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The specificity of the sialic acid-binding lectin from ovine placenta was examined in detail by haemagglutination inhibition assays applying a panel of 32 synthetic sialic acid analogues. The carboxylic acid group is a prerequisite for the interaction with the lectin, the α-anomer of the methyl glycoside is only a little more effective as an inhibitor than the β-anomer and the most potent inhibitor was 9-deoxy-10-carboxylic acid Neu5Ac, followed by 4-oxo-Neu5Ac. In contrast to the majority of known sialic acid-binding lectins, the N-acetyl group of Neu5Ac is not indispensable for binding, neither is the hydroxyl group at C-9 since substitutions at this carbon atom are well tolerated. Furthermore, all sulfur-containing substituents at C-9 enhanced the affinity of the lectin. This is the first sialic acid-binding lectin found to strongly bind thio derivatives.

The siglecs, formerly called sialoadhesins, are a family of I-type lectins binding to sialic acids on the cell surface. Five members of this family have been identified: sialoadhesin, myelin-associated glycoprotein (MAG), Schwann cell myelin protein (SMP), CD22 and CD33. We have investigated the relevance of substituents at position C-9 and in the N-acetyl group of N-acetylneuraminic acid, using a series of synthetic sialic-acid analogues either on resialylated human erythrocytes or as free alpha-glycosides in hapten inhibition. All five siglecs require the hydroxy group at C-9 for binding, suggesting hydrogen bonding of this substituent with the binding site. Remarkable differences were found among the proteins in their specificity for modifications of the N-acetyl group. Whereas sialoadhesin, MAG and SMP do not tolerate a hydroxy group as in N-glycolylneuraminic acid, they bind to halogenated acetyl residues. In the case of MAG, N-fluoroacetylneuraminic acid is bound about 17-fold better than N-acetylneuraminic acid. In contrast, human and murine CD22 both show good affinity for N-glycolylneuraminic acid, but only human CD22 bound the halogenated compounds. In conclusion, our data indicate that interactions of the hydroxy group at position 9 and the N-acyl substituent contribute significantly to the binding strength.

The mammalian hepatic asialoglycoprotein receptor, a member of the C-type animal lectin family, displays preferential binding to N-acetylgalactosamine compared with galactose. The structural basis for selective binding to N-acetylgalactosamine has been investigated. Regions of the carbohydrate-recognition domain of the receptor believed to be important in preferential binding to N-acetylgalactosamine have been inserted into the homologous carbohydrate-recognition domain of a mannose-binding protein mutant that was previously altered to bind galactose. Introduction of a single histidine residue corresponding to residue 256 of the hepatic asialoglycoprotein receptor was found to cause a 14-fold increase in the relative affinity for N-acetylgalactosamine compared with galactose. The relative ability of various acyl derivatives of galactosamine to compete for binding to this modified carbohydrate-recognition domain suggest that it is a good model for the natural N-acetylgalactosamine binding site of the asialoglycoprotein receptor. Crystallographic analysis of this mutant carbohydrate-recognition domain in complex with N-acetylgalactosamine reveals a direct interaction between the inserted histidine residue and the methyl group of the N-acetyl substituent of the sugar. Evidence for the role of the side chain at position 208 of the receptor in positioning this key histidine residue was obtained from structural analysis and mutagenesis experiments. The corresponding serine residue in the modified carbohydrate-recognition domain of mannose-binding protein forms a hydrogen bond to the imidazole side chain. When this serine residue is changed to valine, loss in selectivity for N-acetylgalactosamine is observed. The structure of this mutant reveals that the beta-branched valine side chain interacts directly with the histidine side chain, resulting in an altered imidazole ring orientation.

Reaction of de- N -acetyl gangliosides GM 1 ( 1 ) and GM 3 ( 2b ) with methyl dithioacetate ( 3 ) afforded N -thioacetyl GM 1 ( 1c ) and N -thioacetyl GM 3 ( 2c ) with the oxygen of the acetamido group of N -acetylneuraminic acid residue replaced by sulfur.

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.

N-Thioacylation of neuraminic acid methyl α-glycoside (5) with O-ethyl thioformate (1), methyl dithioacetate (2), and methyl dithiopropionate (3) afforded N-thioformyl, N-thioacetyl, and N-thiopropionyl neuraminic acid derivatives 6a-c in high yield. Cleavage of the glycosides was accomplished either by acid hydrolysis or by sialidase treatment. Alternatively, 5-N-thioacylneuraminic acids were produced from the corresponding N-thioacyl- d -mannosamines 7a-c and pyruvate employing N-acetylneuraminate pyruvate lyase. The sialidase-resistant methyl α-thioglycoside of N-thioacetylneuraminic acid (10) was also prepared. N-Acetyl-9-deoxy-9-thioacetamido neuraminic acid (19) was obtained either directly by reaction of N-acetyl-9-amino-9-deoxyneuraminic acid (17) with methyl dithioacetate (2) or via its methyl α-glycoside 18. Compound 17 was produced from the methyl α-glycoside of 9-O-tosylated methyl ester 12 via the azide 13. — For the 5-N-thioacyl sialic acids 6a-c and 8a-c as well as for the 9-deoxy-9-thioacetamido derivatives 18 and 19 some biological properties are reported.

The binding of influenza A virus hemagglutinin to its cell surface receptor, alpha-linked 5-N-acetylneuraminic acid (sialic acid), was studied in solution. The effect of structural modifications introduced into the N-acetyl group of the sialic acid on the binding was monitored by determining the dissociation constants by proton nuclear magnetic resonance (1H NMR) spectroscopy. Methyl alpha-glycoside of N-thioacetylneuraminic acid showed high, whereas the corresponding N-methylcarbamoylneuraminic acid exhibited relatively low binding affinity towards the hemagglutinin.

Unsubstituted amino sugars readily react with dithiocarboxylic ester or O -ethyl thioformate to yield the hitherto unknown free N -thioacylated sugars which are of biological interest.

Unsubstituted amino sugars readily react with dithiocarboxylic ester or O -ethyl thioformate to yield the hitherto unknown free N -thioacylated sugars which are of biological interest.

N-Thioacylation of neuraminic acid methyl α-glycoside (5) with O-ethyl thioformate (1), methyl dithioacetate (2), and methyl dithiopropionate (3) afforded N-thioformyl, N-thioacetyl, and N-thiopropionyl neuraminic acid derivatives 6a-c in high yield. Cleavage of the glycosides was accomplished either by acid hydrolysis or by sialidase treatment. Alternatively, 5-N-thioacylneuraminic acids were produced from the corresponding N-thioacyl- d -mannosamines 7a-c and pyruvate employing N-acetylneuraminate pyruvate lyase. The sialidase-resistant methyl α-thioglycoside of N-thioacetylneuraminic acid (10) was also prepared. N-Acetyl-9-deoxy-9-thioacetamido neuraminic acid (19) was obtained either directly by reaction of N-acetyl-9-amino-9-deoxyneuraminic acid (17) with methyl dithioacetate (2) or via its methyl α-glycoside 18. Compound 17 was produced from the methyl α-glycoside of 9-O-tosylated methyl ester 12 via the azide 13. — For the 5-N-thioacyl sialic acids 6a-c and 8a-c as well as for the 9-deoxy-9-thioacetamido derivatives 18 and 19 some biological properties are reported.

Sialic acids as terminal residues of oligosaccharide chains play crucial roles in several cellular recognition events. Exploiting the selective affinity of Achatinin-H toward N-acetyl-9-O-acetylneuraminic acid-a2-6-GalNAc, we have demonstrated the presence of 9-O-acetylated sialoglycoproteins (Neu5,9Ac 2 -GPs) on lymphoblasts of 70 children with acute lymphoblastic leukemia (ALL) and on leukemic cell lines by fluorimetric HPLC and flow cytometric analysis. This study aims to assess the structural aspect of the glycotope of Neu5,9Ac 2 -GPs A L L and to evaluate whether these disease-specific molecules can be used to monitor the clinical outcome of ALL. The Neu5,9Ac 2 -GPs A L L were affinity-purified, and three distinct leukemia-specific molecular determinants (135, 120, and 90 kDa) were demonstrated by SDS-PAGE, western blotting, and isoelectric focusing. The carbohydrate epitope of Neu5,9Ac 2 -GPS A L L was confirmed by using synthetic sialic acid analogs. The enhanced presence of anti-Neu5,9Ac 2 -GP A L L antibody in ALL patients prompted us to develop an antigen-ELISA using purified Neus,9Ac 2 -GPS A L L as coating antigens. Purified antigen was able to detect leukemia-specific antibodies at presentation of disease, which gradually decreased with treatment. Longitudinal monitoring of 18 patients revealed that in the early phase of the treatment patients with lower anti-Neu5,9Ac 2 -GPs showed a better prognosis. Minimal cross-reactivity was observed in other hematological disorders (n=50) like chronic myeloid leukemia, acute myelogenous leukemia, chronic lymphocytic leukemia, and non-Hodgkin's lymphoma as well as normal healthy individuals (n=21). This study demonstrated the potential of purified Neu5,9Ac 2 -GPS A L L as an alternate tool for detection of anti-Neu5,9Ac 2 -GP antibodies to be helpful for diagnosis and monitoring of childhood ALL patients.

We describe a synthetic sialic acid analogue, 9-thioacetamido-N-acetylneuraminic acid (9-thioacetamido-Neu5Ac), which is recognized by the receptor-binding activity of influenza C virus, but is resistant to the receptor-destroying enzyme (acetylesterase) of this virus. Following transfer of the analogue to the surface of receptor-negative cells, influenza C virus is able to attach to these cells, but is unable to infect the cells. This result suggests that inactivation of virus receptors by the receptor-destroying enzyme is essential for initiation of infection. Because of their unique properties such analogues promise to be powerful chemotherapeutic agents.

Thioketenes or thioketene equivalents give β-thiolactams in the reaction with C=N systems, but problems arise in the synthesis of highly functionalized derivatives. Therefore to obtain the thione analog of a bactam a thionation approach was chosen.