con A-Sepharose chromatography in the same manner as purified amphiphilic trehalase.

Purification and characterization of kidney and intestinal brush-border membrane trehalases from the rabbit

Trehalase (alpha, alpha-trehalose glucohydrolase, EC 3.2.1.28) was solubilized from the brush-border membrane of rabbit intestine and kidney by Emulphogen BC 720. The intestinal and kidney enzymes were purified 10 400-times and 4457-times respectively, in a five-step procedure, including DEAE-Trisacyl, chromatofocusing, AcA 34 gel filtration, HA Ultrogel and preparative polyacrylamide gel electrophoresis. The purified enzymes were homogeneous on polyacrylamide gel electrophoresis. The specific activities of kidney and intestinal pure trehalase were identical. Kidney and intestinal trehalases have the same molecular weight (about 85 000 by Sephadex G-200 gel filtration and 75 000 by SDS-polyacrylamide gel electrophoresis). A Stokes radius of 38 A was determined. Detergent solubilized trehalase is not retarded by phenyl-Sepharose CL-4B chromatography. The isoelectric point, measured by chromatofocusing, is between pH 3.8 and 4.2 for kidney trehalase and between pH 4.6 and 4.8 for intestinal trehalase. The enzymic properties for both kidney and intestinal trehalases are identical. The optimum pH is between 5.5 and 6.0. Trehalase is heat stable up to 50 degrees C. The apparent Km was found to be 3.5 mM in maleate buffer pH 6.0. The activation energy of trehalase is 11.17 kcal/mol. Tris, sucrose and phloridzin are fully competitive inhibitors with Ki of 3.7 mM, 3.1 mM, 1.1 mM respectively. Schiff's staining on polyacrylamide gel and interaction with Con A-Sepharose indicate that trehalase is a glycoprotein. Trehalase accounts for 0.1% and 0.3% of total brush-border membrane protein of intestine and kidney, respectively.

Distribution and biosynthesis of aminopeptidase N and dipeptidyl aminopeptidase IV in rat small intestine

The regional, cellular and subcellular distribution patterns of aminopeptidase N and dipeptidyl aminopeptidase IV were examined in rat small intestine. Aminopeptidase N of brush border membrane had maximal activity in the upper and middle intestine, while dipeptidyl aminopeptidase IV had a more uniform distribution profile with relatively high activity in the ileum. Along the villus and crypt cell gradient, the activity of both enzymes was maximally expressed in the mid-villus cells. However there was substantial dipeptidyl aminopeptidase IV activity in the crypt cells. Both enzymes were primarily associated with brush border membranes in all segments, however, in the proximal intestine, a significant amount of dipeptidyl aminopeptidase IV activity was associated with the cytosol fraction. The cytosol and brush border membrane forms of dipeptidyl aminopeptidase IV were immunologically identical and had the same electrophoretic mobility on disc gels. In contrast, the soluble and brush border membrane-bound forms of aminopeptidase N were immunologically distinct. When the total amount of aminopeptidase N and dipeptidyl aminopeptidase IV was determined by competitive radioimmunoassay, there were no regional or cellular differences in specific activity (enzyme activity/mg of enzyme protein) of either enzyme in brush border membrane and homogenate. The specific activity of both enzymes in a purified Golgi membrane fraction as measured by radioimmunoassay was about half that of the brush border membrane fraction. These results suggest that (1) aminopeptidase N and dipeptidyl aminopeptidase IV have different regional, cellular and subcellular distribution patterns; (2) there are enzymatically inactive forms of both enzymes present in a constant proportion to active molecules and that (3) a two-fold activation of precursor enzyme forms occurs during transfer from the Golgi membranes to the brush border membranes.