Low expression of Neu2 sialidase in the thymus of SM/J mice—existence of neuraminidase positive cells “Neu-medullocyte” in the murine thymus

We have already reported that the homogenate of the A/J mouse thymus shows a high sialidase activity at the neutral pH region and that in both soluble and membrane fractions optimal pH was 6.5–7 (Kijimoto-Ochiai et al., Glycoconj. J., 20:375–384, 2004). In the present study, we investigated the level of sialidase activities in the thymus of the SM/J mouse, a mouse strain that we know to have a Neu1^a allele that reveals a low level of sialidase activity in the liver. We found that while in the A/J thymus the soluble sialidase activity at pH 6.5 was high, the SM/J thymus lacked all such activity. A QTL analysis of SMXA recombinant inbred strains showed that soluble sialidase activity correlated well with the D1Mit8/9 marker on chromosome 1. The murine whole DNA-sequence data and the results of our FISH analysis (Kotani et al., Biochem. Biophys. Res. Comm., 286:250–258, 2001) showed that this location is consistent with the position of Neu2 gene. We confirmed that it is hard to detect the Neu2 enzyme of the SM/J mouse thymus by an anti-Neu2 antibody using a Western blot analysis. We also found that while the mRNA expression of Neu2 was quite normal in the SM/J mouse liver, it was very low in the SM/J mouse thymus. We therefore conclude that the lack of soluble sialidase activity in the SM/J mouse thymus is due to the thymus-specific low expression level of the Neu2 gene. We have previously shown that the sialidase positive cell which contains the Mac-1 and immunoglobulin, and which is located sparsely in the corticomedullar region or medullary region of the A/J mouse thymus (Kijimoto-Ochiai et al., Glycoconj. J., 20:375–384, 2004). We showed now in this paper that the detection of this cell in the SM/J mouse thymus at pH 7.0 was difficult. We propose, therefore, to name the cell “Neu-medullocyte”.     

Localization of sialidase-positive cells expressing Mac-1 and immunoglobulin in the mouse thymus

We have sought an endogenous membrane bound sialidase acting at neutral pH in immune system, because the removal of sialic acid from cell surfaces will affect the cell-cell interaction directly or indirectly. The levels of activity of unique membrane-bound sialidase at neutral pH and also soluble sialidase are high in the thymus but low in the spleen and lymph nodes. These are thought to be plasma membrane and cytosolic types based on the behavior of inhibition by Cu(2+) and 2-deoxy-2, 3-dehydro-N-acetylneuraminic acid. Newly synthesized 5-bromo-4-chloro-3-indolyl-N-acetylnueraminic acid was used for histochemical staining of sialidase-positive thymic cells, and the results showed positive cells sparsely distributed in the corticomedullar region or medullary region of the thymus. They expressed immunoglobulin and Mac-1 antigen on their surfaces. These cells must therefore be of a B cell lineage, not a T cell lineage. We also found that some vessels in the thymus were sialidase-positive.     

Localization of sialidase-positive cells expressing Mac-1 and immunoglobulin in the mouse thymus

We have sought an endogenous membrane bound sialidase acting at neutral pH in immune system, because the removal of sialic acid from cell surfaces will affect the cell-cell interaction directly or indirectly. The levels of activity of unique membrane-bound sialidase at neutral pH and also soluble sialidase are high in the thymus but low in the spleen and lymph nodes. These are thought to be plasma membrane and cytosolic types based on the behavior of inhibition by Cu²⁺ and 2-deoxy-2, 3-dehydro-N-acetylneuraminic acid. Newly synthesized 5-bromo-4-chloro-3-indolyl-N-acetylnueraminic acid was used for histochemical staining of sialidase-positive thymic cells, and the results showed positive cells sparsely distributed in the corticomedullar region or medullary region of the thymus. They expressed immunoglobulin and Mac-1 antigen on their surfaces. These cells must therefore be of a B cell lineage, not a T cell lineage. We also found that some vessels in the thymus were sialidase-positive. Published in 2004. This revised version was published online in July 2006 with corrections to the Cover Date.     

Molecular characterization of membrane type and ganglioside-specific sialidase (Neu3) expressed in E. coli

Endogenous expression of human membrane type ganglioside sialidase (Neu3) was examined in various cell lines including NB-1, U87MG, SK-MEL-2, SK-N-MC, HepG2, Hep3B, Jurkat, HL-60, K562, ECV304, Hela and MCF-7. Expression was detected in the neuroblastoma cell lines NB-1 and SK-N-MC, and also in erythroleukemia K562 cells, but not in any other cells. We isolated a Neu3 cDNA from K562 cells and expressed a His-tagged derivative in a bacterial expression system. The purified recombinant product of approximately 48 kDa had sialidase activity toward 4-methyl-umbelliferyl-alpha-D-N-acetylneuraminic acid (4MU-NeuAc). The optimal pH of the purified Neu3 protein for GD3 ganglioside was 4.5. The enzyme also efficiently hydrolyzed GD3, GD1a, GD1b and GM3 whereas sialyllactose, 4MU-NeuAc, GM1 and GM2 were poor substrates, and it had no activity against sialylated glycoproteins such as fetuin, transferrin and orosomucoid. We conclude that the sialidase activity of Neu3 is specific for gangliosides.     

The action of sialidases on substrates containing O-acetylsialic acids

O-Acetyl substitution of sialic acids in glycoconjugates reduces the rate of action of sialidases on these substrates. A plasma glycoprotein fraction and an erythrocyte ganglioside containing 4-O-acetylsialic acids were isolated and characterized from equine blood, and a sialyllactose preparation with Neu5,9Ac2 was purified from rat urine. Using the novel substrates II3Neu4Ac5Gc-LacCer and II3Neu5,9Ac2-Lac the influence of individual mono-O-acetylated sialic acids on bacterial and viral sialidases could be clearly shown. This extends and clarifies observations with glycoproteins containing mixtures of mono-, di- and higher O-acetylated sialic acids with substitution at the hydroxyls on carbons 4, 7, 8 and 9. A 4-O-acetyl substitution in sialic acids blocks the action of bacterial sialidases for substrates containing these derivatives, while viral enzymes show low but significant activity, reflected in Km and Vmax values. A small reduction in bacterial sialidase activity was observed for II3Neu5,9Ac2-Lac relative to II3Neu5Ac-Lac in agreement with kinetic analysis. Newcastle disease virus sialidase showed a 50% reduction in hydrolysis rate for the 9-O-acetylated substrate and ten-fold reductions of both Km and Vmax values.     

Specific expression of Neu2 type B in mouse thymus and the existence of a membrane-bound form in COS cells

Neu2 mRNA from the mouse thymus, as we have reported [K. Kotani, A. Kuroiwa, T. Saito, Y. Matsuda, T. Koda, S. Kijimoto-Ochiai, Cloning, chromosomal mapping, and characteristic 5′-UTR sequence of murine cytosolic sialidase, Biochem. Biophys. Res. Commun. 286 (2001) 250-258], has a novel sequence at the 5′ terminus that shows the ability to encode 6 extra amino acids in the N-terminus than that of the muscle. In this paper, we analyzed the cDNA and EST database and found the five types of alternative splicing of Neu2 mRNA: A, B, C, D and N. We studied the expression of these types in the immune tissues and found that the thymus expressed only type B. We constructed 2 types of plasmid that encode long (B) or short (C) form of Neu2 protein, and transfected them into COS7 cells to study them under the same conditions. We found that 30-40% of the both forms of Neu2 activity was located in the crude membrane-fraction, and hydrolyzed ganglioside effectively, while both soluble fraction showed particular behavior with substrate specificity. Microscopic study by active staining with X-NANA showed that they located not only in the cytoplasm but also in areas surrounding the nucleus and in the peripheral ruffled spot.     

A close association of the ganglioside-specific sialidase Neu3 with caveolin in membrane microdomains

The ganglioside-specific sialidase Neu3 has been suggested to play essential roles in regulation of cell surface functions because of its major localization in the plasma membrane and strict substrate preference for gangliosides involved in signal transduction. Here we show that human Neu3 sialidase is enriched in caveolae microdomains and closely associates with caveolin like other caveolin-binding signaling molecules. Using HeLa cells and Neu3-transfected COS-1 cells, endogenous and exogenous Neu3 was found to co-concentrate caveolin-1 in low density Triton X-100-insoluble membrane fractions on sucrose density gradients of the respective cell extracts, as assessed by enzyme activity assays and immunoblotting with a monoclonal antibody to human Neu3. The presence of a putative caveolin-binding motif within Neu3 prompted us to determine whether Neu3 binds to caveolin-1. In transfectants expressing a polyhistidine-tagged form of Neu3, caveolin-1 co-eluted with Neu3 on affinity column chromatography. A mutation with a single amino acid change in the caveolin-binding motif led to inhibition of recruitment of the sialidase to the microdomain, accompanied by reduction of the enzyme activity. Neu3 also failed to associate with caveolin-enriched microdomains by cholesterol depletion with beta-cyclodextrin (with concomitant decrease of the sialidase activity), whereas Neu3 was activated by increased caveolin-1 expression. The tight association of Neu3 with caveolin-1 was supported further by co-immunoprecipitation of Neu3 by anti-caveolin-1 antibody. These results strongly suggest that Neu3 functions as a caveolin-related signaling molecule within caveolin-rich microdomains.     

Initial characterization of human thymocyte sialidase activity: Evidence that this enzymatic system is not altered during the course of T-cell maturation

• 1.1. The sialidase activity of human thymocyte was examined by a fluorogenic assay.
• 2.2. These studies revealed that human thymocyte sialidase activity is essentially acid-active and membrane-bound since 59.6% and 33% of the total activity was recovered in the lysosome-enriched and microsomal fractions, respectively.
• 3.3. A weak activity was also detected in the cytosolic fraction.
• 4.4. However, the acidic optimum pH of this soluble sialidase was at variance with the general concept of mammalian soluble sialidases which are known to be optimally active at more neutral pH.
• 5.5. This acidic soluble sialidase seems to be a general characteristic of the human T-cell lineage since examination of mature circulating T-cells revealed that they contain a soluble sialidase activity similar to that observed in thymocytes.
• 6.6. Analysis of mature and immature thymocyte subpopulation obtained by differential PNA agglutination indicated that this enzymatic system was not altered during the course of thymic maturation.
• 7.7. These results suggest that unlike in T-cell activation where changes in the level of sialidase activity were shown to influence the extent of cell surface sialylation and thereby the cell physiology, this enzymatic system seems not to be involved in the fluctuation of cell surface sialic acid content observed during thymic maturation.

     

Rat-liver lysosomal sialidase. Solubilization, substrate specificity and comparison with the cytosolic sialidase

Purified liver lysosomes, prepared from rats previously injected with Triton WR-1339, exhibited sialidase activity towards sialyllactose, fetuin, submaxillary mucin (bovine) and gangliosides, and could be disrupted hypotonically with little loss in these activities. After centrifugation, the activities with sialyllactose and fetuin were largely recovered in the supernatant, demonstrating that they were originally in the intralysosomal space. The activities towards submaxillary mucin and gangliosides, on the other hand, remained in the pellet. In the supernatant, activity with fetuin or orosomucoid was markedly reduced by protease inhibitors, suggesting that proteolysis of these glycoproteins may be prerequisite to sialidase activity. The intralysosomal sialidase was solubilized from the mitochondrial-lysosomal fraction of rat liver and partially purified by Sephadex G-200, or Sephadex G-200 followed by CM-cellulose. The enzyme was maximally active at pH 4.7 with sialyllactose as substrate and had a minimum relative molecular mass of 60 000 +/- 5000 by gel filtration; it hydrolyzed a variety of sialooligosaccharides , those containing (alpha 2----3)sialyl linkages being better substrates than those with (alpha 2----6)sialyl linkages. The enzyme failed to attack submaxillary mucin and gangliosides. It was also inactive towards fetuin, orosomucoid and transferrin but capable of hydrolyzing glycopeptides from pronase digest of fetuin. In contrast to the intralysosomal sialidase, the sialidase partially purified from rat liver cytosol by (NH4)2SO4 fractionation followed by chromatography on DEAE-cellulose and CM-cellulose hydrolyzed fetuin and orosomucoid to the extent about half that for sialyllactose. The enzyme was maximally active at pH 5.8 and had a relative molecular mass of approximately 60 000. It also hydrolyzed gangliosides but not submaxillary mucin.     

Recovery of function and mass of endogenous beta-cells in streptozotocin-induced diabetic rats treated with islet transplantation

We found that the hepatopancreas of oyster, Crassostrea virginica, contained a sialidase capable of releasing Neu5Gc from the novel polysialic acid chain (-->5-O(glycolyl)Neu5Gcalpha2-->)n more efficiently than from the conventional type of polysialic acid chains, (-->8Neu5Acalpha2-->)n, or (-->8Neu5Gcalpha2-->)n. We have partially purified this novel sialidase and compared its reactivity with that of microbial sialidases using four different sialic acid dimers, Neu5Gcalpha2-->5-O(glycolyl)Neu5Gc (Gg2), Neu5Acalpha2-->8Neu5Ac (A2), Neu5Gcalpha2-->8Neu5Gc (G2), and KDNalpha2-->8KDN (K2) as substrates. Hydrolysis was monitored by high performance anion-exchange chromatography with a CarboPac PA-100 column and pulsed amperometric detection, the method by which we can accurately quantitate both the substrate (sialiac acid dimers) and the product (sialic acid monomers). The oyster sialidase effectively hydrolyzed Gg2 and K2, whereas A2 and G2 were poor substrates. Neu5Ac2en but not KDN2en effectively inhibited the hydrolysis of Gg2 by the oyster sialidase. Likewise, the hydrolysis of K2 by the oyster sialidase was inhibited by a cognate inhibitor, KDN2en, but not by Neu5Ac2en. Using the new analytical method we found that Gg2 was hydrolyzed less efficiently than A2 but much more readily than G2 by Arthrobacter ureafaciens sialidase. This result was at variance with the previous report using the thiobarbituric acid method to detect the released free sialic acid [Kitazume, S., et al. (1994) Biochem. Biophys. Res. Commun. 205, 893-898]. In agreement with previous results, Gg2 was a poor substrate for Clostridium perfringens sialidase, while K2 was refractory to all microbial sialidases tested. Thus, the oyster sialidase is novel and distinct from microbial sialidases with regards to glycon- and linkage-specificity. This finding adds an example of the presence of diverse sialidases, in line with the diverse sialic acids and sialic acid linkages that exist in nature. The new sialidase should become useful for both structural and functional studies of sialoglycoconjugates.     

Characterization of a sialidase (neuraminidase) isolated from Clostridium chauvoei (Jakari strain)

A sialidase from Clostridium chauvoei (Jakari strain), an indigenous bacterial strain that causes blackleg in Nigerian cattle and other ruminants was isolated and partially purified by chromatography on DEAE cellulose, hydroxyapatite and phenyl agarose columns. The enzyme migrated as a 65-kDa protein after electrophoresis on sodium dodecyl sulphate polyacrylamide gels. It was optimally active at pH 4.5 and 40 degrees C with an activation energy (Ea) of 13.40 kJ mol(-1). It had Km and Vmax values of 170 microM and 200 micromole h(-1) mg(-1) respectively with fetuin as substrate. When sialyllactose (Neu5Ac2,3 lactose) was used as substrate the Km and Vmax values were 8 microM and 5 micromoles min(-1) mg(-1) respectively. The Clostridium chauvoei sialidase cleaved sialic acids from RBC ghosts of sheep, horse, goat, cattle, pig and mice as well as mouse brain cells, albeit at different rates. The enzyme was activated by Ca2+ and Mg2+ and inhibited by the group-specific reagents diethylpyrocarbonate (DEP) and N-ethylmalemide (NEM). The sialidase inhibitors, 2,3 didehydroneuraminic acid (Neu5Ac2,3en) and paranitrophenyl oxamic acid (pNPO) inhibited the enzyme competitively with Ki values of 40 and 30 microM respectively.     

Evidence for sialidase activity in K 562 cells: inhibition by adriamycin treatment

Sialidase activity has been studied in the human erythroleukemia K 562 cell line grown in vitro. The total sialidase activity was determined using disialoganglioside GD1a and fetuin as exogenous substrates. The enzymatic activity was stimulated by 0.08% Triton X-100 and reached the highest level at pH 4.0. Results obtained showed that gangliosides are hydrolysed more extensively than glycoproteins by K 562 sialidases. This finding could suggest that endogenous gangliosides may be the main source of metabolically available sialic acid in K 562 cell line. After treatment of K 562 cells by Adriamycin (40 nM), a potent anticancer drug, sialidase activity decreased by 40% as compared to control cells. This decrease occurs early during the first day of incubation with Adriamycin. This inhibition of sialidase activity could explain previous results obtained in our laboratory which show an enhanced sialylation of the membrane glycoconjugates after Adriamycin treatment.     

Selective ganglioside desialylation in the plasma membrane of human neuroblastoma cells*

Gangliosides of the plasma membrane are important modulatorsof cellular functions. Previous work from our laboratory hadsuggested that a plasma membrane sialidase was involved in growthcontrol and differentiation in cultured human neuroblastomacells (SK-N-MC), but its substrates had remained obscure. Wenow performed sialidase specificity studies in subcellular fractionsand found ganglioside GM3 desialylating activity in presenceof Triton X-100 to be associated with the plasma membrane, butabsent in lysosomes. This Triton-activated plasma membrane enzymedesialylated also gangliosides GDla, GD1b, and GT1b, therebyforming GM1; cleavage of GM1 and GM2, however, was not observed.Sialidase activity towards the glycoprotein fetuin with modifiedC-7 sialic acids and towards 4-methylumbelliferyl neuraminatewas solely found in lysosomal, but not in plasma membrane fractions. The role of the plasma membrane sialidase in ganglioside desialylationof living cells was examined by following the fate of [³H]galactose-labelledindividual gangliosides in pulse-chase experiments in absenceand presence of the extracellular sialidase inhibitor 2-deoxy-2,3-dehydro-N-acetylneuraminicacid. When the plasma membrane sialidase was inhibited, radioactivityof all gangliosides chased at the same rate. In the absenceof inhibitor, GM3, GD1a, GD1b, GD2, GD3 and GT1b were degradedat a considerably faster rate in confluent cultures, whereasthe GM1-pool seemed to be filled by the desialylation of highergangliosides. The results thus suggest that the plasma membranesialidase causes selective ganglioside desialylation, and thatsuch surface glycolipid modification triggers growth controland differentiation in human neuroblastoma cells. ganglioside neuroblastoma cells plasma membrane sialidase     

Specificity of the N1 and N2 sialidase subtypes of human influenza A virus for natural and synthetic gangliosides

Sialyl-linkage specificity of sialidases of the human influenza A virus strains, A/Aichi/2/68 (H3N2) and A/PR/8/34 (H1N1) were studied using natural and synthetic gangliosides. The sialidase of the A/Aichi/2/68 strain hydrolyzed the terminal Neu5Acalpha2-3Gal sequence but not the Neu5Acalpha2-3 linkage on the inner Gal of GM1a, which is a ganglioside that has the gangliotetraose chain (Galbeta1-3GalNAcbeta1-4- (Neu5Acalpha2-3)Galbeta1++ +-4Glcbeta1-Cer). The sialidase hydrolyzed the Neu5Ac on the inner Gal of GM2, which had a shorter gangliotriose chain. GM4, which had the shortest chain (Neu5Acalpha2-3Galbeta1-Cer) of the gangliosides, had a lower substrate specificity. The N1 and N2 sialidase subtypes of the human influenza A virus had no significant variation in their substrate specificity for the gangliosides. Analysis of 11 synthetic gangliosides, which contained various ceramide or sialic acid moieties, demonstrated that A/Aichi/2/68 (H3N2) sialidase recognized the ceramide and sialic acid moiety and the length and structure of the sialyl sugar chain.      

Enzymatic 4-O-acetylation of N-acetylneuraminic acid in guinea-pig liver

Sialic acids from the liver and serum of guinea-pig are composed of N-acetylneuraminic acid (Neu5Ac; 85% and 61%, respectively), N-acetyl-4-O-acetylneuraminic acid (Neu4,5Ac2; 10% and 32%, respectively) and N-glycolylneuraminic acid (Neu5Gc; 5% and 7%, respectively), besides traces of N-glycolyl-4-O-acetylneuraminic acid in serum. The analysis was carried out using thin-layer chromatography, high-performance liquid chromatography, electron impact ionization mass spectrometry, and different enzymes (sialidase, sialate esterase, and sialate-pyruvate lyase after hydrolysis and purification of the sialic acids by ion-exchange chromatography). We showed that this O-acetylation of sialic acids is due to the activity of an acetyl-coenzyme A:sialate-4-O-acetyltransferase (EC 2.3.1.44), which occurs together with sialyltransferase activity in Golgi-enriched membrane fractions of guinea-pig liver. The enzyme operates optimally at 30°C in 70 mM potassium phosphate buffer at pH 6.7 and in the presence of 90 mM KCl with an apparent KM for AcCoA of 0.6 1M and a Vmax of 20 pmol/mg protein x min. The enzyme is inhibited by coenzyme A in a mixed-competitive manner (Ki = 4.2 M), as well as by para-chloromercuribenzoate, MnCl2, saponin and Triton X-100.     

Different behavior of ghost-linked acidic and neutral sialidases during human erythrocyte ageing

Acidic and neutral sialidases (pH optimum 4.7 and 7.2, respectively) were assayed on human circulating erythrocytes during ageing. The assays were performed on intact erythrocytes and resealed erythrocyte ghost membranes. From young to senescent erythrocytes the acidic sialidase featured a 2.7-fold and 2.5-fold decrease in specific activity when measured on intact cells or resealed ghost membranes, whereas the neutral sialidase a 5-fold and 7-fold increase, respectively.The Ca²⁺-loading procedure was employed to mimic the vesiculation process occurring during erythrocyte ageing. Under these conditions the released vesicles displayed an elevated content of acidic sialidase, almost completely linked through a glycan phosphoinositide (GPI) anchor but no neutral sialidase activity, that was completely retained by remnant erythrocytes together with almost all the starting content of sialoglycoconjugates. The loss with vesiculation of acidic sialidase with a concomitant relative increase of neutral sialidase was more marked in young than senescent erythrocytes.The data presented suggest that during ageing erythrocytes loose acidic sialidase, and get enriched in the neutral enzyme, the vesiculation process, possibly involving GPI-anchors-rich membrane microdomains, being likely responsible for these changes. The enhanced neutral sialidase activity might account for the sialic acid loss occurring during erythrocyte ageing.     

Differential expression of endogenous sialidases of human monocytes during cellular differentiation into macrophages

Sialidases are enzymes that influence cellular activity by removing terminal sialic acid from glycolipids and glycoproteins. Four genetically distinct sialidases have been identified in mammalian cells. In this study, we demonstrate that three of these sialidases, lysosomal Neu1 and Neu4 and plasma membrane-associated Neu3, are expressed in human monocytes. When measured using the artificial substrate 2'-(4-methylumbelliferyl)-alpha-d-N-acetylneuraminic acid (4-MU-NANA), sialidase activity of monocytes increased up to 14-fold per milligram of total protein after cells had differentiated into macrophages. In these same cells, the specific activity of other cellular proteins (e.g. beta-galactosidase, cathepsin A and alkaline phosphatase) increased only two- to fourfold during differentiation of monocytes. Sialidase activity measured with 4-MU-NANA resulted from increased expression of Neu1, as removal of Neu1 from the cell lysate by immunoprecipitation eliminated more than 99% of detectable sialidase activity. When exogenous mixed bovine gangliosides were used as substrates, there was a twofold increase in sialidase activity per milligram of total protein in monocyte-derived macrophages in comparison to monocytes. The increased activity measured with mixed gangliosides was not affected by removal of Neu1, suggesting that the expression of a sialidase other than Neu1 was present in macrophages. The amount of Neu1 and Neu3 RNAs detected by real time RT-PCR increased as monocytes differentiated into macrophages, whereas the amount of Neu4 RNA decreased. No RNA encoding the cytosolic sialidase (Neu2) was detected in monocytes or macrophages. Western blot analysis using specific antibodies showed that the amount of Neu1 and Neu3 proteins increased during monocyte differentiation. Thus, the differentiation of monocytes into macrophages is associated with regulation of the expression of at least three distinct cellular sialidases, with specific up-regulation of the enzyme activity of only Neu1.     

Degradation of Alzheimer's ß-Amyloid Protein by Human and Rat Brain Peptidases: Involvement of Insulin-Degrading Enzyme

We examined the degradation of Alzheimer's ß-amyloid protein (1–40) by soluble and synaptic membrane fractions from post mortem human and fresh rat brain using HPLC. Most of the activity at neutral pH was in the soluble fraction. The activity was thiol and metal dependent, with a similar inhibition profile to insulin-degrading enzyme. Immunoprecipitation of insulin-degrading enzyme from the human soluble fraction using a monoclonal antibody removed over 85% of the ß-amyloid protein degrading activity. Thus insulin-degrading enzyme is the main soluble ß-amyloid degrading enzyme at neutral pH in human brain. The highest ß-amyloid protein degrading activity in the soluble fractions occurred between pH 4–5, and this activity was inhibited by pepstatin, implicating an aspartyl protease. Synaptic membranes had much lower ß-amyloid protein degrading activity than the soluble fraction. EDTA (2mM) caused over 85% inhibition of the degrading activity but inhibitors of endopeptidases –24.11, –24.15, –24.16, angiotensin converting enzyme, aminopeptidases, and carboxypeptidases had little or no effect.     

4-Methylumbelliferyl-alpha-glycosides of partially O-acetylated N-acetylneuraminic acids as substrates of bacterial and viral sialidases

4-O-Acetylated, 7-O-acetylated, and 9-O-acetylated 4-methylumbelliferyl-alpha-N-acetyl-neuraminic acids (Neu4,5Ac2-MU, Neu5,7Ac2-MU, Neu5,9Ac2-MU) were tested as substrates of sialidases of Vibrio cholerae and of Clostridium perfringens. Both sialidases were unable to hydrolyse Neu4,5Ac2-MU. This compound at 1 mM concentration did not inhibit significantly the cleavage of Neu5Ac-MU, the best substrate tested. The 4-O-acetylated sialic acid glycoside is hydrolysed slowly by the sialidase from fowl plague virus. The relative substrate specificity, reflected in V/Km of the Vibrio cholerae sialidase is Neu5Ac-MU much greater than Neu5,7Ac2-MU approximately Neu5,9Ac2-MU and of the clostridial enzyme it is Neu5Ac-MU greater than Neu5,9Ac2-MU greater than Neu5,7Ac2-MU. The affinities of both enzymes for the side-chain O-acetylated sialic acid derivatives are higher than for Neu5Ac-MU. The artificial, well-defined substrates, described here, provide the opportunity to quantify the influence of sialic acid O-acetylation on the hydrolysis of sialoglycoconjugates without the side effects introduced by other parts of more complex glycans.