Synthesis of Lewis X and three Lewis X trisaccharide analogues in which glucose and rhamnose replace N-acetylglucosamine and fucose, respectively

Three analogues of the Le(x) trisaccharide: alpha-L-Fucp-(1-->3)-[beta-D-Galp-(1-->4)]-D-GlcNAcp as well as the Le(x) trisaccharide itself were synthesized as methyl glycosides. In the analogues, either only the fucose residue is replaced by rhamnose or both the N-acetylglucosamine and the fucosyl residues are replaced by glucose and rhamnose, respectively. Our synthetic strategy relied on the use of lactoside and 2-azido lactoside derivatives as disaccharide acceptors, which were submitted to either fucosylation or rhamnosylation. Our results confirm that the reactivity of lactose in protection and glycosylation reactions is greatly affected by (1) the structure of the aglycone and (2) the presence of an azido substituent at C-2 of the glucose moiety. Thus, a methyl lactoside acceptor was easily glycosylated at O-3 with perbenzylated beta-thiophenyl fucoside and rhamnoside to give anomerically pure alpha-fucosylated and alpha-rhamnosylated trisaccharides, respectively. In contrast, the same reactions on a 2-azido methyl lactoside acceptor led to the formation of anomeric mixtures. While the alpha- and beta-fucosylated 2-azido trisaccharides could be separated by RP-HPLC, such separation was not possible for the rhamnosylated anomers. The desired rhamnosylated trisaccharide was finally obtained anomerically pure using an isopropylidene-protected rhamnosyl donor. The deprotection sequences also showed that the presence of a 2-azido substituent at C-2 of the glucose residue conferred stability to the vicinal fucosidic linkage at C-3. To test their relative affinity for anti-Le(x) Abs the Le(x) analogues will be used as competitive inhibitors against methyl Le(x). In addition, their conformational behavior will be studied by NMR spectroscopy and molecular modeling experiments.     

Synthesis of propyl and 2-aminoethyl glycosides of alpha-D-galactosyl-(1-->3')-beta-lactoside.

Propyl and 2-aminoethyl alpha-D-galactopyranosyl-(1-->3')-beta-lactosides (1 and 2) were prepared from the corresponding perbenzylated trisaccharide allyl glycoside 6 which, in turn, was obtained by methyl triflate promoted alpha-galactosylation of benzylated allyl lactoside acceptor 4 with thiogalactoside 3. Transformation of the allyl moiety in compound 6 into 2-azidoethyl one was achieved by cleavage of the double bond followed by reduction into alcohol 9, subsequent mesylation, and mesylate-->azide substitution. Alternatively trisaccharide 2 was synthesized using alpha-galactosylation of selectively benzoylated 2-azidoethyl lactoside 19 with 3 as the key step.     

Active-latent glycosylation strategy toward Lewis X pentasaccharide in a form suitable for neoglycoconjugate syntheses.

Glycosylation of 4-nitrophenyl 2-acetamido-6-O-tert-butyldiphenylsilyl-2-deoxy-1-thio-beta-D-gluc opyranoside with phenyl 2,3,4,6-tetra-O-benzoyl-1-thio-beta-D-galactopyranoside in the presence of NIS and TfOH as catalyst gave the lactosamine derivative regiospecifically in high yield. Further 3-O-fucosylation with phenyl 2,3,4-tri-O-benzyl-1-thio-beta-L-fucopyranoside using DMTST as promoter afforded the Lex trisaccharide intermediate. The latent glycosyl donor was transformed into its active form (p-acetamidothiophenyl) by reduction with zinc in acetic acid and N-acetylation. Glycosidation with p-nitrothiophenyl lactoside acceptor in the presence of NIS/TfOH as catalyst gave the Lex pentasaccharide in 71% yield.     

Construction, and in vitro and in vivo analyses of tetravalent immunoadhesins

Previous observations demonstrated that various immunosuppressive agents and their combination therapies can increase allograft survival rates. However, these treatments may have serious side effects and cannot substantially improve or prolong graft survival in acute graft-versus-host disease (GVHD). To improve the therapeutic potency of divalent immunoadhesins, we have constructed and produced several tetravalent forms of immunoadhesins comprising each of cytotoxic T-lymphocyte-associated antigen-4 (CTLA4), CD2, and lymphocyte activation gene-3 (LAG3). Flow cytometric and T cell proliferation analyses displayed that tetravalent immunoadhesins have a higher binding affinity and more potent efficacy than divalent immunoadhesins. Although all tetravalent immunoadhesins possess better efficacies, tetravalent forms of CTLA4-Ig and LAG3-Ig revealed higher inhibitory effects on T cell proliferation than tetravalent forms of TNFR2-Ig and CD2-Ig. In vitro mixed lymphocytes reaction (MLR) showed that combined treatment with tetravalent CTLA4-Ig and tetravalent LAG3-Ig was highly effective for inhibiting T cell proliferation in both human and murine allogeneic stimulation. In addition, both single tetravalent-form and combination treatments can prevent the lethality of murine acute GVHD. The results of this study demonstrated that co-blockade of the major histocompatibility complex class (MHC)II:T cell receptor (TCR) and CD28:B7 pathways by using tetravalent human LAG3-Ig and CTLA4-Ig synergistically prevented murine acute GVHD.     

Chemo-bacterial synthesis and immunoreactivity of a brain HNK-1 analogue

This work reports the synthesis and the biological validation of a trisaccharide analogue of the HNK-1 epitope. The 3-O-sulfo-β-d-GlcpA-(1→3)-β-d-Galp-(1→4)-β-d-Glcp-allyl has been prepared by enzymatic glucuronylation of allyl lactoside by an engineered recombinant Escherichia coli strain followed by a chemoselective sulfation. Subsequent covalent attachment of the ozone-oxidised trisaccharide to bovine serum albumin provided a neo-glycoconjugate, which has been interrogated with antibodies specific to the human natural killer carbohydrate epitope HNK-1. ELISA assays confirmed the absolute requirement of the sulfate group for protein recognition and the potential application of this synthetic oligosaccharide as HNK-1 surrogate.     

Facile synthesis of cyclopeptide-centered multivalent glycoclusters with ‘click chemistry’ and molecular recognition study by surface plasmon resonance

A facile synthesis of cyclopeptide-centered multivalent glycoclusters using Cu(I) catalyzed Huisgen 1,3-dipolar cycloaddition of azides and terminal alkynes, so called ‘click chemistry’, has been developed. The affinities of mannose-specific protein Concanavalin A (Con A) toward two synthetic glycoclusters respectively bearing divalent or tetravalent mannoses were investigated by surface plasmon resonance (SPR). It is founded that the tetravalent glycocluster has 3.0-fold increase in binding affinity relative to the divalent glycoluster (valency-corrected values), which indicates the potential of this system in investigating carbohydrate–protein interactions.     

Synthesis of multivalent lactose derivatives by 1,3-dipolar cycloadditions: selective galectin-1 inhibition

Acetylene derivatives of phenylalanine, phenethylamine and the multifunctional unnatural amino acids, phenyl-bis-alanine and phenyl-tris-alanine, were synthesized and functionalized with 2-azidoethyl beta-D-galactopyranosyl-(1-->4)-beta-D-glucopyranoside via regioselective copper(I)-mediated 1,3-dipolar cycloaddition to give a panel of mono-, di- and trivalent lactoside derivatives. Evaluation of the compounds as inhibitors against the tumour- and inflammation-related galectin-1, -3, -4N, -4C, -4, -7, -8N and -9N revealed a divalent compound with a Kd value as low as 3.2 microM for galectin-1, which corresponded to a relative potency of 30 per lactose unit as compared to the natural disaccharide ligand lactose. This divalent compound had at least one order of magnitude higher affinity for galectin-1 than for any of the other galectins investigated.     

Role of the valence of concanavalin A in the activation of guinea pig polymorphonuclear leukocytes

Stimulation of polymorphonuclear leukocytes (PMN) by tetravalent concanavalin A (alpha-ConA) induces membrane depolarization preceding the onset of superoxide anion (O2-) production. Both divalent and monovalent ConA analogues were studied to evaluate the role of valence. Monovalent ConA (m-ConA) was inactive in stimulating O2- production and divalent derivatives were less active than native alpha-ConA. Similarly, membrane depolarization was dependent on the valency of ConA. m-ConA did not induce a marked change in membrane potential, whereas sustained depolarization occurred with multivalent ConA. The formation of multiple linked interactions between surface receptors may be an important early event in the activation of PMN by ConA.     

Different effects of concanavalin A and its succinylated derivative on superoxide release in peritoneal macrophages.

The effects of tetravalent conconavalin A and its succinylated derivative on the intracellular production of superoxide anion (O-2) and its release into cell exterior of peritoneal macrophages were observed. Both tetravalent concanavalin A and its succinylated derivative induced marked enhancement of intracellular reduction of nitroblue tetrazolium, which could be inhibited by alpha-methyl-D-glucoside. The extent of activation of nitroblue tetrazolium reduction induced by both types of the lectin paralleled the activation ratio of oxygen consumption. There was little difference in the extent of intracellular O-2 production induced by two types of the lectin. Nitroblue tetrazolium reduction was not affected significantly by pretreatment with colchicine, rotenone or malonate, inhibitors of the cytoskeletal system and of the electron transport system. In contrast, tetravalent concanavalin A induced a higher rate of superoxide release than did succinylated divalent concanavalin A, which lacks the cross-linking activity of surface glycoproteins. These results indicate that superoxide production following oxygen consumption and superoxide release into cell exterior are controlled independently by a separate membrane mechanism and that superoxide production system is not essentially dependent on the involvement of the cytoskeletal system.     

Remarkable drug-release enhancement with an elimination-based AB3 self-immolative dendritic amplifier

Self-immolative dendritic prodrugs, activated through a single catalytic reaction by a specific enzyme, could offer significant advantages in inhibition of tumor growth relative to monomeric prodrug, especially if the targeted or secreted enzyme exists at relatively low levels in the malignant tissue. We have designed and synthesized new AB(3) self-immolative dendritic prodrug system that releases three active drugs by a single cleavage of the enzyme penicillin-G-amidase. The cleavage signal is transferred from the dendron focal point to its periphery through fast elimination reactions and the design leads to three-fold signal amplification. In cell-growth inhibition assays, the elimination-based AB(3) self-immolative dendritic prodrug was significantly more effective than a cyclization-based AB(3) dendritic prodrug.     

Synthesis of fluorescent alkyl lactoside derivatives

Fluorescent-labeled alkyl lactoside (NBD-alkyl lactoside) derivatives were synthesized by the reaction of NBD alkyl alcohol with heptaacetyllactosyl trichloroacetimidate in the presence of BF(3).Et(2)O.     

Chemoenzymatic synthesis of the sialyl-alpha-(2-->3')-lactosamine trisaccharide with a 3-aminopropyl group as a spacer at the reducing end

The trisaccharide, 3-aminopropyl 5-acetamido-3,5-dideoxy-D-glycero-alpha-D-galacto-2-nonulopyranosylonic acid-(2-->3)-beta-D-galactopyranosyl-(1-->4)-2-acetamido-2-deoxy-beta-D-glucopyranoside has been synthesized chemoenzymatically for the first time. First, the acceptor, 3-aminopropyl beta-D-galactopyranosyl-(1-->4)-2-acetamido-2-deoxy-beta-D-glucopyranoside was synthesized in a conventional chemical manner, and then it was coupled with CMP-sialic acid using alpha-(2-->3)-(N)-sialyltransferase to afford the desired trisaccharide by an enzymatically stereocontrolled manner.     

Enhanced membrane pore formation by multimeric/oligomeric antimicrobial peptides

The pore-forming antibacterial peptide magainin 2 was made divalent, tetravalent, and octavalent via a copper(I)-mediated 1-3 dipolar cycloaddition reaction ("click" chemistry). This series of pore-forming compounds was tested in vitro for their ability to form pores in large unilamillar vesicles (LUVs). A large increase in the pore-forming capability was especially observed with the tetravalent and octavalent magainin compounds in the LUVs consisting of DOPC, and the octavalent magainin compound showed a marked increase with the DOPC/DOPG LUVs. Activity was observed in the low nanomolar range for these compounds.     

The role of weak protein-protein interactions in multivalent lectin-carbohydrate binding: crystal structure of cross-linked FRIL

Binding of multivalent glycoconjugates by lectins often leads to the formation of cross-linked complexes. Type I cross-links, which are one-dimensional, are formed by a divalent lectin and a divalent glycoconjugate. Type II cross-links, which are two or three-dimensional, occur when a lectin or glycoconjugate has a valence greater than two. Type II complexes are a source of additional specificity, since homogeneous type II complexes are formed in the presence of mixtures of lectins and glycoconjugates. This additional specificity is thought to become important when a lectin interacts with clusters of glycoconjugates, e.g. as is present on the cell surface. The cryst1al structure of the Glc/Man binding legume lectin FRIL in complex with a trisaccharide provides a molecular snapshot of how weak protein-protein interactions, which are not observed in solution, can become important when a cross-linked complex is formed. In solution, FRIL is a divalent dimer, but in the crystal FRIL forms a tetramer, which allows for the formation of an intricate type II cross-linked complex with the divalent trisaccharide. The dependence on weak protein-protein interactions can ensure that a specific type II cross-linked complex and its associated specificity can occur only under stringent conditions, which explains why lectins are often found forming higher-order oligomers.     

Covalently cross-linked monovalent, divalent, and tetravalent derivatives of concanavalin A.

Dimeric succinyl-concanavalin A was cross-linked with ethylenediamine using 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide a condensing agent. Thus, a divalent dimer and a tetravalent tetramer composed mostly of covalently cross-linked subunits bearing altered net charges were obtained. Photoaffinity labeling of the cross-linked dimer with p-azidophenyl alpha-D-mannopyranoside resulted in a specific label for its saccharide-binding site and yielded a nonvalent dimer and a monovalent dimer (showing no subunit exchange). However, hemagglutination and glycogen precipitation data suggested that the labeled binding site is shielded but not destroyed by the label and can still bind weakly an external saccharide ligand possibly due to unsteadiness of the shielding label. Although nonvalent and monovalent derivatives were mitogenic as well as divalent and tetravalent derivatives for mouse splenic lymphocytes, binding and stimulation experiments indicated that their stimulating efficiencies after binding to the cells were far lower than those of the multivalent counterparts. Their activities were inhibited by methyl alpha-D-mannopyranoside, suggesting that the weak activities of nonvalent and monovalent derivatives were due to the labeled sites entirely and partly, respectively. We suggest that the triggering of lymphocyte mitogenesis by concanavalin A may depend on cross-linkage of cell surface receptors.     

Identification of a Wide Range of Motifs Inhibitory to Shiga Toxin by Affinity-Driven Screening of Customized Divalent Peptides Synthesized on a Membrane

Shiga toxin (Stx), a major virulence factor of enterohemorrhagic Escherichia coli, binds to target cells through a multivalent interaction between its B-subunit pentamer and the cell surface receptor globotriaosylceramide, resulting in a remarkable increase in its binding affinity. This phenomenon is referred to as the “clustering effect.” Previously, we developed a multivalent peptide library that can exert the clustering effect and identified Stx neutralizers with tetravalent peptides by screening this library for high-affinity binding to the specific receptor-binding site of the B subunit. However, this technique yielded only a limited number of binding motifs, with some redundancy in amino acid selectivity. In this study, we established a novel technique to synthesize up to 384 divalent peptides whose structures were customized to exert the clustering effect on the B subunit on a single cellulose membrane. By targeting Stx1a, a major Stx subtype, the customized divalent peptides were screened to identify high-affinity binding motifs. The sequences of the peptides were designed based on information obtained from the multivalent peptide library technique. A total of 64 candidate motifs were successfully identified, and 11 of these were selected to synthesize tetravalent forms of the peptides. All of the synthesized tetravalent peptides bound to the B subunit with high affinities and effectively inhibited the cytotoxicity of Stx1a in Vero cells. Thus, the combination of the two techniques results in greatly improved efficiency in identifying biologically active neutralizers of Stx.     

Different effects of concanavalin A and its succinylated derivative on superoxide release in peritoneal macrophages

The effects of tetravalent concanavalin A and its succinylated derivative on the intracellular production of superoxide anion (O₂^?) and its release into cell exterior of peritoneal macrophages were observed. Both tetravalent concanavalin A and its succinylated derivative induced marked enhancement of intracellular reduction of nitroblue tetrazolium, which could be inhibited by α-methyl-D-glucoside. The extent of activation of nitroblue tetrazolium reduction induced by both types of the lectin paralleled the activation ratio of oxygen consumption.There was littele difference in the extent of intracellular O₂^? production induced by two types of the lectin. Nitroblue tetrazolium reduction was not affected significantly by pretreatment with colchicine, rotenone or malonate, inhibitors of the cytoskeletal system and of the electron transport system. In contrast, tetravalent concanavalin A induced a higher rate of superoxide release than did succinylated divalent concanavalin A, which lacks the cross-linking activity of surface glycoproteins.These results indicate that superoxide production following oxygen consumption and superoxide release into cell exterior are controlled independently by a separate membrane mechanism and that superoxide production system is not essentially dependent on the involvement of the cytoskeletal system.     

New and more efficient multivalent glyco-ligands for asialoglycoprotein receptor of mammalian hepatocytes

New multi-valent, carbohydrate ligands that contain terminal N-acetylgalactosamine (GalNAc) or lactose (Lac) were prepared using a nitrilotriacetic acid (NTA) derivative of L-lysine as scaffold. Tri-valent structures were prepared by attaching an ω-amino glycoside of GalNAc or Lac to each of the three carboxyl groups of N(ε)-protected N(α)-dicarboxymethyl-L-lysine. In addition, a hexa-valent lactoside was synthesized by attaching N(ε)-deprotected trivalent lactoside to each of the carboxyl group of N(α)-(trifluoroacetamido)hexanoyl L-aspartic acid. Tri-valent GalNAc glycosides and the hexa-valent lactoside had high affinity (dissociation constants approaching nM) for rat hepatocytes. The hexa-valent lactoside, after de-N(ε)-protection, was modified with a chelator, diethylenetriaminepentaacetic acid (DTPA), through which a fluorescent or radioactive tag, such as europium or indium, can be firmly attached. Intravenous infusion of (111)Indium-tagged hexa-valent lactoside to rats and mice resulted in nearly exclusive accumulation of radioactivity in the liver.     

IONIZING INFLUENCE OF SALTS WITH TRIVALENT AND TETRAVALENT IONS ON CRYSTALLINE EGG ALBUMIN AT THE ISOELECTRIC POINT

1. While crystalline egg albumin is highly soluble in water at low temperature at the pH of its isoelectric point, it is coagulated by heating. It has long been known that this coagulation can be prevented by adding either acid or alkali, whereby the protein is ionized. 2. It is shown in this paper that salts with trivalent or tetravalent ions, e.g. LaCl₃ or Na₄Fe(CN)₆, are also able to prevent the heat coagulation of albumin at the isoelectric point (i.e. pH 4.8), while salts with a divalent ion, e.g. CaCl₂, BaCl₄, Na₂SO₄, or salts like NaCl, have no such effect. 3. This is in harmony with the fact shown in a preceding paper that salts with trivalent or tetravalent ions can cause the ionization of proteins at its isoelectric point and thus give rise to a membrane potential between micellæ of isoelectric protein and surrounding aqueous solution, while the above mentioned salts with divalent and monovalent ions have apparently no such effect.     

Aggregation of nucleosomes by divalent cations.

Conditions of precipitation of nucleosome core particles (NCP) by divalent cations (Ca(2+) and Mg(2+)) have been explored over a large range of nucleosome and cation concentrations. Precipitation of NCP occurs for a threshold of divalent cation concentration, and redissolution is observed for further addition of salt. The phase diagram looks similar to those obtained with DNA and synthetic polyelectrolytes in the presence of multivalent cations, which supports the idea that NCP/NCP interactions are driven by cation condensation. In the phase separation domain the effective charge of the aggregates was determined by measurements of their electrophoretic mobility. Aggregates formed in the presence of divalent cations (Mg(2+)) remain negatively charged over the whole concentration range. They turn positively charged when aggregation is induced by trivalent (spermidine) or tetravalent (spermine) cations. The higher the valency of the counterions, the more significant is the reversal of the effective charge of the aggregates. The sign of the effective charge has no influence on the aspect of the phase diagram. We discuss the possible reasons for this charge reversal in the light of actual theoretical approaches.