An unique specificity of a sialic acid binding lectin AchatininH, from the hemolymph of Achatina fulica snail

Detection Strategies for Sialic Acid and Sialoglycoconjugates

Glycoconjugates are a vast class of biomolecules implicated in biological processes important for human health and disease. The structural complexity of glycoconjugates remains a challenge to deciphering their precise biological roles and for their development as biomarkers and therapeutics. Human glycoconjugates on the outside of the cell are modified with sialic (neuraminic) acid residues at their termini. The enzymes that install sialic acids are sialyltransferases (SiaTs), a family of 20 different isoenzymes. The removal and degradation of sialic acids is mediated by neuraminidase (NEU; sialidase) enzymes, of which there are four isoenzymes. In this review, we discuss chemical and biochemical approaches for the detection and analysis of sialoglycoconjugate (SGC) structures and their enzymatic products. The most common methods include affinity probes and synthetic substrates. Fluorogenic and radiolabelled substrates are also important tools for many applications, including screening for enzyme inhibitors. Strategies that give insight into the native substrate-specificity of enzymes that regulate SGCs (SiaT & NEU) are necessary to improve our understanding of the role of sialic acid metabolism in health and disease.

Development and applications of sialoglycan-recognizing probes (SGRPs) with defined specificities: exploring the dynamic mammalian sialoglycome

Glycans that are abundantly displayed on vertebrate cell surface and secreted molecules are often capped with terminal sialic acids (Sias). These diverse 9-carbon-backbone monosaccharides are involved in numerous intrinsic biological processes. They also interact with commensals and pathogens, while undergoing dynamic changes in time and space, often influenced by environmental conditions. However, most of this sialoglycan complexity and variation remains poorly characterized by conventional techniques, which often tend to destroy or overlook crucial aspects of Sia diversity and/or fail to elucidate native structures in biological systems, i.e. in the intact sialome. To date, in situ detection and analysis of sialoglycans has largely relied on the use of plant lectins, sialidases, or antibodies, whose preferences (with certain exceptions) are limited and/or uncertain. We took advantage of naturally evolved microbial molecules (bacterial adhesins, toxin subunits, and viral hemagglutinin-esterases) that recognize sialoglycans with defined specificity to delineate 9 classes of sialoglycan recognizing probes (SGRPs: SGRP1-SGRP9) that can be used to explore mammalian sialome changes in a simple and systematic manner, using techniques common in most laboratories. SGRP candidates with specificity defined by sialoglycan microarray studies were engineered as tagged probes, each with a corresponding nonbinding mutant probe as a simple and reliable negative control. The optimized panel of SGRPs can be used in methods commonly available in most bioscience labs, such as ELISA, western blot, flow cytometry, and histochemistry. To demonstrate the utility of this approach, we provide examples of sialoglycome differences in tissues from C57BL/6 wild-type mice and human-like Cmah-/- mice.

Post-Glycosylation Modification of Sialic Acid and Its Role in Virus Pathogenesis

Sialic acids are a family of nine carbon keto-aldononulosonic acids presented at the terminal ends of glycans on cellular membranes. α-Linked sialoglycoconjugates often undergo post-glycosylation modifications, among which O-acetylation of N-acetyl neuraminic acid (Neu5Ac) is the most common in mammalian cells. Isoforms of sialic acid are critical determinants of virus pathogenesis. To date, the focus of viral receptor-mediated attachment has been on Neu5Ac. O-Acetylated Neu5Acs have been largely ignored as receptor determinants of virus pathogenesis, although it is ubiquitous across species. Significantly, the array of structures resulting from site-specific O-acetylation by sialic acid O-acetyltransferases (SOATs) provides a means to examine specificity of viral binding to host cells. Specifically, C₄ O-acetylated Neu5Ac can influence virus pathogenicity. However, the biological implications of only O-acetylated Neu5Ac at C_7-9 have been explored extensively. This review will highlight the biological significance, extraction methods, and synthetic modifications of C₄ O-acetylated Neu5Ac that may provide value in therapeutic developments and targets to prevent virus related diseases.

Unique Directional Motility of Influenza C Virus Controlled by Its Filamentous Morphology and Short-Range Motions

Influenza virus motility is based on cooperation between two viral spike proteins, hemagglutinin (HA) and neuraminidase (NA), and is a major determinant of virus infectivity. To translocate a virus particle on the cell surface, HA molecules exchange viral receptors and NA molecules accelerate the receptor exchange of HA. This type of virus motility was recently identified in influenza A virus (IAV). To determine if other influenza virus types have a similar receptor exchange mechanism-driven motility, we investigated influenza C virus (ICV) motility on a receptor-fixed glass surface. This system excludes receptor mobility, which makes it more desirable than a cell surface for demonstrating virus motility by receptor exchange. Like IAV, ICV was observed to move across the receptor-fixed surface. However, in contrast to the random movement of IAV, a filamentous ICV strain, Ann Arbor/1/50 (AA), moved in a straight line, in a directed manner, and at a constant rate, whereas a spherical ICV strain, Taylor/1233/47 (Taylor), moved randomly, similar to IAV. The AA and Taylor viruses each moved with a combination of gradual (crawling) and rapid (gliding) motions, but the distances of crawling and gliding for the AA virus were shorter than those of the Taylor virus. Our findings indicate that like IAV, ICV also has a motility that is driven by the receptor exchange mechanism. However, compared with IAV movement, filamentous ICV movement is highly regulated in both direction and speed. Control of ICV movement is based on its specific motility employing short crawling and gliding motions as well as its own filamentous morphology.IMPORTANCE Influenza virus enters into a host cell for infection via cellular endocytosis. Human influenza virus infects epithelial cells of the respiratory tract, the surfaces of which are hidden by abundant cilia that are inactive in endocytosis. An open question is the manner by which the virus migrates to endocytosis-active domains. In analyzing individual virus behaviors through single-virus tracking, we identified a novel function of the hemagglutinin and esterase of influenza C virus (ICV) as the motility machinery. Hemagglutinin iteratively exchanges a viral receptor, causing virus movement. Esterase degrades the receptors along the trajectory traveled by the virus and prevents the virus from moving backward, causing directional movement. We propose that ICV has a unique motile machinery directionally controlled via hemagglutinin sensing the receptor density manipulated by esterase.

Identification and analysis of o-acetylated sialoglycoproteins

5-N-acetylneuraminic acid, commonly known as sialic acid (Sia), constitutes a family of N- and O-substituted 9-carbon monosaccharides. Frequent modification of O-acetylations at positions C-7, C-8, or C-9 of Sias generates a family of O-acetylated sialic acid (O-AcSia) and plays crucial roles in many cellular events like cell-cell adhesion, proliferation, migration, etc. Therefore, identification and analysis of O-acetylated sialoglycoproteins (O-AcSGPs) are important. In this chapter, we describe several approaches for successful identification of O-AcSGPs. We broadly divide them into two categories, i.e., invasive and noninvasive methods. Several O-AcSias-binding probes are used for this purpose. Detailed methodologies for step-by-step identification using these probes have been discussed. We have also included a few invasive analytical methods for identification and quantitation of O-AcSias. Several indirect methods are also elaborated for such purpose, in which O-acetyl group from sialic acids is initially removed followed by detection of Sias by several approaches. For molecular identification, we have described methods for affinity purification of O-AcSGPs using an O-AcSias-binding lectin as an affinity matrix followed by sequencing using matrix-assisted laser desorption/ionization-time of flight (MALDI-TOF-TOF) mass spectroscopy (MS). In spite of special attention, loss of O-acetyl groups due to its sensitivity towards alkaline pH and high temperature along with migration of labile O-acetyl groups from C7-C8-C9 during sample preparation is difficult to avoid. Therefore there is always a risk for underestimation of O-AcSias.

Cloning and transcriptional analysis of two sialic acid-binding lectins (SABLs) from razor clam Solen grandis

Sialic acid-binding lectin (SABL) plays crucial role in both innate and adaptive immune responses benefiting from its predominant affinity toward glycan. In the present study, two SABLs from razor clam Solen grandis (designated as SgSABL-1 and SgSABL-2) were identified, and their expression patterns, both in tissues and towards microorganism glycan stimulation, were then characterized. The cDNA of SgSABL-1 and SgSABL-2 was 988 and 1281 bp, containing an open reading frame (ORF) of 744 and 570 bp, respectively, and deduced amino acid sequences showed high similarity to other invertebrates SABLs. Both SgSABL-1 and SgSABL-2 encoded a C1q domain. SgSABL-1 and SgSABL-2 were found to be constitutively expressed in a wide range of tissues with different levels, including mantle, gill, gonad, hemocyte, muscle, and hepatopancreas, and both of them were highly expressed in hepatopancreas. SgSABL-1 and SgSABL-2 could be significantly induced after razor clams were stimulated by acetylated subunits-containing glycan LPS and PGN, suggesting the two SgSABLs might perform potential function of glycan recognition. In addition, SgSABL-2 could also be induced by β-1,3-glucan. All these results indicated that SgSABL-1 and SgSABL-2 might be involved in the immune response against microbe infection and contributed to the pathogens recognition.

Functions and Biosynthesis of O-Acetylated Sialic Acids

Sialic acids have a pivotal functional impact in many biological interactions such as virus attachment, cellular adhesion, regulation of proliferation, and apoptosis. A common modification of sialic acids is O-acetylation. O-Acetylated sialic acids occur in bacteria and parasites and are also receptor determinants for a number of viruses. Moreover, they have important functions in embryogenesis, development, and immunological processes. O-Acetylated sialic acids represent cancer markers, as shown for acute lymphoblastic leukemia, and they are known to play significant roles in the regulation of ganglioside-mediated apoptosis. Expression of O-acetylated sialoglycans is regulated by sialic acid-specific O-acetyltransferases and O-acetylesterases. Recent developments in the identification of the enigmatic sialic acid-specific O-acetyltransferase are discussed.

O-acetylation of sialic acids is required for the survival of lymphoblasts in childhood acute lymphoblastic leukemia (ALL)

Exploiting the selective affinity of Achatinin-H towards 9-O-acetylneuraminic acid(alpha2-6)GalNAc, we have demonstrated the presence of 9-O-acetylated sialoglycoproteins (Neu5,9Ac(2)-GPs) on hematopoietic cells of children suffering from acute lymphoblastic leukemia (ALL), indicative of defective sialylation associated with this disease. The carbohydrate epitope of Neu5,9Ac(2)-GPs(ALL) was confirmed by using several synthetic sialic acid analogues. They are functionally active signaling molecules as demonstrated by their role in mediating lymphoproliferative responses and consequential increased production of IFN-gamma due to specific stimulation of Neu5,9Ac(2)-GPs on PBMC(ALL) with Achatinin-H. Cells devoid of 9-O-acetylations (9-O-AcSA(-)) revealed decreased nitric oxide production as compared to 9-O-AcSA(+) cells on exposure to IFN-gamma. Under this condition, a decrease in viability of 9-O-AcSA(-) cells as compared to 9-O-AcSA(+) cells was also observed which was reflected from increased caspase 3 activity and apoptosis suggesting the protective role of this glycotope. These Neu5,9Ac(2)-GPs are also capable of inducing disease-specific anti-Neu5,9Ac(2)-GPs antibodies in ALL children. Additionally, we have observed that disease-specific anti-Neu5,9Ac(2)-GPs have altered glycosylation profile, and they are incapable of exerting a few Fc-glycosylation-sensitive effector functions. These observations hint toward a disbalanced homeostasis, thereby enabling the cancer cells to escape host defense. Taken together, it may be hypothesized that Neu5,9Ac(2)-GPs and their antibodies play a prominent role in promoting the survival of lymphoblasts in ALL.

Interferon gamma promotes survival of lymphoblasts overexpressing 9-O-acetylated sialoglycoconjugates in childhood acute lymphoblastic leukaemia (ALL)

An enhanced linkage-specific 9-O-acetylated sialic acid (9-O-AcSA) on peripheral blood mononuclear cells (PBMC) of children with acute lymphoblastic leukaemia, ALL (PBMC(ALL), 9-O-AcSA+ cells) was demonstrated by using a lectin, Achatinin-H, whose lectinogenic epitope was 9-O-AcSAalpha2-6GalNAc. Our aim was to evaluate the in vitro contributory role of this glycotope (9-O-AcSAalpha2-6GalNAc) towards the survival of these 9-O-AcSA+ cells in ALL patients. For direct comparison, 9-O-AcSA- cells were generated by removing O-acetyl group of 9-O-AcSA present on PBMC(ALL) using O-acetyl esterase. An elevated level of serum interferon gamma (IFN-gamma) in affected children led us to think that PBMC(ALL) are continuously exposed specifically to this cytokine. Accordingly, 9-O-AcSA+ and 9-O-AcSA- cells were exposed in vitro to IFN-gamma. A twofold increased NO release along with inducible NO synthase (iNOS) mRNA expression by the 9-O-AcSA+ cells was observed as compared to the 9-O-AcSA- cells. The decreased viability of IFN-gamma exposed 9-O-AcSA- cells as compared to 9-O-AcSA+ cells were reflected from a 5.0-fold increased caspase-3-like activity and a 10.0-fold increased apoptosis in the 9-O-AcSA- cells when production of NO was lowered by adding competitive inhibitor of iNOS in reaction mixture. Therefore, it may be envisaged that a link exists between induction of this glycotope and their role in regulating viability of PBMC(ALL). Taken together, it is reasonable to hypothesise that O-acetylation of sialic acids on PBMC(ALL) may be an additional mechanism that promotes the survival of lymphoblasts by avoiding apoptosis via IFN-gamma-induced NO production.

Identification of 9-O-acetylated sialoglycans on peripheral blood mononuclear cells in Indian Visceral Leishmaniasis

Although the existence of O -acetylated sialic acids is well known, it is only in recent years that steady refinement of analytical techniques have enabled detailed mapping of their structural diversity [1]. Fluorimetric analysis of peripheral blood mononuclear cells (PBMC) of patients with Visceral Leishmaniasis (VL) showed six fold increase in the percentage of surface 9-O-acetylated sialoglycoconjugates (9-O-AcSGs) as compared to normal human donors. Using Achatinin-H, a 9-O-acetyl sialic acid- binding lectin, an enhanced presence of 9-O-AcSGs in an alpha2 --> 6 linkage was demonstrated by flow cytometry; abolition of its binding by pretreatment with a recombinant 9-O-acetylesterase corroborated the presence of this glycotope. Western blotting of PBMC from VL patients indicated the presence of five O-acetylated sialoglycans corresponding to 144, 65, 56, 36 and 19 kDa as compared to 144 and 36 kDa in normal individuals. Taken together our data indicates that during active disease, there is an overexpression of 9AcSGs on the surface of PBMC of VL patients, thus opening up new research avenues wherein the expression of this biomarker could be exploited to monitor the clinical status of VL patients.

Increased interferon gamma production by peripheral blood mononuclear cells in response to stimulation of overexpressed disease-specific 9-O-acetylated sialoglycoconjugates in children suffering from acute lymphoblastic leukaemia

Disease-specific over-expression of 9-O-acetylated sialoglycoconjugates (9-O-AcSGs) on peripheral blood mononuclear cells (PBMC) of children with acute lymphoblastic leukaemia (ALL, PBMC(ALL)) has been demonstrated using a lectin, Achatinin-H, with specificity towards 9-O-AcSAalpha2-6GalNAc. This study investigated the contributory role of 9-O-AcSGs induced on PBMC(ALL). Stimulation of PBMC(ALL) with Achatinin-H through 9-O-AcSGs led to a lymphoproliferative response with a significantly increased interferon-gamma (IFN-gamma) production when compared with unstimulated cells as demonstrated by enzyme-linked immunosorbent assay and mRNA expression. Under identical conditions, PBMC(ALL) ablated of O-acetylations did not respond to such stimulation. In summary, it may be concluded that stimulation of over-expressed 9-O-AcSGs regulate signalling for proliferation, leading to the release of IFN-gamma. Controlled expression of these molecules may be exploited as potential targets for therapy, promising beneficial effects to children with ALL.

Detection of immune-complexed 9-O-acetylated sialoglycoconjugates in the sera of patients with pediatric acute lymphoblastic leukemia

Although childhood acute lymphoblastic leukemia (ALL) is highly responsive to chemotherapy, reliable techniques are needed to determine treatment outcome. Over expression of 9-O-acetylated sialoglycoconjugates (9-OAcSGs) on lymphoblasts and concomitant anti-9-OAcSGs was found to have a diagnostic and prognostic potential. However, the presence of circulatory immune-complexed antigens remains unknown. The present study was aimed to evaluate whether immune-complexed 9-OAcSGs can be harnessed for better disease management. Immune-complexed antigens were evaluated in ALL sera (n=262) by a Dot-blot using a 9-OAcSAalpha2-6GalNAc-specific lectin, Achatinin-H. Using three serum samples, the inter- and intra-assay imprecision was evaluated as 11-13% and 7-11%, respectively. The recovery of spiked 9-OAcSGs was 84.2-95.4%. The central 95% reference interval for immune-complexed 9-OAcSGs in normal human sera (NHS, n=144) was 2.9-3.4 mug/ml irrespective of sex and age. At disease presentation, the immune-complexed 9-OAcSGs were fivefold higher than NHS, decreased with remission induction and importantly, reappeared with clinical relapse. Sera from patients with other hematological disorders (n=86) showed negligible levels. The Dot-blot demonstrated the potential application of immune-complexed antigen as a disease-specific marker and its efficacy as a sensitive and specific method that could serve as an economical yet effective index for monitoring disease status.

Purification, characterization of O-acetylated sialoglycoconjugates-specific IgM, and development of an enzyme-linked immunosorbent assay for diagnosis and follow-up of indian visceral leishmaniasis patients

The surface expression of 9-O-acetylated sialic acid (9-OAcSA) is elevated on hematopoietic cells and erythrocytes of visceral leishmaniasis (VL) patients. In this study, we show that VL patients contain elevated levels of IgM antibodies directed against 9-O-acetylated sialoglycoconjugates (9-OAcSG). These antibodies were affinity purified with bovine submaxillary protein as the affinity matrix containing the terminal epitope, 9-OAcSAalpha2-6GalNAc. They also bound to 9-OAcSGs on hematopoietic cells of patients with VL and to epitopes in the cytosol of Leishmania donovani promastigotes. A novel enzyme-linked immunosorbent assay was employed that showed 4-fold higher anti-OAcSG titers in VL patients (n=38), mean +/- S.E.M. being 0.83 +/- 0.09 vs. 0.21 +/- 0.04 detected in normal donors (n=20) and patients with cross-reactive diseases such as malaria (n=4) or tuberculosis (n=4). Assay specificity and sensitivity was 100% and 92%, respectively, whereas positive and negative predictive values were 100% and 90%, respectively. Significantly, anti-OAcSG titers declined 30 days after completion of anti-leishmanial treatment, indicating that monitoring of anti-9-OAcSGs may be a valuable alternative toward increasing the efficiency of diagnosis and follow-up of VL.

Altered erythrocyte membrane characteristics during anemia in childhood acute lymphoblastic leukemia

Anemia is a prominent feature in children with acute lymphoblastic leukemia (ALL). To investigate the erythrocyte features during anemia in these patients, we studied the altered characters of these cells and oxidative stress imposed in their serum. This investigation reveals that erythrocytes from ALL patients show (1) increased membrane fluidity detected by fluorescence anisotropy studies, increased osmotic fragility detected by hemolysis of erythrocytes in different saline concentrations, and increased hydrophobicity as measured by binding with 8-anilino-1-naphthalenesulfonic acid, (2) enhanced (approximately threefold) glycosylation and sialylation, monitored by digoxigenin enzyme assay, and (3) expression of disease-specific 210, 105, 83, 54, and 28 kDa 9-O-acetyl sialoglycoconjugates (9-O-AcSGs) demonstrated by Western blot analysis and fluorescence-activated cell sorter (FACS) analysis studies using Achatinin-H with specificity towards 9-O-AcSAalpha2-6GalNAc as the analytical probe. (4) In addition, induced oxidative stress was observed in the sera of these children as indicated by increased nitric oxide (approximately fourfold) and thiobarbituric acid (TBA) reactive species (twofold) as detected by Griess reaction and TBA assay, respectively. For all the experiments, erythrocytes from normal individuals served as controls. Thus, the altered membrane characteristics together with their exposure to induced oxidative stress in serum are found to be a few features restricted to diseased erythrocytes. Taken together, our results are suggestive of their interplay in the contribution to the observed anemia in these patients, which may be exploited for better management of the disease.

Differential expression of 9-O-acetylated sialoglycoconjugates on leukemic blasts: a potential tool for long-term monitoring of children with acute lymphoblastic leukemia

Earlier studies have demonstrated overexpression of 9-O-acetylated sialoglycoconjugates (9-O-AcSGs) on lymphoblasts, concomitant with high titers of anti-9-O-AcSG antibodies in childhood acute lymphoblastic leukemia (ALL). Our aim was to evaluate the correlation between expression of different 9-O-AcSGs during chemotherapeutic treatment. Accordingly, expression of 9-O-AcSGs on lymphoblasts of ALL patients (n = 70) were longitudinally monitored for 6 years (1997-2002), using Achatinin-H, a 9-O-acetylated sialic acid (9-O-AcSA) binding lectin with preferential affinity for 9-O-AcSGs with terminal 9-O-AcSA alpha 2-->6GalNAc. Western blot analysis of patients (n = 30) showed that 3 ALL-specific 9-O-AcSGs (90, 120 and 135 kDa) were induced at presentation; all these bands disappeared after treatment in patients (n = 22) who had disease-free survival. The 90 kDa band persisted in 8 patients who subsequently relapsed with reexpression of the 120 kDa band. FACS analysis revealed that at presentation (n = 70) 90.1 +/- 5.0% cells expressed 9-O-AcSGs, which decreased progressively with chemotherapy, remained <5% during clinical remission and reappeared in relapse (80 +/- 10%, n = 18). Early clearance of 9-O-AcSG(+) cells, during 4-8 weeks of treatment showed a good correlation with low risk of relapse. Sensitivity of detection of 9-O-AcSG(+) cells was 0.1%. Numbers of both high- and low-affinity binding sites were maximum at presentation, decreased with treatment and increased again in clinical relapse. We propose that close monitoring of 90 and 120 kDa 9-O-AcSGs may serve as a reliable index for long-term management of childhood ALL and merits therapeutic consideration.

Identification of sialic acids on Leishmania donovani amastigotes

The presence of Neu5Ac on promastigotes of Leishmania donovani, the causative organism of Indian visceral leishmaniasis, has been reported recently. Here we report the occurrence of Neu5Ac as a major component on amastigotes, as well as Neu5Gc, Neu5,9Ac2 and Neu9Ac5Gc as indicated by fluorimetric high performance liquid chromatography and gas liquid chromatography/electron impact mass spectrometry. Furthermore, binding studies with Sambucus nigra agglutinin (SNA), Maackia amurensis agglutinin (MAA), and various Siglecs, showed the presence of both (alpha2-6) and (alpha2-3)linked sialic acids; their binding was reduced after sialidase pretreatment. Western blotting of amastigote membrane glycoproteins with SNA demonstrated the presence of two sialoglycoconjugates of Mr values of 164 000 and 150 000. Similarly, binding of MAA demonstrated the presence of five distinct sialoglycans corresponding to molecular masses of 188, 162, 136, 137 and 124 kDa. Achatinin-H, a lectin that preferentially identifies 9-Oacetylated sialic acid (alpha2-6)linked to GalNAc, demonstrated the occurrence of two 9-Oacetylated sialoglycans with Mr 158 000 and 150 000, and was corroborated by flow cytometry; this binding was abolished by recombinant 9-Oacetylesterase pretreatment. Our results indicate that Neu5Ac w(alpha2-6) and (alpha2-3)linked, as well as Neu5Gc and their 9-Oacetyl derivatives, constitute components of the amastigote cell surface of L. donovani.

Development of an assay for quantification of linkage-specific O-acetylated sialoglycans on erythrocytes; its application in Indian visceral leishmaniasis

We have developed a noninvasive approach for the quantification of linkage-specific 9-O-acetylated sialoglycans on mammalian erythrocytes using a lectin, Achatinin-H, whose lectinogenic epitope has previously been defined as 9-O-acetylated sialoglycoconjugates (9-O-AcSGs) alpha 2-->6 linked to subterminal GalNAc. Titration and checkerboard analysis were performed to optimize the assay using rabbit, rat and human erythrocytes that contain differing amounts of this glycotope. Assay specificity was established by decreased binding of erythrocytes to immobilised Achatinin-H when pre-incubated with excess lectin. The intra-assay coefficient of variation (CV) for rat and human erythrocytes was 8.6-9.2% and 11.1-13.0%, respectively. The inter-assay CV for rat and human erythrocytes was 9.9-10.1% and 15.2-16.6%, respectively. In previous studies, we have identified an enhanced presence of cell surface 9-O-AcSGs on the erythrocytes of patients with visceral leishmaniasis (VL) [Am. J. Trop. Med. Hyg. 58 (1998) 551]. Our assay when evaluated on erythrocytes from VL patients (n=30) showed a fourfold increase in lectin binding as compared to endemic controls. The mean +/- S.E.M. of the A(405) nm value was 1.14 +/- 0.04 vs. 0.23 +/- 0.03, respectively (p<0.0001). Following effective chemotherapy, a significant reduction of this glycotope on the erythrocytes of VL patients indicates that this assay has both a diagnostic and prognostic potential. Taken together, we conclude that this antigen-based assay is a specific and reproducible method for monitoring the disease status of VL patients and could be used in retrospective and prospective trials.

O-acetyl sialic acid specific IgM in childhood acute lymphoblastic leukaemia

Initial studies have revealed an enhanced surface expression of O-acetylated sialoglycoconjugates (O-AcSGs) on lymphoblasts concomitant with high titres of IgG in childhood Acute Lymphoblastic Leukaemia (ALL) (Mandal C, Chatterjee M, Sinha D, Br J Haematol 110, 801-12, 2000). In our efforts to identify disease specific markers for ALL, we have affinity-purified IgM directed against O-AcSGs that reacts with three disease specific O-AcSGs present on membrane proteins derived from peripheral blood mononuclear cells (PBMC) of ALL patients. Antibody specificity towards O-AcSGs was confirmed by selective binding to erythrocytes bearing surface O-AcSGs, decreased binding with de-O-acetylated BSM and following pretreatment with O-acetyl esterase. Competitive inhibition ELISA demonstrated a higher avidity of IgM for O-AcSG than IgG. Flow cytometry demonstrated the diagnostic potential of purified O-AcSA IgM as binding was specific with ALL patients and minimal with other haematological disorders and normal individuals. It therefore may be adopted as a non-invasive approach for detection of childhood ALL. Taken together, the data indicates that carbohydrate epitopes having terminal O-AcSA alpha2 --> 6 GalNAc determinants induce disease specific IgG and IgM, potentially useful molecular markers for childhood ALL.

Role of linkage specific 9-O-acetylated sialoglycoconjugates in activation of the alternate complement pathway in mammalian erythrocytes

Substitution of the -OH group at C-9 of sialic acid by an O-acetyl ester has been suggested to modify various biological phenomena that are regulated by sialic acids. Amongst them, enhancement of erythrocyte lysis by 9-O-acetylated sialic acid determinants through modulation of the alternate pathway of complement has been extensively studied on murine erythrocytes [1]. A variable expression of linkage specific 9-O-acetylated sialoglycoconjugates as defined by the lectinogenic epitope of Achatinin-H namely 9-O-acetylated sialic acid alpha2-->6Gal NAc was identified on rabbit, guinea pig, hamster, rat, mouse and human erythrocytes. This differential expression of linkage specific 9-O-acetylated sialoglycoconjugates strongly correlated with the susceptibility of mammalian erythrocytes to lysis by the alternate pathway of complement. Additionally, low levels of antibodies directed against O-acetylated sialic acids in these mammalian species suggested that these constitutively present determinants have low immunogenicity. Taken together, our results indicate that complement mediated hemolysis depends not simply upon the extent of surface 9-O-acetylated sialic acids present but more importantly upon the specific linkage.

Identification and purification of cytolytic antibodies directed against O-acetylated sialic acid in childhood acute lymphoblastic leukemia

Sialic acids typically present as terminal sugars of oligo-saccharides are reported to be modified by O-acetylation at the C-9 position on lymphoblasts of childhood acute lymphoblastic leukemia (ALL) patients (Sinha et al., 1999a, Leukaemia, 13, 119-125). We now report high titers of IgG antibodies directed against O-acetylated derivatives of sialic acids (O-AcSA) in serum of ALL patients. These antibodies were purified using bovine submaxillary mucin (BSM) and the IgG distribution was confined to IgG(1)and IgG(2)subclasses; their binding was totally abolished with de-O-acetylation confirming their specificity towards O-AcSA determinants. Flow cytometry demonstrated binding of these antibody fractions to peripheral blood mononuclear cells (PBMC) of both T- and B-ALL patients having increased cell surface 9-O-AcSA determinants. Western blotting of membranes derived from PBMC of ALL patients confirmed binding of the antibody to O-acetylated sialoglycoconjugates corresponding to 144, 135, 120, 90, and 36 kDa whereas binding to PBMC from normal individuals corresponded to 144 and 36 kDa. Specificity of the antibody fraction towards 9-O-AcSA was substantiated by hemagglutination and hemagglutination-inhibition assays. The antibody purified from ALL serum selectively mediates complement dependent cytolysis of lymphoblasts expressing O-AcSAs and thereby possibly confers passive protection. The enhanced anti O-AcSA antibody levels allowed for development of a serodiagnostic assay (BSM-ELISA) specific for ALL. Minimal crossreactivity was observed with other hematological disorders like acute myeloid leukemia (n = 16), chronic myeloid leukemia (n = 6), chronic lymphocytic leukemia (n = 7) and non-Hodgkin's lymphoma (n = 3) as well as normal healthy individuals (n = 28). The BSM-ELISA therefore provides a simple, noninvasive alternative diagnostic approach for ALL and merits clinical consideration.

A colorimetric assay to evaluate the chemotherapeutic response of children with acute lymphoblastic leukemia (ALL) employing achatininH: a 9-O-acetyl sialic acid binding lectin

Employing a 9-O-acetyl sialic acid binding lectin, Achatinin(H) (ATNH), we have reported a non-invasive, blood based lymphoproliferation assay which measures the maximal lymphoproliferative dose (MLD) of ATN(H) to assess the status of 9-O-acetylated sialoglycoconjugates (9-OAcSGs) in patients with Acute lymphoblastic leukemia (ALL) (Mandal C, Sinha D, Sharma V, Bhattacharya DK. O-acetyl sialic acid binding lectin, as a probe for detection of subtle changes on the cell surface induced during acute lymphoblastic leukemia [ALL] and its clinical application. Ind J Biochem Biophys 1997;34:82; Sinha D, Mandal C, Bhattacharya DK. Development of a simple blood based lymphoproliferation assay to assess the clinical status of patients with acute lymphoblastic leukemia. Leuk Res 1999;13:309-312; Sinha D, Mandal C, Bhattacharya DK. A novel method for prognostic evaluation of childhood acute lymphoblastic leukemia. Leukemia 1999;13[in press]). Although the expression of 9-OAcSGs clearly serves as an index of treatment outcome, the assay has limitations in that it requires radioisotopes, i.e. [3H]-TdR. Therefore a colorimetric assay was developed as an alternative approach. The pre-treatment MLD, as measured by the colorimetric assay, was 0.15 +/- 0.02 microg which progressively increased during consolidation therapy (1.40 +/- 0.39 microg), maintenance therapy (4.20 +/- 1.60 microg) and in followed-up cases (5.20 +/- 0.43 microg) but sharply declined following relapse (0.25 +/- 0.02 microg). The colorimetric assay also showed a good correlation with radiometric assay (r = + 0.93) and their mean coefficient of inter-assay precision were also comparable (15.53% versus 14.86%). We therefore propose that the colorimetric assay is a safe, non-radiometric, user-friendly alternative for assessing individual chemotherapeutic responses in childhood ALL.

Diagnostic and prognostic potential of antibodies against O-acetylated sialic acids in canine visceral leishmaniasis

Employing bovine submaxillary mucin (BSM) as the coating agent, an enzyme-linked immunosorbent assay (BSM-ELISA) was developed to detect antibodies directed against O-acetylated sialic acids (O-AcSA) in canine visceral leishmaniasis (CVL). Serum samples were collected from 50 dogs previously screened by a parasite-ELISA to detect anti-leishmanial antibodies and designated as seropositive (n = 30) and seronegative (n = 20). The BSM-ELISA detected anti-O-AcSA antibodies in 29 out of 30 seropositive dogs and was negative in 15 out of 20 seronegative dogs; the sensitivity and specificity of the assay being 96.6% and 75%, respectively. Seven dogs from an endemic area in central Israel were longitudinally monitored for 15 months clinically, serologically and cultured for parasite. The levels of antibodies directed against O-AcSA increased with the appearance of clinical symptoms and/or seropositivity, disappeared when the disease was self-limiting as also with chemotherapeutic response and reappeared with relapse. The BSM-ELISA, therefore, represents a valuable tool for assessment of disease progression.

Development of a simple, blood based lymphoproliferation assay to assess the clinical status of patients with acute lymphoblastic leukemia

Although childhood acute lymphoblastic leukemia (ALL) is highly responsive to chemotherapy, reliable techniques are needed to determine treatment outcome and predict relapse. Employing a 9-O-acetyl sialic acid binding lectin, ATN(H), we have identified two 9-O-acetylated sialogycoconjugates (9-OAcSGs) as novel biomarkers expressed selectively on leukemic blasts of ALL patients. Presently, we report a non-invasive, blood based lymphoproliferation assay, which employs the maximal lymphoproliferative dose of ATN(H) (MLD) to assess the status of 9-OAcSGs with progressive therapy. A low MLD (0.18 +/- 0.01 microg) in untreated patients reflects increased expression of 9-OAcSGs which decline following therapy (MLD = 2.10 +/- 0.60 microg), persist during maintenance therapy (MLD = 4.50 +/- 1.60 microg)/follow-up (MLD = 5.50 +/- 0.85 microg) and are re-induced with relapse (MLD = 0.25 +/- 0.01 microg). Since the assay detects lymphoblasts with a sensitivity of 10(-4), shows no cross-reactivity with other hematological disorders (n = 48) and has been tested in 212 patients, it meets clinical consideration.

Identification of antibodies directed against O-acetylated sialic acids in visceral leishmaniasis: its diagnostic and prognostic role

A significantly increased O-acetylated sialic acid (O-AcSA) binding fraction was purified from serum of visceral leishmaniasis (VL) patients by affinity chromatography on immobilized bovine submaxillary mucin (BSM) and found to be immunoglobulin in origin. The serodiagnostic and prognostic potential of BSM as a capture antigen was established by ELISA with no cross reactivity with coendemic diseases like malaria, tuberculosis, leprosy, chagas disease and cutaneous leishmaniasis; however, a strong cross reactivity was present with trypanosomiasis patients. In 56 clinically diagnosed VL patients, the BSM-ELISA was compared with diagnosis by microscopy using Giemsa stained tissue smears and direct ELISA using crude parasite antigen (parasite-ELISA); 49/56(87.5%) and 5/56(9.0%) were positive and negative respectively by all 3 methods. The BSM-ELISA failed to diagnose 2/56(3.5%) patients which were biopsy and parasite-ELISA positive. The prognostic potential of the BSM-ELISA in 18 longitudinally monitored VL patients before and after conventional antimonial treatment showed a significant decrease in anti O-AcSA titres in drug responsive patients whereas anti O-AcSA levels persisted in drug unresponsive patients. The IgG subclass distribution of antibodies directed against O-AcSA showed increased IgG2 levels in VL patients as compared to healthy controls. The BSM-based ELISA holds great promise as a serodiagnostic and prognostic assay for VL.

The lectin from the crustacean Liocarcinus depurator recognizes O-acetylsialic acids

A lectin that recognized sialic acids and aggultinated mouse erythrocytes was purified from hemolymph of the crab Liocarcinus depurator. It consisted of 38-kDa subunits and had a pI about 6.0. The specificity of the lectin was assayed by hemagglutination inhibition. N-acetylneuraminic acid (Neu5Ac) was a good inhibitor and its N-acetyl group at C-5 was critical for lectin-ligand interaction. Substitution of the C-9 hydroxyl on Neu5Ac with an O-acetyl group (9-O-Ac-Neu5Ac) increased the inhibitory potency of this molecule. Furthermore, O-acetyl substitution of all the hydroxyl groups yielded even better inhibitors (2,4,7,8,9-O-Ac-Neu5Ac and its 1-O-methyl ester). Removal of the hydroxyl or O-acetyl group connected to C-2 reduced the potency of these inhibitors. The lectin agglutinated and stimulated human but not mouse lymphocytes. It was also inhibited by Escherichia coli (O111:B4) lipopolysaccharide and agglutinated specific gram-negative bacteria. In vitro labeling with [35S]methionine indicated that the lectin was synthesized in hepatopangreas of L. depurator. Immunofluorescence showed that among hemocytes it localized mainly in the large-granule population.

Binding specificity of influenza C-virus to variably O-acetylated glycoconjugates and its use for histochemical detection of N-acetyl-9-O-acetylneuraminic acid in mammalian tissues

The specificity of influenza C-virus binding to sialoglycoconjugates was tested with various naturally O-acetylated gangliosides or synthetically O-acetylated sialic acid thioketosides, which revealed binding to 9-O-acetylated N-acetylneuraminic acid. Binding was also observed with a sample of Neu5,7Ac2-GD3, however at a lower degree. Sialic acids with two or three O-acetyl groups in the side chain of synthetic sialic acid derivatives are not recognized by the virus. In these experiments, bound viruses were detected with esterase substrates. Influenza C-virus was also used for the histological identification of mono-O-acetylated sialic acids in combination with an immunological visualization of the virus bound to thin-sections. The occurrence of these sialic acids was demonstrated in bovine submandibular gland, rat liver, human normal adult and fetal colon and diseased colon, as well as in human sweat gland. Submandibular gland and colon also contain significant amounts of glycoconjugates with two or three acetyl esters in the sialic acid side chain, demonstrating the value of the virus in discriminating between mono- and higher O-acetylation at the same site. The patterns of staining showed differences between healthy persons and patients with colon carcinoma, ulcerative colitis or Crohn's disease. Remarkably, some human colon samples did not show O-acetyl sialic acid-specific staining. The histochemical observations were controlled by chemical analysis of tissue sialic acids.

The specificity of the binding site of AchatininH, a sialic acid-binding lectin from Achatina fulica

A sialic acid-binding lectin, AchatininH (ATNH), having unique specificity towards 9-O-acetylneuraminic acid, has been purified and characterized. The specificity of this lectin for O-acetylsialic acids was studied in detail, using various sialic acid derivatives and sialoglycoproteins. The potent inhibition of hemagglutination by bovine submaxillary mucin (BSM), which contains 9(7,8)-O-acetylsialic acid and by free 9-O-acetylneuraminic acid confirms the preferential affinity towards this sugar. Further support for the role of O-acetylsialic acid was obtained by sialidase treatment of BSM. O-Deacetylation of the sialic acid residue abolished its inhibitory potency. Moreover, when the trihydroxypropyl side chain of the sialic acid molecule was modified by periodate-borohydride treatment, the truncated C7-sialic acid was unable to bind ATNH. This result suggests that the glycerol side chain of Neu5Ac, especially the C-8 and/or C-9 portion is an important determinant for ATNH. The hemagglutination-inhibition results using several mono-, di-, and tri-saccharides containing terminal sialic acid and various sialoglycoproteins reveals that ATNH preferentially binds the alpha-(2-->6)-linked sialic acid. Furthermore, beta-D-GlcNAc-(1-->3)-[alpha-NeuGc-(2-->6)]-GalNAc-ol was found to be the best ligand for ATNH.

O-acetylated sialic acid as a distinct marker for differentiation between several leukemia erythrocytes

AchatininH (ATNH) is a lectin, isolated from the hemolymph of Achatina fulica snail, which has been shown to have narrow specificity towards 9-O-acetyl sialic acid. Usually ATNH does not agglutinate normal human erythrocytes, however, it is capable of agglutinating erythrocytes of patients suffering from acute lymphocytic and acute myelogenous leukemia. Determination of binding constants, numbers of binding sites and lectin overlay experiments using patients' erythrocytes ghost, have suggested that some alterations in erythrocyte cell surface sialoglycoproteins or more precisely appearance of some O-acetylated sialoglycoprotein as a result of pathological transformations has caused this change in the binding of ATNH.

Cellular sialoglycoconjugates: a histochemical perspective

Sialic acids are found at the non-reducing terminus of oligosaccharide side-chains of most glycoproteins and glycolipids. Their expression is often developmentally regulated during organogenesis and in differentiating cells. Further, differential expression of various sialyltransferases results in organ- and cell-type specific expression of sialylated glycosylation sequences. The availability of highly specific lectins and of monoclonal antibodies has permitted histochemical investigations of the various cellular and subcellular aspects of the process of sialylation. This review provides a survey on the developmentally regulated and organ- as well as cell-type specific expression of various sialoglycoconjugates and their tumour-associated occurrence with special reference to intestine and kidney.

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.

Synthesis of N-acetyl-9-O-acetylneuraminic acid alpha-p-amino-phenylthioketoside and its application as ligand in the affinity chromatography of a lectin with preferential affinity to O-acetylated sialic acids

The N-acetyl-9-O-acetylneuraminic acid-alpha-p-aminophenylthioketoside 7 was synthesized as a sialidase-stable ligand for the affinity chromatography of a lectin with preferential affinity to O-acetylated sialic acids. The thioketoside was prepared by phase-transfer-catalysed glycosidation followed by Zemplen deacetylation. Regioselective acetylation of the completely de-O-acetylated derivative was practised by two different methods. The acetylation with trimethylorthoacetate did not show the desired selectivity for hydroxyl groups; in addition to the acetylation in position 9 extensive formation of an acetimidate ester derivative with the amino-group in the aminophenyl-moiety was observed. However the esterification with N,N-dimethylacetamide dimethyl acetal resulted in an exclusive acetylation of the hydroxyl-group in position 9. After catalytic hydrogenation this ligand was immobilized both directly and by a six-carbon long spacer group to the agarose matrix. The adsorbents were applied in the affinity chromatography of the lectin and their binding capacity and selectivity compared to those of the formerly used mucin matrix. In both respects the thioketoside coupled by the spacer turned out to be a better ligand for the isolation of the lectin than the mucin.

Albumen gland of the snail Achatina fulica is the site for synthesis of AchatininH, a sialic acid binding lectin

A sialic acid binding lectin, AchatininH was purified from the hemolymph of Achatina fulica snail. To identify the site of synthesis of AchatininH, in vitro incubation studies in presence of labelled amino acid precursor were performed. Different organs from the snail were sliced and incubated in methionine-deficient Eagle's minimum essential medium containing [35S]-methionine at 25 degrees C for 5 h. After termination of incubation, tissues were homogenized, centrifuged and the de novo synthesized protein was immunoprecipitated with specific AchatininH antibody, followed by protein-A. The precipitated antigen-antibody complex was analysed by SDS-PAGE. Data obtained from native gel electrophoresis and SDS-PAGE radioautographic analysis indicates that AchatininH is synthesized in the albumen gland.

Use of influenza C virus for detection of 9-O-acetylated sialic acids on immobilized glycoconjugates by esterase activity

An overlay and a solid-phase assay are presented which allow the specific detection of 9-O-acetylated sialic acids on sialoglycoconjugates immobilized on microtiter plates, nitrocellulose or separated on thin-layer chromatograms. The assay takes advantage of two different biological properties of influenza C virus, its high-affinity binding to 9-O-acetylated sialic acids and its sialate 9-O-acetylesterase that is used for detection of bound virus with fluorogenic or chromogenic substrates. Though simple and rapid, the assay is highly sensitive with a detection limit of 65 fmol 9-O-acetylated sialic acid in 9-O-acetylated ganglioside GD1a. Influenza C virus is able to bind to a wide spectrum of sialoglycoconjugates like mucins, serum glycoproteins or gangliosides containing naturally or synthetically O-acetylated sialic acids. 9-O-Acetyl-N-glycoloylneuraminic acid can also function as a high-affinity receptor determinant for influenza C virus. While the acetyl ester at the 9 position is essential for virus binding in all cases, a 4-O-acetyl group is not recognized. In addition to alpha(2.3) or alpha(2.6) bonds, 9-O-acetyl-N-acetylneuraminic acid in alpha(2.8) linkage to N-acetylneuraminic acid is also functionally active.

The polyclonal activation of lymphocytes and T cell mitogenicity by a unique sialic-acid-binding lectin from the hemolymph of Achatina fulica snail

A unique sialic-acid-binding lectin, AchatininH, isolated from the hemolymph of Achatina fulica snail was found to be strongly mitogenic, as monitored by [3H]thymidine incorporation assay, to rat and human lymphocytes. However, the degree of mitogenic response varied with the type of cell population. As indicated, this lectin induced proliferation of purified T lymphocytes and rat thymocytes, whereas it was less mitogenic towards peripheral lymphocytes of pregnant rats and was not mitogenic towards B lymphocytes. Furthermore, the mitogenic response was inhibited by the sialic-acid-containing disaccharide, a strong inhibitor of this lectin. This suggests that lymphocyte cell surface molecules containing sialic acid residues are important for this interaction and may have a structure similar to that of AchatininH receptors. Although this lectin showed strong mitogenicity towards lymphocytes, it showed very weak leucoagglutination. Surprisingly, PHA-induced blastogenesis was inhibited by the same dose of AchatininH as caused mitogenic activity in resting lymphocyte culture. The degree of suppression was higher for the lymphocytes isolated from pregnant rat blood compared to the non-pregnant control. Since the 51Cr-uptake assay and the cell viability test results negate the potential cytotoxic activity of this protein, the immunosuppression induced in the presence of PHA by AchatininH may be the additive effect of two mitogens causing an increased cell-density-dependent arrest. AchatininH does not have complement-like activity, but induces potent polyclonal activation of B cells as measured by the reverse hemolytic plaque assay.

Sialic acid binding lectins

The literature contains several reviews on lectins in general, covering mainly those from plants and invertebrates. However, the sialic acid binding lectins have not been reviewed so far. Considering the importance of sialic acids in cell sociology, lectins which specifically recognize terminal sialic acid residues are potentially useful as analytical tools in studying the biological functions of sialoglycoconjugates. These lectins, along with monoclonal antibodies raised against sialoglycoconjugates, have been used in the detection, affinity purification, cytochemical localization and quantitation of such glycoconjugates. In this review the main emphasis has been placed on the occurrence, general purification procedures, macromolecular properties, sugar specificities and applications of these lectins.

Physiochemical studies on achatininH, a novel sialic acid-binding lectin

We have purified a sialic acid-binding lectin, achatininH, in a single step by affinity chromatography, having high affinity for 9-O-acetylneuraminic acid. The physicochemical characterization of the interaction of achatininH with bivalent metal ions and sialic acid derivatives by the use of spectrofluorimetry, spectropolarimetry and precipitin reaction is reported. From fluorescence quenching studies the binding of Ca2+ (Ka = 251 +/- 9 M-1) and of Mn2+ (Ka = 86 +/- 5 M-1) was found to be weak, but their presence is absolutely necessary for sugar binding as well as biological activity. The nature and position of the substituent group play a very important role in the binding affinity. AchatininH shows a high affinity for 9-O-acetylneuraminic acid (Ka = 1.20 x 10(3) +/- 0.07 x 10(3) M-1) compared with that for the 4-O-acetyl derivative. In oligomers the binding strength increases in the order monosaccharide less than disaccharide less than trisaccharide. The binding affinity of achatininH for the disaccharide was found to reach a peak around pH 8. From c.d. spectral studies achatininH was found to have a high beta-sheet content (46%) and a low alpha-helix content (24%). From precipitin analysis at least one sugar-binding site on each of the 16 monomer subunits of the protein is indicated.

Elderberry bark lectin--gold techniques for the detection of Neu5Ac (alpha 2,6) Gal/GalNAc sequences: applications and limitations

The lectin from the elderberry (Sambucus nigra L.) bark, shown to recognize the sequence neuraminic acid (alpha 2,6) galactose/N-acetylgalactosamine, was applied for detecting binding sites in Lowicryl K4M sections by light and electron microscopy. The lectin was used either directly complexed to colloidal gold or in a two-step cytochemical affinity technique. The lectin-gold complex proved to be superior and thus was extensively tested on rat liver, kidney and hepatoma cells as well as on sheep and bovine submandibular glands. Controls to establish specificity of lectin-gold binding included sugar and glycoprotein inhibition tests and enzymic removal of sialic acid. In agreement with biochemical data demonstrating the potentiating effect of sialic acid on the binding of the lectin to oligosaccharides, enzymic removal of sialic acid from liver sections resulted in abolition of lectin staining. However, in the submandibular glands, neuraminidase pretreatment of the sections had no effect on the subsequent lectin-gold binding. In rat kidney some structures became negative while others retained the lectin-gold staining due to binding to penultimate N-acetylgalactosamine exposed after sialic acid removal. In line with this, spot blot analysis demonstrated that the lectin-gold complex reacted with both fetuin and asialofetuin. Taken together, these results suggest that, for cytochemical staining, the sialic acid and the galactose/N-acetylgalactosamine lectin combining subsites of Sambucus nigra L. lectin are equally reactive with cellular glycoconjugates and that neuraminidase predigestion of tissue sections is of utmost importance to ensure specificity of staining for the sequence neuraminic acid (alpha 2,6) galactose/N-acetylgalactosamine.

Chemical-modification studies of a unique sialic acid-binding lectin from the snail Achatina fulica. Involvement of tryptophan and histidine residues in biological activity

A unique sialic acid-binding lectin, achatininH (ATNH) was purified in single step from the haemolymph of the snail Achatina fulica by affinity chromatography on sheep submaxillary-gland mucin coupled to Sepharose 4B. The homogeneity was checked by alkaline gel electrophoresis, immunodiffusion and immunoelectrophoresis. Amino acid analysis showed that the lectin has a fairly high content of acidic amino acid residues (22% of the total). About 1.3% of the residues are half-cystine. The glycoprotein contains 21% carbohydrate. The unusually high content of xylose (6%) and fucose (2.7%) in this snail lectin is quite interesting. The protein was subjected to various chemical modifications in order to detect the amino acid residues and carbohydrate residues present in its binding sites. Modification of tyrosine and arginine residues did not affect the binding activity of ATNH; however, modification of tryptophan and histidine residues led to a complete loss of its biological activity. A marked decrease in the fluorescence emission was found as the tryptophan residues of ATNH were modified. The c.d. data showed the presence of an identical type of conformation in the native and modified agglutinin. The modification of lysine and carboxy residues partially diminished the biological activity. The activity was completely lost after a beta-elimination reaction, indicating that the sugars are O-glycosidically linked to the glycoprotein's protein moiety. This result confirms that the carbohydrate moiety also plays an important role in the agglutination property of this lectin.