Sialic acids expression in newborn rat lungs: implications for pulmonary developmental biology

Mammalian lung development proceeds during the postnatal period and continues throughout life. Intricate tubular systems of airways and vessels lined by epithelial cells are developed during this process. All cells, and particularly epithelial cells, carry an array of glycans on their surfaces. N-acetylneuraminic (Neu5Ac) and N-glycolylneuraminic (Neu5Gc) acids, two most frequently-occurring sialic acid residues, are essential determinants during development and in the homeostasis of cells and organisms. However, systematic data about the presence of cell surface sialic acids in the postnatal lung and their content is still scarce. In the present study, we addressed the histochemical localization of Neu5Ac > Neu5Gc in 0-day-old rat lungs. Furthermore, both residues were separated, identified and quantified in lung membranes isolated from 0-day-old rat lungs using high-performance liquid chromatography (HPLC) methodologies. Finally, we compared these results with those previously reported by us for adult rat lungs. The Neu5Ac > Neu5Gc residues were located on the surface of ciliated and non-ciliated cells and the median values for both residues in the purified lung membranes of newborn rats were 5.365 and 1.935 μg/mg prot., respectively. Comparing these results with those reported for the adults, it was possible to observe a significant difference between the levels of Neu5Ac and Neu5Gc (p < 0.001). A more substantial change was found for the case of Neu5Ac. The preponderance of Neu5Ac and its expressive increase during the postnatal development points towards a more prominent role of this residue. Bearing in mind that sialic acids are negatively charged molecules, the high content of Neu5Ac could contribute to the formation of an anion "shield" and have a role in pulmonary development and physiology.

Design, synthesis, and structural elucidation of novel NmeNANAS inhibitors for the treatment of meningococcal infection

Neisseria meningitidis is the primary cause of bacterial meningitis in many parts of the world, with considerable mortality rates among neonates and adults. In Saudi Arabia, serious outbreaks of N. meningitidis affecting several hundreds of pilgrims attending Hajj in Makkah were recorded in the 2000–2001 season. Evidence shows increased rates of bacterial resistance to penicillin and other antimicrobial agents that are used in the treatment of the meningococcal disease. The host’s immune system becomes unable to recognize the polysialic acid capsule of the resistant N. meningitidis that mimics the mammalian cell surface. The biosynthetic pathways of sialic acid (i.e., N-acetylneuraminic acid [NANA]) in bacteria, however, are somewhat different from those in mammals. The largest obstacle facing previously identified inhibitors of NANA synthase (NANAS) in N. meningitidis is that these inhibitors feature undesired chemical and pharmacological characteristics. To better comprehend the binding mechanism underlying these inhibitors at the catalytic site of NANAS, we performed molecular modeling studies to uncover essential structural aspects for the ultimate recognition at the catalytic site required for optimal inhibitory activity. Applying two virtual screening candidate molecules and one designed molecule showed promising structural scaffolds. Here, we report ethyl 3-benzoyl-2,7-dimethyl indolizine-1-carboxylate (INLZ) as a novel molecule with high energetic fitness scores at the catalytic site of the NmeNANAS enzyme. INLZ represents a promising scaffold for NmeNANAS enzyme inhibitors, with new prospects for further structural development and activity optimization.

A glycal-based photoaffinity probe that enriches sialic acid binding proteins

To identify sialic acid binding proteins from complex proteomes, three photocrosslinking affinity-based probes were constructed using Neu5Ac (5 and 6) and Neu5Ac2en (7) scaffolds. Kinetic inhibition assays and Western blotting revealed the Neu5Ac2en-based 7 to be an effective probe for the labeling of a purified gut microbial sialidase (BDI_2946) and a purified human sialic acid binding protein (hCD33). Additionally, LC-MS/MS affinity-based protein profiling verified the ability of 7 to enrich a low-abundance sialic acid binding protein (complement factor H) from human serum thus validating the utility of this probe in a complex context.

Dissecting the influence of oxazolidinones and cyclic carbonates in sialic acid chemistry

At a moment's notice: Thermal equilibration of 1 and mass spectral analysis of sialyl phosphates suggest that the 4O,5N-oxazolidinone and the 4,5-O-carbonate systems influence the anomeric effect and the mechanisms of sialidation by virtue of their dipole moment in the mean plane of the pyranose ring. The electron-withdrawing effect destabilizes 2 and promotes associative glycosylation mechanisms. TEMPO = 2,2,6,6-tetramethylpiperidine N-oxide.

Targeting of CD22-positive B-cell lymphoma cells by synthetic divalent sialic acid analogues

CD22 is an inhibitory co-receptor of the B-cell receptor (BCR) on B cells. Since CD22 is ubiquitously expressed in the B-cell lineage and CD22 endocytosis can be triggered efficiently, antibodies and antibody-based immunotoxins against CD22 are used to target B cells both in B-cell lymphomas and leukemias, as well as in autoimmune diseases. CD22 recognizes α2,6-linked sialic acids as endogenous ligands. We have developed new synthetic sialosides as ligands for human CD22. These sialosides bind CD22 on human B cells with high affinity and can efficiently enhance IgM-triggered Ca(2+) signaling. We coupled these sialosides to Pseudomonas exotoxin A to generate a novel CD22 ligand-based immunotoxin. This sialoside-exotoxin-A construct can specifically kill CD22-positive B-cell lymphoma cells. It binds specifically to CD22-positive B-cell lymphoma cells and is dominant over endogenous cis-ligands on the B-cell surface. The sialoside-exotoxin-A construct is efficiently internalized by endocytosis into B-cell lymphoma cell lines. Thus we show the development of a new therapeutic compound for targeting CD22 on human B cells, both for B-cell lymphoma, as well as for B-cell-mediated autoimmune diseases.

General consideration on sialic acid chemistry

Sialic acids, also known as neuraminic acids, are a family of negatively charged α-keto acids with a nine-carbon backbone. These unique sugars have been found at the termini of many glycan chains of vertebrate cell surface, which play pivotal roles in mediating or modulating a variety of physiological and pathological processes. This brief review covers general approaches for synthesizing sialic acid containing structures. Recently developed synthetic methods along with structural diversities and biological functions of sialic acid are discussed.

Synthesis and inhibitory activity of sialic acid derivatives targeted at viral sialate-O-acetylesterases

A series of sialosides modified at the 4- and 9-hydroxy group were synthesised and tested for inhibition of the viral haemagglutinin-esterase activity from various Orthomyxoviruses and Coronaviruses. While no inhibition of the sialate-4-O-acetylesterases from mouse hepatitis virus strain S or sialodacryoadenitis virus was found, a 9-O-methyl derivative displayed inhibitory activity against recombinant sialate-9-O-acetylesterase from influenza C virus.

New stabilized FastPrep-CLEAs for sialic acid synthesis

N-acetyl-D-neuraminic acid aldolase, a key enzyme in the biotechnological production of N-acetyl-D-neuraminic acid (sialic acid) from N-acetyl-D-mannosamine and pyruvate, was immobilized as cross-linked enzyme aggregates (CLEAs) by precipitation with 90% ammonium sulfate and crosslinking with 1% glutaraldehyde. Because dispersion in a reciprocating disruptor (FastPrep) was only able to recover 40% of the activity, improved CLEAs were then prepared by co-aggregation of the enzyme with 10mg/mL bovine serum albumin followed by a sodium borohydride treatment and final disruption by FastPrep (FastPrep-CLEAs). This produced a twofold increase in activity up to 86%, which is a 30% more than that reported for this aldolase in cross-linked inclusion bodies (CLIBs). In addition, these FastPrep-CLEAs presented remarkable biotechnological features for Neu5Ac synthesis, including, good activity and stability at alkaline pHs, a high K(M) for ManNAc (lower for pyruvate) and good operational stability. These results reinforce the practicability of using FastPrep-CLEAs in biocatalysis, thus reducing production costs and favoring reusability.

Development of miracle medicines from sialic acids

Sialic acids are electronegatively charged C9-sugars and are considered to play important roles in higher animals and some microorganisms. Denoting their significance, understanding and exploiting the complexity of the sialic acids has been referred to as the "the third language of life". In essence, "sialic acid derivatives possess a harmonious shape and good balance between two opposing hydrophilic and hydrophobic parts, meaning that they should display various kinds of potentially unique and possibly conflicting physiological activities (glycolipoids)". Consequently, there are good omens that unprecedented 'miracle' medicines could be developed from sialic acid derivatives. In this review, the first problem, the preparation of sialic acids, is covered, the synthesis of sialic acid derivatives and confirmation of their structures obviously being of critical significance. In addition we needed to confirm their precise stereochemistry and a hydrolysis method has been developed for confirmation of the anomeric position. Several of the compounds have already demonstrated interesting bioactivity.

Toward fully synthetic homogeneous beta-human follicle-stimulating hormone (beta-hFSH) with a biantennary N-linked dodecasaccharide. synthesis of beta-hFSH with chitobiose units at the natural linkage sites

A highly convergent synthesis of the sialic acid-rich biantennary N-linked glycan found in human glycoprotein hormones and its use in the synthesis of a fragment derived from the beta-domain of human Follicle-Stimulating Hormone (hFSH) are described. The synthesis highlights the use of the Sinay radical glycosidation protocol for the simultaneous installation of both biantennary side-chains of the dodecasaccharide as well as the use of glycal chemistry to construct the tetrasaccharide core in an efficient manner. The synthetic glycan was used to prepare the glycosylated 20-27aa domain of the beta-subunit of hFSH under a Lansbury aspartylation protocol. The proposed strategy for incorporating the prepared N-linked dodecasaccharide-containing 20-27aa domain into beta-hFSH subunit was validated in the context of a model system, providing protected beta-hFSH subunit functionalized with chitobiose at positions 7 and 24.

Regioselective synthesis of 4azido-Neu2en5,7Ac(2)1Me and its intramolecular transformation to 4azido-Neu2en5,9Ac(2)1Me

Methyl 5-N-acetyl-7-O-acetyl-4-azido-2,3-didehydro-2,4-dideoxy-neuraminic acid (4azido-Neu2en5,7 Ac(2)1Me) was synthesized regioselectively starting from 4azido-Neu2en5Ac1Me in high yield. The transformation of 4azido-Neu2en5,7Ac(2)1Me to the corresponding thermodynamically stable 4azido-Neu2en5,9Ac(2)1Me via intramolecular acetyl migration was confirmed by single-crystal X-ray diffraction analysis. The proposed rearrangement mechanism is discussed.

Mimetics of the tri- and tetrasaccharide epitope of GQ1bα as myelin-associated glycoprotein (MAG) ligands

The synthesis of phenoxyphenyl, phenoxybenzyl, biphenyl, and phenyltriazole substituted sialic acid derivatives as mimics of the tri- and tetrasaccharide epitopes of GQ1balpha is described. These synthetically easily available sialosides show comparable or even enhanced affinity to MAG compared with the natural tri- and tetrasaccharide epitopes and form a new class of potential MAG antagonists.

Synthesis of sialic acid derivatives as ligands for the myelin-associated glycoprotein (MAG)

The trisaccharide substructure 13 of the ganglioside GQ1balpha shows a remarkable affinity for the myelin-associated glycoprotein (MAG). In the search for structurally simplified and pharmacokinetically improved mimics of 13, sialosides with modifications at the reducing and non-reducing end were synthesized. The biological evaluation of mimics 12a-o was performed in a competitive target-based assay. It was found that the relative inhibitory potency (rIP) of antagonist 12h was enhanced by more than 1000-fold in comparison to the reference trisaccharide 13, despite the former having a much simpler structure. In addition, the sialic acid derivatives, for example, 12h, have clearly improved pharmacokinetic properties due to the presence of aromatic moieties, a lower molecular weight, and a reduced number of polar hydroxy functions compared to the reference compound 13.

Synthesis of a sialic acid alpha(2-3) galactose building block and its use in a linear synthesis of sialyl Lewis X

[reaction: see text] The ubiquity of the sialic acid alpha(2-3) galactose linkage in oligosaccharides of biological relevance necessitates a building block for the incorporation of this motif into oligosaccharides prepared by modular synthesis. The linear synthesis of the sialyl Lewis X tumor-associated antigen (1) has been accomplished in good yield using a sialic acid alpha(2-3) galactose disaccharide building block. The disaccharide building block was synthesized efficiently from readily available galactal by a high-yielding and selective sialylation reaction.

Synthesis and evaluation of 4-O-alkylated 2-deoxy-2,3-didehydro-N-acetylneuraminic acid derivatives as inhibitors of human parainfluenza virus type-3 sialidase activity

The X-ray crystal structure of the paramyxoviral surface glycoprotein haemagglutinin-neuraminidase (HN) from Newcastle Disease virus was used as a template to design inhibitors of the HN from human parainfluenza virus type-3 (hPIV-3). 4-O-Alkylated derivatives of 2-deoxy-2,3-didehydro-N-acetylneuraminic acid (Neu5Ac2en), accessed from 8,9-O-isopropylidenated-Neu5Ac2en1Me, were found to inhibit the sialidase (neuraminidase) activity of hPIV-3 (strain C243) in the range of 3-30muM. This is comparable or improved activity compared to the parent 4-hydroxy compound.

2-Deoxy-2,3-didehydro-N-acetylneuraminic acid analogues structurally modified at the C-4 position: Synthesis and biological evaluation as inhibitors of human parainfluenza virus type 1

To explore the influence of binding to human parainfluenza virus type 1 (hPIV-1), a series of 4-O-substituted Neu5Ac2en derivatives 6a-e was synthesized and tested for their ability to inhibit hPIV-1 sialidase. Among compounds 6a-e, the 4-O-ethyl-Neu5Ac2en derivative 6b showed the most potent inhibitory activity (IC50 6.3 microM) against hPIV-1 sialidase.

Preparation of a fluorous protecting group and its application to the chemoenzymatic synthesis of sialidase inhibitor

2-(Perfluorohexyl)ethoxymethyl chloride was prepared as a novel fluorous protecting reagent. Neu5Ac aldolase-catalyzed chemoenzymatic transformation of N-acetyl-D-mannosamine to Neu5Ac derivatives was achieved successfully by using the fluorous reagent not only for hydroxy group protection but also for fluorous tagging. This chemoenzymatic method was applied to the synthesis of 2-deoxy-2,3-didehydrosialic acid 1 known as a potent sialidase inhibitor.

Polysulfated sialic acid derivatives as anti-human immunodeficiency virus

We report the synthesis of a novel alkyl polysulfated sialic acid derivative denoted as NMSO3. NMSO3 exhibited potent inhibition against both laboratory and clinical human immunodeficiency virus type 1 (HIV-1). The anti-viral activity of this compound (1 uM) was compared to dextran sulfate (3 uM), and was found to be more potent against HIV-1IIIb than AZT (10 uM). The anti-coagulation time was more than 15-fold shorter than that of dextran sulfate. An in vivo anti-viral study of NMSO3 in NOD-SCID-PBL mice HIV model showed complete protection of the animals from virus challenge at the concentration of 10 mg/kg. This suggests that NMSO3 can be effective in the treatment of HIV-infected individuals.

Homomultimeric complexes of CD22 in B cells revealed by protein-glycan cross-linking

CD22 is a negative regulator of B-cell receptor signaling, an activity mediated by recruitment of SH2 domain-containing phosphatase 1 through a phosphorylated immunoreceptor tyrosine inhibitory motif in its cytoplasmic domain. As in other members of the sialic acid-binding immunoglobulin-like lectin, or siglec, family, the extracellular N-terminal immunoglobulin domain of CD22 binds to glycan ligands containing sialic acid, which are highly expressed on B-cell glycoproteins. B-cell glycoproteins bind to CD22 in cis and 'mask' the ligand-binding domain, modulating its activity as a regulator of B-cell signaling. To assess cell-surface cis ligand interactions, we developed a new method for in situ photoaffinity cross-linking of glycan ligands to CD22. Notably, CD45, surfaceIgM (sIgM) and other glycoproteins that bind to CD22 in vitro do not appear to be important cis ligands of CD22 in situ. Instead, CD22 seems to recognize glycans of neighboring CD22 molecules as cis ligands, forming homomultimeric complexes.

Synthesis of aryl sialosides using Mitsunobu conditions

Mitsunobu conditions for the efficient synthesis of aryl alpha/beta-sialosides were developed. An oxidative work-up procedure was employed to avoid a cumbersome chromatographic separation from the 2,3-dehydro derivative of sialic acid, which is formed as a side-product.

Synthesis of a new stable conformationally constrained 2,7-anhydrosialic acid derivative

A stereoseletive synthesis of 2,7-anhydrosialic acid derivative 18 was achieved from d-glucono-delta-lactone. A highly syn-selective addition of Grignard reagent to theN-benzylimine 8 served as a key step.

Synthesis of sialic acids via desymmetrization by ring-closing metathesis

Formal total syntheses of the naturally occurring deaminated sialic acids KDN (2), a potential oncofetal antigen, and N-acetylneuraminic acid (Neu5Ac, 1), the most naturally abundant sialic acid, have been accomplished in 46% and 9.3% overall yield, respectively, via a novel ketalization/ring-closing metathesis sequence. The rapid introduction of all oxygen and nitrogen functionality in a completely stereocontrolled manner exploited a rigid 6,8-dioxabicyclo[3.2.1]oct-2-ene template. The 2,7-anhydro-KDN derivative 40 served as an advanced intermediate in each of the two syntheses.

Synthesis of C-glycosyl compounds of N-acetylneuraminic acid from D-gluconolactone

A general strategy towards the synthesis of C-glycosyl compounds of N-acetylneuraminic acid (Neu5Ac) has been developed and successfully applied to the synthesis of C-methyl and C-phenyl derivatives. The key strategic elements are (i) chain extension of a D-gluconolactone derivative as C(6)-precursor with an allyl Grignard reagent as C(3)-precursor having in 2 position the C-linked aglycon moiety, (ii) stereoselective C-4/C-5 erythro-diol formation, (iii) 6-exo-trig selective heterocyclization, and (iv) installment of the 5-acetylamino and C-1 carboxylate functionalities. The efficiency and potential versatility of this approach was exemplified in the synthesis of C-methyl derivative 1 as target molecule.

Oxidation of 3-C-(2-amino-2-deoxy-D-glucopyranosyl)-1-propene compounds and the structure of 3-C-(2-amino-2-deoxy-D-glucopyranosyl)-1,2-propanediol derivatives for a synthesis of 2,3-didehydro-2,7-dideoxy-N-acetylneuraminic acid

Oxidation of 5-acetamido-4,8-anhydro-1,2,3,5-tetradeoxy-D-glycero-D-ido-non-1-enitol [3-C-(2-amino-2-deoxy-beta-D-glucopyranosyl)-1-propene] was studied to search for preparative routes to aminodeoxy didehydro nonulosonic acid derivatives. Since only moderate chiral induction was observed with osmium tetroxide dihydroxylation as well as with peracid epoxidation, the catalytic asymmetric dihydroxylation conditions were applied to give the stereocontrolled formation of 1,2-propanediol derivatives. The structures of these diastereoisomeric 1,2-propanediol derivatives were determined by X-ray crystallographic analyses. The formation of diastereoisomeric 1,2-propanediols also varied with the nature of 2-substituent on the aminodoexy glycosyl moiety. Thus 5-acetamido-4,8-anhydro-3,5-dideoxy-D-erythro-L-ido-nonitol [(2S)-3-C-(2-acetamido-2-deoxy-beta-D-glucopyranosyl)-1,2-propanediol] was obtained predominantly up to 70% from 3-C-(2-acetamido-2-deoxyglycosyl)-1-propene by the use of ADmixbeta reagent. The (2S)-propanediol derivative was transformed in a five-step reaction sequence to 2,3-didehydro-2,7-dideoxy-N-acetylneuraminic acid.

An improved fluorometric high-performance liquid chromatography method for sialic acid determination: an internal standard method and its application to sialic acid analysis of human apolipoprotein E

An improved fluorometric HPLC method for sialic acid determination was developed by employing synthetic N-propionylneuraminic acid (NPNA) as an internal standard. A fixed amount of NPNA was added to a sialoglycoconjugate sample. After hydrolyzing sialioglycoconjugates with diluted sulfuric acid, the released sialic acids and NPNA were derivatized with a fluorogenic compound, 1,2-diamino-4,5-(methylenedioxy)benzene (DMB), followed by fluorometric HPLC. The fluorescent derivative of NPNA was separated from those of N-acetylneuraminic acid, N-glycolylneuraminic acid, 2-keto-3-deoxy-D-glycero-D-galacto-nonoic acid, and 2-keto-3-deoxyoctanoate on HPLC. The separation of NPNA derivative on HPLC was not interfered by components of biological samples such as human sera. Using this internal standard method, low amounts of NANA (0.15-1.0 ng) were quantified with the coefficient of variation values below 4%. Using this method, the sialic acid content of human apolipoprotein E was successfully determined. The present method is useful for sensitive and accurate quantification of sialic acids of different molecular species in biological samples.

Synthesis and biological evaluation of sialylmimetics as rotavirus inhibitors

Rotaviruses cause severe gastroenteritis in infants and are estimated to be responsible for over 600 000 deaths annually, primarily in developing countries. The development of potential inhibitors of this virus is therefore of great interest, particularly since the safety and efficacy of rotaviral vaccines has recently been questioned. This study describes the synthesis of a variety of compounds that can be considered as mimetics of N-acetylneuraminic acid thioglycosides and the subsequent in vitro biological evaluation of these sialylmimetics as inhibitors of rotaviral infection. Our results show that readily accessible carbohydrate-based compounds have the potential to act as inhibitors of rotaviral replication in vitro, presumably through inhibition of the rotaviral adhesion process.

Synthesis of 2-deoxy-2,3-didehydro-N-acetylneuraminic acid analogues modified at the C-4 and C-9 positions and their behaviour towards sialidase from influenza virus and pig liver membrane

The synthesis of novel 2-deoxy-2,3-didehydro-N-acetylneuraminic acid analogues structurally varied at C-4 and C-9 by transformation from versatile key intermediates and their inhibitory activity against sialidase from influenza virus A and pig liver membrane are described.

Use of an altered sugar-nucleotide to unmask the transition state for alpha(2-->6) sialyltransferase

Rat liver alpha(2-->6) sialyltransferase catalyzes the formation of a glycosidic bond between N-acetylneuraminic acid and the 6-hydroxyl group of a galactose residue at the nonreducing terminus of an oligosaccharide. This reaction has been investigated through the use of the novel sugar-nucleotide donor substrate UMP-NeuAc. A series of UMP-NeuAc radioisotopomers were prepared by chemical deamination of the corresponding CMP-NeuAc precursors. Kinetic isotope effects (KIEs) on V/K were measured using mixtures of radiolabeled UMP-NeuAc's as the donor substrate and N-acetyllactosamine as the acceptor. The secondary beta-(2)H KIE was 1.218 +/- 0.010, and the primary (14)C KIE was 1.030 +/- 0.010. A large inverse (3)H binding isotope effect of 0.944 +/- 0.010 was measured at the terminal carbon of the NeuAc glycerol side chain. These KIEs observed using UMP-NeuAc are much larger than those previously measured with CMP-NeuAc [Bruner, M., and Horenstein, B. A. (1998) Biochemistry 37, 289-297]. Solvent deuterium isotope effects of 1.3 and 2.6 on V/K and V(max) were observed with CMP-NeuAc as the donor, and it is revealing that these isotope effects vanished with use of the slow donor substrate UMP-NeuAc. Bell-shaped pH versus rate profiles were observed for V(max) (pK(a) values = 5.5, 9.0) and V/K(UMP)(-)(NeuAc) (pK(a)values = 6.2, 9.0). The results are considered in terms of a mechanism involving an isotopically sensitive conformational change which is independent of the glycosyl transfer step. The isotope effects reveal that the enzyme-bound transition state bears considerable charge on the N-acetylneuraminic acid residue, and this and other features of this mechanism provide new directions for sialyltransferase inhibitor design.

Synthesis and evaluation of C-9 modified N-acetylneuraminic acid derivatives as substrates for N-acetylneuraminic acid aldolase

Several C-9 modified N-acetylneuraminic acid derivatives have been synthesised and evaluated as substrates of N-acetylneuraminic acid aldolase. Simple C-9 acyl or ether modified derivatives of N-acetylneuraminic acid were found to be accepted as substrates by the enzyme, albeit being transformed more slowly than Neu5Ac itself. 1H NMR spectroscopy was used to evaluate the extent of the enzyme catalysed transformation of these compounds. Interestingly, the chain-extended Neu5Ac derivative 16 is not a substrate for N-acetylneuraminate lyase and behaves as an inhibitor of the enzyme.

A facile synthesis of 2-deoxy-2,3-didehydroneuraminic acid derivatives

The 2-thio- or 2-selenoglycosides of N-acetylneuraminic acid methyl ester were transformed by successive treatment with dimethyl(methylthio)sulfonium triflate (DMTST) and 1,8-diazabicyclo[5.4.0]-7-undecene (DBU) to give the corresponding methyl 2-deoxy-2,3-didehydroneuraminates in excellent yields. Their acids and their analogues are sialidase inhibitors of pharmaceutical interest.

Alkaline biocatalysis for the direct synthesis of N-acetyl-D-neuraminic acid (Neu5Ac) from N-acetyl-D-glucosamine (GlcNAc)

Integration between the alkaline epimerization of N-acetyl-D-glucosamine (GlcNAc) to N-Acetyl-D-mannosamine (ManNAc) and the N-acetyl-D-neuraminic acid (Neu5Ac) aldolase-catalyzed biotransformation has been assessed experimentally. GlcNAc epimerization took place above pH 9.0, and the initial rate of ManNAc formation increased exponentially to 10.37 mmol/L per hour at pH 12. However, above this pH, severe degradation of pyruvate occurred. A value of 31.3% molar conversion on Pyr was achieved in an integrated biotransformation. The "pseudo"-steady state at the end of the reaction was comparable to the equilibrium achieved with a combination of an epimerase and aldolase enzymes. The integrated reaction proved feasible, but at the expense of pyruvate and Neu5Ac aldolase degradation.

Synthesis of bromoindolyl 4,7-di-O-methyl-Neu5Ac: specificity toward influenza A and B viruses

N-Acetylneuraminic acid (Neu5Ac) was converted into the methyl ester methyl ketoside-8,9-epoxy derivative (8). Methylation of 8 followed by deprotection gave 4,7-di-O-methyl-Neu5Ac (10). Compound 10 was converted into the corresponding methyl ester-chloroacetate derivative, which was subsequently coupled to 5-bromo-indol-3-ol to give the chromogenic product (13). Deprotection of 13 gave 5-bromo-indol-3-yl 4,7-di-O-methyl-Neu5Ac (5). The product 5 was specifically cleaved by sialidase from either influenza A or influenza B virus to give an indigo-blue precipitate, but was not cleaved by several bacterial or viral sialidases tested. The properties of product 5 relative to a fluorescent substrate for sialidase were also documented.

Lipase-catalyzed acetylation of N-acetylneuraminic acid derivative

A facile preparation of triacetylated derivative of 2-phenylthioglycoside of N-acetylneuraminic acid (4) was achieved by treatment with lipase PS in vinyl acetate. The major product 4 has a free hydroxyl group at C-7. Results of time-course HPLC analysis indicate that the reactivity of the hydroxyl groups under this condition is in the following order; C-9 > C-4 > C-8 > C-7.

Synthesis of a Neu2en5Ac analog hapten and isolation of monoclonal antibody to Neu2en5Ac

Neu2en5Ac is a minor component of body fluids and is abundant in sialuria, but no antibody to detect it has been reported. 5-Acetamido-2,6-anhydro-9-glutaramido-3,5,9-trideoxy-D-glycero-D- galacto-non-2-enonic acid has been synthesized and conjugated with keyhole limpet hemocyanin (KLH) for immunization. A hybridoma named SIC172 was obtained that produces a monoclonal antibody (MAb) to Neu2en5Ac. SIC172 MAb in culture supernatant bound strongly to the hapten conjugated to BSA in ELISA, but slightly to fetuin, a glycoprotein which is rich in Neu5Ac. SIC172 MAb (IgG3(kappa)), purified with a protein A/G affinity column, bound strongly to fetuin. Neu2en5Ac competed with the MAb in binding in amounts as low as 3 microM, while the competition of Neu5Ac appeared at amounts of more than 300 microM. SIC172 MAb is a unique MAb specific to Neu2en5Ac and might be useful for detecting Neu2en5Ac, which occurs naturally and in sialuria.

Chemoenzymatic synthesis of an N-acetylneuraminic acid analogue having a carbamoylmethyl group at C-4 as an inhibitor of sialidase from influenza virus

5,9-Diacetamido-2,6-anhydro-O-4-carbamoylmethyl-3,5,9-trideo xy-D-glycero- D-galacto-non-2-enonic acid (1) was synthesized via a key intemediate 2 through the Neu5Ac aldolase [E.C.4.1.3.3]-catalyzed aldol reaction of 2-acetamido-2,6-dideoxy-6-azido-D-glucose with sodium pyruvate operating under alkaline conditions (pH 10.5) in order to accelerate epimerization C-2 of N-acetyl-D-glucosamine (D-GlcNAc) derivatives. Compound 1 showed inhibitory activity against sialidase.

Monovalent and polymeric 5N-thioacetamido sialosides as tightly-bound receptor analogs of influenza viruses

A possible approach to the development of synthetic inhibitors of influenza virus attachment to host cells is based on the anchoring of the minimum receptor determinant of influenza virus, sialic acid, to a polymeric carrier. In this study, the effect of substitution of oxygen by sulphur in the 5N-acetyl moiety of sialic acid on the binding of monovalent and polymeric sialosides by A and B influenza virus strains was investigated. The polymeric inhibitor with pendant 5N-thioacetylneuraminic acid residues was found to be more broadly active against different virus stains that the one prepared from the Neu5Ac ligand.

In vivo modulated N-acyl side chain of N-acetylneuraminic acid modulates the cell contact-dependent inhibition of growth

Sialylation of plasma membrane glycoproteins is thought to be involved in the regulation of differentiation and in the process of tumorigenesis. Here we show that sialylation also affects cell-cell contact-dependent growth regulation. When cultured in the presence of non-physiological synthetic sialic acid precursors, human diploid fibroblasts no longer exhibited density-dependent inhibition of growth. Concomitantly, increased sialylation of contactinhibin, a glycoprotein involved in density-dependent inhibition of growth, was observed. These results indicate that sialidase-resistant sialic acid modifications lead to dysregulated growth control. The modifications have been induced by N-propanoyl and other N-acyl derivatives of D-mannosamine.