GalNAc-alpha-O-benzyl inhibits NeuAcalpha2-3 glycosylation and blocks the intracellular transport of apical glycoproteins and mucus in differentiated HT-29 cells

Exposure for 24 h of mucus-secreting HT-29 cells to the sugar analogue GalNAc-alpha-O-benzyl results in inhibition of Galbeta1-3GalNAc:alpha2,3-sialyltransferase, reduced mucin sialylation, and inhibition of their secretion (Huet, G., I. Kim, C. de Bolos, J.M. Loguidice, O. Moreau, B. Hémon, C. Richet, P. Delannoy, F.X. Real., and P. Degand. 1995. J. Cell Sci. 108:1275-1285). To determine the effects of prolonged inhibition of sialylation, differentiated HT-29 populations were grown under permanent exposure to GalNAc-alpha-O-benzyl. This results in not only inhibition of mucus secretion, but also in a dramatic swelling of the cells and the accumulation in intracytoplasmic vesicles of brush border-associated glycoproteins like dipeptidylpeptidase-IV, the mucin-like glycoprotein MUC1, and carcinoembryonic antigen which are no longer expressed at the apical membrane. The block occurs beyond the cis-Golgi as substantiated by endoglycosidase treatment and biosynthesis analysis. In contrast, the polarized expression of the basolateral glycoprotein GP 120 is not modified. Underlying these effects we found that (a) like in mucins, NeuAcalpha2-3Gal-R is expressed in the terminal position of the oligosaccharide species associated with the apical, but not the basolateral glycoproteins of the cells, and (b) treatment with GalNAc-alpha-O-benzyl results in an impairment of their sialylation. These effects are reversible upon removal of the drug. It is suggested that alpha2-3 sialylation is involved in apical targeting of brush border membrane glycoproteins and mucus secretion in HT-29 cells.

Synthesis of neoglycoconjugates containing deaminated neuraminic acid (KDN) using rat liver alpha2,6-sialyltransferase

2-Keto-3-deoxy-D- glycero -D- galacto -nononic acid (KDN) was introduced into asialotransferrin and N -acetyllactosamine (LacNAc) from CMP-KDN by using rat liver Galbeta1-->4GlcNAc alpha2, 6-sialyltransferase to form KDN-transferrin and KDN-LacNAc. These structures contain terminal KDNalpha2-->6Gal-residues, a glycotope that has not yet been described in natural glycoconjugates. KDN was transferred to all four Gal residues in asialotransferrin by this enzyme. The incorporation efficiency of KDN from CMP-KDN into asialotransferrin was about half that of Neu5Ac from CMP-Neu5Ac, based on the V max/ K m values for these donor substrates, 0.0527 min-1and 0.119 min-1, respectively. The KDNalpha2-->6Gal linkage was resistant to exosialidase treatment, in contrast to the sensitivity of the Neu5Acalpha2-->6Gal linkage. Interestingly, Sambucus sieboldiana agglutinin (SSA) was shown to prefer KDN-transferrin to the corresponding Neu5Ac-transferrin, as estimated by slot-blot analysis. The use of an alpha2,6-sialyltransferase to synthesize neoglycoproteins containing KDN has not been previously reported. Their facile synthesis using CMP-KDN and sialyltransferases with different specificities offers new possibilities to study the function of neo-KDN-glycoconjugates, and to explore their use in glycotechnology.

NeuAc alpha 2,3gal-glycoconjugate expression determines cell susceptibility to the porcine rubulavirus LPMV

Relevance of membrane sialoglycoconjugates as receptors for infection by the porcine rubulavirus has been determined in vitro by sugar and lectin competition assays and by inhibition of glycosylation. Our results show that NeuAc alpha 2,3Gal but not NeuAc alpha 2,6Gal inhibits the virus infectivity of Vero cells, and the virus was effectively blocked with the lectin Maackia amurensis, specific for NeuAc alpha 2,3Gal. Inhibition of the cellular glycosylation with tunicamycin, deoxinojirimycin as well as neuraminidase treatment diminishes the viral capacity to bind and infect this cell line. Dexamethasone, which promotes the activity of sialyl alpha 2,6 glycosyltransferase, also diminishes the cell susceptibility for infection. This is the first report confirming that NeuAc alpha-2,3Gal recognition is determinant in the pathogenesis of the porcine rubulavirus.

Developmental changes in UDP-N-acetylglucosamine 2-epimerase activity of rat and guinea-pig liver

The activity of UDP-N-acetylglucosamine 2-epimerase was determined in the liver of rats and guinea-pigs of different ages. The activity of this enzyme in rats was low at birth, increased to a maximum value on day 15, and fell gradually until day 30. Thereafter, it increased up to the 60th day. The activity profile of the enzyme from guinea-pig liver was very similar. However, guinea-pig activity was 2-5 times lower than in rats. Both rats and guinea-pigs displayed similar liver sialic acid contents which increased from birth to 2 months of age. Rats also showed a N-glycolylneuraminic acid content that decreased from birth to 2 months. From these results we can inferred that postnatal UDP-N-acetylglucosamine 2-epimerase activity seems to be correlated with age and the developmental states of rats and guinea-pigs.

Is the lectin binding pattern of human breast and colon cancer cells influenced by modulators of sialic acid metabolism?

Sialic acid residues are the most abundant terminal carbohydrate residues of mammalian cells. Modification of the sialic acid residues by exposure of cells in culture to sialic acid precursor analogues resulted in a modified susceptibility to polyoma viruses. In the present study, human breast and colon cancer cell lines were exposed for 65 h to these acid precursor analogues at 5 mM and their lectin binding pattern was analysed. Use of a panel of several different lectins indicated that the pretreatment of these cell lines with the sialic acid analogues did not change their lectin binding profile. The incorporation of these precursors into membrane glycoproteins was assessed by reversed phase high-performance liquid chromatography, which clearly demonstrated that the precursors were incorporated. The results therefore indicate that these analogues are highly specific for sialic acid and do not interfere with other biosynthetic pathways of membrane glycoconjugates.

Adhesion of Helicobacter pylori strains to alpha-2,3-linked sialic acids

Helicobacter pylori is a human pathogen associated with gastritis and peptic ulcer. Adhesion properties of H. pylori to various structures have been described in the literature, including evidence for sialic acid-binding. To study the specificity and frequency of sialic acid-binding, fourteen H. pylori strains were investigated using haemagglutination with derivatized erythrocytes carrying sialic acids only on defined glycans and using haemagglutination inhibition assays. From these studies H. pylori strains can be grouped into sialic acid-dependent and sialic acid-independent classes. The sialic acid-dependent strains require alpha-2,3-linked sialic acid for haemagglutination. The potential roles of sialic acid-dependent adhesions for H. pylori-related infections are discussed.

M-cells in the rabbit tonsil exhibit distinctive glycoconjugates in their apical membranes

The tonsil crypt epithelium contains membranous (M)-cells that transport antigens from the lumen to underlying lymphoid cells, thereby initiating specific immune responses. Mechanisms mediating the adhesion of antigens to the M-cell surface are important for effective and selective uptake of potential pathogens but are still poorly understood. Therefore, the carbohydrates present on crypt epithelial cells of the rabbit palatine tonsil were studied by lectin histochemistry. Ultrathin LR White sections were incubated with a panel of eight lectins conjugated to colloidal gold or biotin. The glycocalyx of the apical membrane of M-cells was selectively labeled by UEA-I, LTA, HPA, and VVA, whereas that of the remaining squamous epithelial cells preferentially bound RCA-I and PNA. WGA and ConA showed only little binding, with no discernible preference for any of the cell types. Double labeling of UEA-1 together with anti-vimentin antibodies revealed that UEA-I-positive epithelial cells also contained the rabbit M-cell marker vimentin, and vice versa. The results show that a specific composition of glycoconjugates, which differs from that on squamous epithelial cells, is found on M-cells of the rabbit tonsil. The M-cell-specific glycoproteins and glycolipids could be selectively targeted by microorganisms that adhere to M-cells and enter the host along this pathway.

Ecto-sialyltransferase of human B lymphocytes reconstitutes differentiation markers in the presence of exogenous CMP-N-acetyl neuraminic acid

The existence of an ecto-sialyltransferase (ecto-ST) on B lymphocytes with increasing activity at late maturation stages is shown using a novel flow cytometric enzyme assay. This ecto-ST is effective in reconstituting different surface glycoconjugates on desialylated B cells in the presence of exogenous CMP-NeuAc. We found that this ecto-ST is distinct in its activity from soluble ST released into the culture supernatant. Surface sialylation was independent of the amount of ST secreted into the culture supernatant and followed different kinetics than sialylation of exogenous substrate by soluble ST. Four human B-cell lines representing different maturation stages were analyzed for secreted and ecto-ST activity. The myeloma cell line U266 and the lymphoblastoid cell line JOK-1 showed higher activity of both ST forms than the acute lymphoblastic leukemia B-cell line Nalm-6. ST activity in culture supernatants of U266, JOK-1, and Nalm-6 cells consisted predominantly of the alpha 2,6 ST type with specificity for N-linked oligosaccharides. As an exception, the myeloma cell line IM-9, deficient of alpha 2,6 ST activity, secreted only small amounts of ST and showed low activity of ecto-ST. Sialylation of surface-expressed glycoconjugates by ecto-ST was measured by incubating B-cell lines in the presence of fluorescent CMP-sialic acid. Surface structures labeled with fluorescent sialic acid under this condition were visualized by confocal laser scanning microscopy and fluorescent label was quantitatively assessed by flow cytometric analysis on live cells. Incubation of cells in acidified culture medium, to release possibly receptor-bound ST, did not alter the intensity of cell surface sialylation. Inhibition of internalization and membrane traffic by various approaches (reduced incubation temperature and chloroquine or brefeldin A treatment) did not block surface sialylation. Together, these observations point to cell surface sialylation in B lymphocytes mediated by a cell surface-expressed ecto-ST distinct from the secreted ST form. On desialylated JOK-1 cells, ecto-ST in the presence of exogenous CMP-NeuAc was able to resialylate the B-cell surface sialoglycans CDw75 and HB-6 and major surface glycoproteins of B cells, such as HLA class I and II antigens, transferrin receptor, and surface IgM. In contrast, cell surface glycans of coincubated desialylated erythrocytes were not sialylated by the B-cell ecto-ST. Ecto-alpha 2,6 ST of B cells may be involved in the sialylation of distinct differentiation glycan antigens.

Isolation and characterization of sialate lyase from pig kidney

Sialate lyase (sialate aldolase; systematic name N-acetylneuraminate pyruvate-lyase, EC 4.1.3.3) was isolated as soluble enzyme from pig kidney and purified 630-fold using a heating step, gel filtration, and chromatography on immobilized neuraminic acid beta-methyl glycoside in 14% yield to apparent homogeneity as tested by SDS-gel electrophoresis. The molecular mass is 58 kDa and the pH-optimum is at pH 7.2. Kinetic parameters were determined with N-acetyl-neuraminic acid as substrate: Km 3.7 mM and Vmax 37.1 mU. The lyase cleaves only free sialic acids with relative rates of 100% for N-acetylneuraminic acid, 55% for N-glycolylneuraminic acid and 32% for N-acetyl-9-O-acetylneuraminic acid, whereas N-acetyl-4-O-acetylneuraminic acid or 2-deoxy-2,3-didehydro-N-acetylneuraminic acid are not substrates. Enzyme activity was inhibited with p-chloromercuribenzoate, o-phenanthroline, cyanide, 5-diazonium-1-H-tetrazole, 5,5'-dithiobis(2-nitrobenzoic acid), diethylpyro-carbonate, and Rose Bengal in the presence of light and O2. Reduction with sodium borohydride in the presence of N-acetylneuraminic acid or pyruvate resulted in irreversible inhibition of enzyme activity. The inhibition experiments suggest the involvement of histidine, lysine and SH-residues in enzyme catalysis. Thus, this mammalian lyase most probably belongs to the Class I aldolases, and has properties similar to the same enzyme from Clostridium perfringens and is active with the alpha-form of N-acetylneuraminic acid.

Influence of host cell infiltration on the glycolipid content of mouse brain tumors

Previous studies showed that levels of some glycosphingolipids (GSLs) expressed in solid brain tumors grown in vivo were reduced or undetectable in cultured cells prepared from the tumors. This phenomenon has been attributed either to suppressed glycolipid synthesis from unknown forces of the tissue culture environment or to the absence of host cells that normally infiltrate the solid tumors growing in vivo. To test further the host cell hypothesis, we examined host cell markers in two experimental mouse brain tumors, the ependymoblastoma and the CT-2A, that were grown as subcutaneous solid tumors in the flank of C57BL/6J (B6) mice or as cultured cells in vitro. The markers included ganglioside N-glycolylneuraminic acid (NeuGc), GA1 (asialo-GM1), and Fc receptor-bearing cells. NeuGc-containing gangliosides, GA1, and Fc receptors are expressed by macrophages and lymphoid-type cells of the mouse host immune system but are not normally expressed by mouse neural cells. Differences in the relative content of Fc receptor-bearing cells in ependymoblastoma and CT-2A tumors grown in vivo (8.3 and 16.8%, respectively) were proportional to differences in the relative content of NeuGc-containing gangliosides (25.5 and 45.1%) and GA1 (8.5 and 13.8%), respectively. Neither cultured tumor cell line expressed Fc receptors, GA1, or NeuGc-containing gangliosides. These findings suggest that non-neoplastic host infiltrating cells (macrophages) contribute significantly to the GSL composition of solid tumors growing in vivo.

Implication of altered levels of plasma alpha(1)-acid glycoprotein and its derived sialic acid on plasma protein binding of trimethoprim in pigs in physiological and pathological states

In growing pigs (0 to 158 days after birth), pregnant sows (0 to 90 days), and in pigs suffering from respiratory disease (Actinobacillus pleuropneumoniae), the plasma levels of alpha(1)-acid glycoprotein (AGP) and its derived sialic acid were determined, as was the meaning of their altered levels on the plasma protein binding of trimethoprim. The AGP level was very high immediately after birth (more than 10,000 mu g/ml), decreased markedly during 2 weeks after birth (about 700 mu g/ml) and thereafter stayed at a constant level (about 400 mu g/ml). Pregnant sows had a low AGP level with a narrow variation throughout pregnancy (about 190 to 260 mu g/ml). Pigs infected with A. pleuropneumoniae showed an increased AGP level (mean value; 732 mu g/ml) with a wide variation (range: 170-1,840 mu g/ml). N-acetylneuraminic acid (NANA) and N-glycolylneuraminic acid (NGNA) were sialic acid subtypes detected in porcine plasma. In growing pigs, the time course of changes in NANA concentrations was consistent with that of AGP, whereas that of NGNA was different, implying that NANA is a sialic acid subtype derived from porcine AGP, in contrast to NGNA. The relationship between AGP and NANA levels in growing pigs could be expressed by the following equation: NANA=0.14 x AGP + 159 mu g/ml, whereas that in pigs with respiratory disease could be expressed by NANA=0.O67 x AGP + 357 mu g/ml, indicating a low fraction of NANA in AGP in diseased pigs. The regression lines between the AGP level and the plasma protein binding of trimethoprim < or = 2,000 mu g of AGP/ml were similar as follows: binding(%)=0.O23 x GP + 34 in growing pigs and binding(%)=0.O22 x GP+29 in diseased pigs, implying a minor role of sialic acid residues in the binding of basic drugs to AGP. In conclusion, the wide change in plasma AGP levels in diseased pigs as well as during the initial growth phase can alter the plasma protein binding of basic drugs such as trimethoprim, probably leading to a change in drug disposition. The low sialylation of AGP in diseased pigs may not have a great influence on the binding of basic drugs to AGP, implying the quantitative importance of AGP.

Suppression of influenza virus infection by an N-thioacetylneuraminic acid acrylamide copolymer resistant to neuraminidase

We have previously shown that alpha-2-O-methyl-5-N-thioacetylneuraminic acid (alpha-Neu5thioAc2Me) has a higher affinity to bromelain-treated hemagglutinin (HA) of influenza A virus than sialic acid from natural sources (Machytka et al., 1993, FEBS Lett. 334, 117-120). We have now compared the inhibitory effects of alpha-Neu5thioAc2Me and other sialic acid analogs on receptor binding and plaque formation of intact influenza A viruses. When alpha-Neu5thioAc2Me was polymerized by conjugation to polyacrylamide, its affinity to HA increased 10(3)-fold. When analyzed by plaque reduction, the alpha-Neu5thioAc2 polymer was about 10 times more efficient as an inhibitor of virus replication than the alpha-Neu5Ac2 polymer, stressing the importance of sulfur at C5. The S-glycoside alpha-2-S-methyl-5-N-thioacetylneuraminic acid (alpha-Neu5thioAc2SMe) had the same affinity to HA as alpha-Neu5thioAc2Me, but was resistant to neuraminidase. The alpha-Neu5thioAc2S polymer interfered with the replication of a wider spectrum of influenza A virus subtypes than the alpha-Neu5thioAc2 polymer. The results indicate that the alpha-Neu5thioAc2S polymer has the potential to be used as an inhibitor of influenza virus infection.

Molecular cloning of cytidine monophospho-N-acetylneuraminic acid hydroxylase. Regulation of species- and tissue-specific expression of N-glycolylneuraminic acid

Cytidine monophospho-N-acetylneuraminic acid (CMP-NeuAc) hydroxylase, which is the key enzyme for the synthesis of N-glycolylneuraminic acid (NeuGc), has been purified from the cytosolic fraction of mouse liver, as described in our previous paper. The amino acid sequences of the purified CMP-NeuAc hydroxylase, and peptides obtained by lysylendopeptidase digestion, were used to synthesize specific oligonucleotide primers. A mouse cDNA clone of the enzyme was obtained by a combination of the polymerase chain reaction and rapid amplification of cDNA ends. The sequence of the clone contained an open reading frame coding for a protein of 577 amino acids with a predicted molecular mass of 66 kDa. The deduced sequence included the amino acid sequences obtained for the purified enzyme and peptides, and a complete match was obtained for 159 residues. The enzyme has neither a signal peptide sequence nor a membrane spanning domain, which is consistent with localization of the enzyme in the cytosol. Transfection of a cDNA construct to COS-1 cells increased the enzyme activity and the amount of NeuGc. Comparison of the sequence with GenBank data indicated that no similar sequence has been reported so far. Northern blot analysis of various mouse tissues with the enzyme cDNA as a probe indicated that expression of NeuGc is related to the level of CMP-NeuAc hydroxylase mRNA. On Southern blot analysis with the same probe, cross-hybridizing bands were detected in the human and fish genomes.

Donor substrate specificities of Gal beta 1,4GlcNAc alpha 2,6-sialyltransferase and Gal beta 1,3GalNAc alpha 2,3-sialyltransferase: comparison of N-acetyl and N-glycolylneuraminic acids

Using cloned sialyltransferases, Gal beta 1,3GalNAc alpha 2,3-sialyltransferase (ST3Gal I) and Gal beta 1,4GlcNAc alpha 2,6-sialyltransferase (ST6Gal I) from both chicken and mouse, CMP-NeuAc and CMP-NeuGc were compared as donor substrates with pyridylamino-oligo-saccharides as acceptors. ST6Gal I showed 4-7-times higher activity toward CMP-NeuGc than CMP-NeuAc, while for ST3Gal I there was no significant difference between them, irrespective of the origin of the enzymes. Also, the difference in donor substrate (i.e., NeuAc and NeuGc) had little effect on the preference to acceptor substrates of these enzymes. Thus, the results showed that the cloned sialyltransferases can utilize both CMP-NeuAc and CMP-NeuGc as donor substrates, and that the preference difference between the sialyltransferases to CMP-NeuGc and CMP-NeuAc could, at least partly, explain the discrepancy in the ratio of NeuAc and NeuGc in glycolipids and glycoproteins in individual tissues.

Evidence for a free N-acetylneuraminic acid-hydroxylating enzyme in pig mandibular gland soluble fraction

The activity of a free N-acetylneuraminic acid (Neu5Ac)-hydroxylating enzyme which converted Neu5Ac into N-glycolyl-neuraminic acid (Neu5Gc) was demonstrated in the soluble fraction of pig mandibular gland. The hydroxylation was possible only with NADPH as the electron donor. The apparent Km was 4.5 mM Neu5Ac. At 0.5 mM monovalent cations had no effect on the hydroxylation of Neu5Ac whereas bivalent cations gave varied inhibition capacities ranging from 14 to 75%. EDTA gave a time-dependent enhancement of activity. It was concluded that the enzyme does not require an exogenously added inorganic cofactor. Results from salt fractionation of the soluble fraction and the use of inhibitors such as mercurials suggested that the hydroxylation of Neu5Ac to Neu5Gc may involve other, as yet unknown, component(s) and the possibility of electrons donated by NADPH being transferred to activated molecular oxygen (second substrate). We propose to name this enzyme N-acetyl-neuraminic acid hydroxylase.

Natural ligands of the B cell adhesion molecule CD22 beta can be masked by 9-O-acetylation of sialic acids

CD22 beta is a B cell-restricted phosphoprotein expressed on the surface of mature resting B cells. It mediates interactions with other cells partly or exclusively via recognition of alpha 2-6-linked sialic acids on glycoconjugates. The sialylated N-linked oligosaccharides recognized best by CD22 beta are common to many glycoproteins, suggesting that additional regulatory mechanisms may exist. Since the exocyclic side chain of sialic acid is required for recognition, we explored the effects of a naturally occurring modification of the side chain, 9-O-acetylation. Semisynthetic N-linked oligosaccharides terminating with 9-O-acetylated, alpha 2-6-linked sialic acids showed markedly reduced binding to CD22 beta relative to their non-O-acetylated counterparts. Murine lymphoid cells were probed for natural CD22 beta ligands that might be O-acetylated using recombinant soluble forms of CD22 beta (CD22 beta Rg) and influenza C esterase (CHE-Fc, which specifically removes 9-O-acetyl esters from sialic acids). By flow cytometry analysis, CD22 beta Rg binding to splenic B cells and a subset of T cells was increased by pretreatment with CHE-Fc, indicating that some potential CD22 beta ligands are naturally "masked" by 9-O-acetylation. Unmasking of these CD22 beta ligands by removal of 9-O-acetyl esters from intact splenocytes substantially increases their CD22 beta-dependent adhesion in an in vitro adhesion assay. Probing of murine lymphoid tissue sections by CD22 beta Rg and CHE-Fc treatment demonstrates regionally restricted and differentially expressed patterns of distribution between masked and unmasked ligands. For example, lymph node-associated follicular B cells express high levels of CD22 beta ligands, none of which are masked by 9-O-acetylation. In contrast, the ligands on lymph node-associated dendritic cells are almost completely masked by 9-O-acetylation, suggesting that masking may regulate interactions between CD22 beta-positive B cells and dendritic cells. In the thymus, only medullary cells express CD22 beta ligands, and a significant portion of these are masked by 9-O-acetylation, particularly at the cortical-medullary junction. Thus, 9-O-acetylation of sialic acids on immune cells is in a position to negatively regulate CD22 beta adhesion events in a manner depending on both cell type and tissue localization.

Generation of Chinese hamster ovary cell glycosylation mutants by retroviral insertional mutagenesis. Integration into a discrete locus generates mutants expressing high levels of N-glycolylneuraminic acid

Retroviral insertional mutagenesis can both generate somatic cell mutants and pinpoint the genomic locus associated with a mutant phenotype. In the present study, this approach was applied to Chinese hamster ovary cells (CHO) made susceptible to Moloney murine leukemia virus (MoMuLV) infection by stable expression of an ecotropic retrovirus receptor. These CHO cells were infected with a replication incompetent MoMuLV construct with a promoterless hygromycin phosphotransferase (hygro) gene inserted into the U3 region of the long terminal repeat and a second selectable marker, neomycin phosphotransferase (neo), expressed from an internal promoter. CHO clones containing integrated proviruses were selected with hygromycin or G418, and the subset of these with reduced cell surface Neu5Ac were then selected with wheat germ agglutinin (WGA). The majority of the resulting clones had a phenotype not previously described for WGA-resistant CHO mutants arising spontaneously or from chemical mutagenesis: Neu5Ac was almost completely replaced by Neu5Gc. We have provisionally termed these clones SAP mutants, for sialic acid phenotype. Southern analysis of HindIII digested DNA from four SAP mutants revealed that the MoMuLV provirus is present in a 10.4-kilobase (kb) fragment. Probing with a flanking CHO sequence resulted in equivalent hybridization to a 4.6-kb fragment and the 10.4-kb provirus-containing fragment in all four cases, while uninfected parental cells and non-SAP glycosylation mutants generated in the same retrovirus insertional mutagenesis experiments yielded only the 4.6-kb fragment. Sequencing of the 3'-flanking DNA revealed that each of the four SAP mutants had a unique provirus integration site falling within a 796 bp region of the CHO genome. The frequency with which SAP mutants arise suggests that this may be a preferred site for retrovirus integration.

Changes in ganglioside and sialic acid contents of goat milk during lactation

The ganglioside content of goat milk has been determined from d 1 after parturition to d 60 of lactation. Marked changes occurred in milk over the course of lactation; the highest ganglioside content occurred in d-1 colostrum and then decreased to the end of the period studied. At least seven different ganglioside species were detected; three gangliosides containing sialyllactosylceramide accounted for 66 to 92% of the total lipid-bound sialic acid; this result reflected a very simple core structure of goat milk gangliosides. The most abundant ganglioside, II3(N-acetylneuraminic acid)2-lactosylceramide, was about 35 to 56%. The sialic acid content exhibited a trend similar to that of gangliosides; during early lactation sialic acid content was higher than in mature milk. Fat, protein, and total solids were high at initiation of lactation and decreased thereafter. However, lactose content remained almost unchanged during the period studied.

Purification and partial characterization of a N-glycolylneuraminic acid specific lectin from the clam Anadara granosa (L)

The N-glycolylneuraminic acid-specific lectin (AFL) from the foot muscles of the marine clam Anadara granosa has been purified to homogeneity by affinity chromatography on bovine submaxillary mucin-Sepharose 4B. The Ca(2+)-dependent lectin agglutinates rabbit erythrocytes. The purified lectin is a tetrameric protein of native M(r) 254 kDa having a pI value of 6.65. The M(r) of two subunits is 65 kDa each and that of the remaining two is 62 kDa each. The dominant amino acids of the lectin are aspartic acid, glutamic acid, serine and glycine. The lectin activity is inhibited only by N-glycolylneuraminic acid specially when it is present in the macromolecular structure of mucin viz., porcine submaxillary mucin, which is the most potent inhibitor. The binding site does not recognize N-acetylneuraminic acid. Due to this strict specificity, the lectin appears to be unique.

Purification and characterization of N-glycolyneuraminic-acid-specific lectin from Scylla serrata

A sialic-acid-binding lectin with specificity for N-glycolylneuraminic acid (NeuGc) was purified from the hemolymph of the marine crab Scylla serrata by affinity chromatography using thyroglobulin-coupled agarose. The binding specificity of Scylla lectin distinguishes it from other known sialic-acid-specific lectins found in Limulus polyphemus and Limax flavus, which show a broader range of specificity for sialic acids. The molecular mass of the purified lectin is about 55 kDa. Under reducing conditions (SDS/PAGE), it resolved into two subunits of 30 kDa and 25 kDa. NeuGc inhibited hemagglutination activity of the purified lectin at a concentration as low as 0.6 mM, whereas N-acetylneuraminic acid (NeuAc) even at a concentration of 100 mM, failed to inhibit hemagglutination. This finding was supported by potent inhibition of hemagglutination by bovine and porcine thyroglobulins, which contain a NeuGc alpha 2-6Gal as terminal component of oligosaccharide residues. Neither glycoproteins (glycophorin NN; porcine submaxillary mucin), which contain NeuAc alpha 2-3Gal/GalNAc and NeuAc alpha 2-6GalNAc, nor human acid glycoprotein, which contains NeuAc alpha 2-3/alpha 2-6 Gal, or colominic acid, a sialopolymer with NeuAc alpha 2-8NeuAc, inhibited the lectin activity. The specificity of the lectin for NeuGc appears to account for the fact that it agglutinates rabbit and mice erythrocytes, but not human A, O, AB, rat or chicken erythrocytes, which contain NeuAc. The inability of the lectin to agglutinate erythrocytes (horse) that prominently express NeuGc could be due to O-acetylation of NeuGc. In support of this, bovine submaxillary mucin, which contains O-acetylated NeuGc inhibited the hemagglutination of the lectin better after removal of O-acetyl groups by base treatment. The unique specificity of Scylla lectin is of diagnostic potential for human cancer tissues expressing NeuGc, since NeuGc is not found in normal human tissues.

X-Neu5Ac: a novel substrate for chromogenic assay of neuraminidase activity in bacterial expression systems

A chromogenic substrate 1, 5-bromo-4-chloroindol-3-yl 5-acetamido-3,5-dideoxy-alpha-D-glycero-D-galacto-2-nonulopyranosidon ic acid (X-Neu5Ac), has been synthesized to facilitate the screening of bacterial colonies or plaques for the detection of either natural or mutant neuraminidase activity. Substrate 1 was hydrolyzed by neuraminidase isolated from Clostridium perfringens to release a halogenated indol-3-ol 2 that undergoes rapid aerobic oxidation to form the dark blue pigment, 5,5'-dibromo-4,-4'-dichloroindigo 3. Preliminary kinetic studies indicate that this compound is a good substrate (Km 0.89 x 10(-3) M) for neuraminidase and is quite stable under identical conditions in the absence of enzyme. These results suggest that X-Neu5Ac 1 can be useful to screen for bacterially-encoded enzyme production directly on agar plates.

Neuraminic acid specific lectins as markers of early cortical plate neurons

Early cortical plate and subplate cells in the developing neocortex of many animal species and humans contain one specific plasma protein, fetuin. Fetuin is heavily glycosylated and it is possible that due to the large amount of sugars, this molecule may play a part in cellular recognition during brain development. Cellular and extracellular carbohydrates in the developing brain of the sheep were studied histochemically using a battery of fluorescein-labelled lectins. Two neuraminic acid specific lectins, Sambucus nigra and Maackia amurensis, labelled consistently the fetuin-positive cells as demonstrated by double labelling with lectins and antifetuin antibodies. Brain sections from other species, known to contain fetuin-positive cells (fetal cow, postnatal tammar wallaby) showed a similar lectin staining pattern to that of the sheep fetus. Additionally, sections from species thought to contain fetuin in their developing brains that failed to cross-react with available antifetuin antibodies (postnatal Monodelphis, fetal cat) also demonstrated lectin-positive staining in the same neuronal cell population. Thus, neuraminic acid is a common and well conserved terminal carbohydrate in cortical plate and subplate neurons of the developing brain. Neuraminic-specific lectins are useful markers for these neurons in addition to the more traditional use of immunocytochemical methods in studies of formation of the neocortex.

Lactational changes in the N-glycoloylneuraminic acid content of bovine milk gangliosides

The N-glycoloylneuraminic acid (Neu5Gc) contents of milk and milk gangliosides from bovines were investigated during the different stages of lactation. The Neu5Gc content of milk is high in the colostrum (32% of the total sialic acid content of milk) and decreases thereafter until the end of the period considered (6% on day 30). When the Neu5Gc content of gangliosides was evaluated a similar profile to that of Neu5Gc in total sialic acids was found. Gangliosides from colostrum showed the highest Neu5Gc content (21-22% of the total sialic acid content of milk gangliosides). This content dropped towards the end of the period studied (8% on day 90). Our results indicate that a significant supply of Neu5Gc by the milk could be important for the newborn during the first days after parturition.