Studies toward the site specific incorporation of sugars into proteins: synthesis of glycosylated aminoacyl-tRNAs

A series of glycosylated serine derivatives was synthesized from peracetylated sugars and Fmoc-protected serine; these were chemically esterified with the tris-(tetrabutylammonium) salt of pdCpA. The fully protected and deprotected glycosylated aminoacyl pdCpAs were ligated enzymatically to an abbreviated tRNA (tRNA-C(OH)) to provide the title compounds that are key intermediates in the elaboration of glycoproteins using readthrough of a nonsense codon.

Synthesis of neoglycoproteins containing D-glycero-D-talo-oct-2-ulopyranosylonic acid (Ko) ligands corresponding to core units from Burkholderia and Acinetobacter lipopolysaccharide

Glycal esters of Kdo derivatives were converted into 2,3-anhydro intermediates, which were transformed into D-glycero-D-talo-oct-2-ulopyranosylonic acid (Ko), as well as 3-O- and 4-O-p-nitrobenzoyl-Ko derivatives. The exo-allyl orthoester derivative, methyl [5,7,8-tri-O-acetyl-4-O-(4-nitrobenzoyl)-2,3-O-[(1-exo-allyloxy)-ethylidene]-D-glycero-beta-D-talo-oct-2-ulopyranos]onate, prepared from the 4-O-pNBz-protected Ko derivative, was elaborated into the alpha-Ko allyl ketoside, the reducing disaccharide alpha-Kdop-(2-->4)-Ko and the disaccharide alpha-Kdop-(2-->4)-Kop-(2-->OAll). Conversely, methyl[4,5,7,8-tetra-O-acetyl-3-O-(4-nitrobenzoyl)-alpha-D-glycero-D-talo-2-octulopyranosyl bromide]onate [Carbohydr. Res., 244 (1993) 69-84], was coupled with a Kdo acceptor to give the disaccharide alpha-Kop-(2-->4)-Kdop-(2-->OAll) after orthoester rearrangement and deprotection. The allyl glycosides were treated with cysteamine and converted into neoglycoproteins. The ligands correspond to inner core units from Acinetobacter haemolyticus and Burkholderia cepacia lipopolysaccharides.

Complex carbohydrate synthesis tools for glycobiologists: enzyme-based approach and programmable one-pot strategies

The ultimate goal in complex carbohydrate synthesis is to develop synthetic tools which are simple and easily accessible to glycobiologists. This review will describe methods which have the potential to reach this goal, with particular focus on enzymatic and computer-based one-pot approaches for the preparation of complex carbohydrates and glycoconjugates.

Prosaposin-derived peptides enhanced sprouting of sensory neurons in vitro and induced sprouting at motor endplates in vivo

Prosaposin exhibits neurotrophic factor properties that are localized to a 12-amino acid sequence located in the amino terminal portion of the saposin C domain. Prosaptides are peptides derived from the neurotrophic portion of prosaposin; these have been previously reported to be bioactive in neuroblastoma cell lines in vitro. We report that prosaptides were also bioactive in explants of adult primary sensory neurons by dose-dependently increasing both the number (3- to 4-fold) and elongation of these neurites by 50%. Local injection of prosaptides into the gluteus muscle of adult mice also induced sprouting at the motor endplate. Our results indicate that prosaptides are potent neuritogenic factors for both sensory and motor neurons of adult peripheral nerve.

Controlled site-selective protein glycosylation for precise glycan structure-catalytic activity relationships

Glycoproteins occur naturally as complex mixtures of differently glycosylated forms which are difficult to separate. To explore their individual properties, there is a need for homogeneous sources of carbohydrate-protein conjugates and this has recently prompted us to develop a novel method for the site-selective glycosylation of proteins. The potential of the method was illustrated by site-selective glycosylations of subtilisin Bacillus lentus (SBL) as a model protein. A representative library of mono- and disaccharide MTS reagents were synthesized from their parent carbohydrates and used to modify cysteine mutants of SBL at positions 62 in the S2 site, 156 and 166 in the S1 site and 217 in the S1' site. These were the first examples of preparations of homogeneous neoglycoproteins in which both the site of glycosylation and structure of the introduced glycan were predetermined. The scope of this versatile method was expanded further through the combined use of peracetylated MTS reagents and careful pH adjustment to introduce glycans containing different numbers of acetate groups. This method provides a highly controlled and versatile route that is virtually unlimited in the scope of the sites and glycans that may be conjugated, and opens up hitherto inaccessible opportunities for the systematic determination of the properties of glycosylated proteins. This potential has been clearly demonstrated by the determination of detailed glycan structure-hydrolytic activity relationships for SBL. The 48 glycosylated CMMs formed display kcat/KM values that range from 1.1-fold higher than WT to 7-fold lower than WT. The anomeric stereochemistry of the glycans introduced modulates changes in kcat/KM upon acetylation. At positions 62 and 217 acetylation enhances the activity of alpha-glycosylated CMMs but decreases that of beta-glycosylated. This trend is reversed at position 166 where, in contrast, acetylation enhances the kcat/KMs of beta-glycosylated CMMs but decreases those of alpha-glycosylated. Consistent with its surface exposed nature changes at position 156 are more modest, but still allow control of activity, particularly through glycosylation with disaccharide lactose.

Site-selective glycosylation of subtilisin Bacillus lentus causes dramatic increases in esterase activity

Using site directed mutagenesis combined with chemical modification, we have developed a general and versatile method for the glycosylation of proteins which is virtually unlimited in the scope of proteins and glycans that may be conjugated and in which the site of glycosylation and the nature of the introduced glycan can be carefully controlled. We have demonstrated the applicability of this method through the synthesis of a library of 48 glycosylated forms of the serine protease subtilisin Bacillus lentus (SBL) as single, pure species. As part of our ongoing program to tailor the activity of SBL for use in peptide synthesis, we have screened these enzymes for activity against the esterase substrate succinyl-Ala-Ala-Pro-Phe-S-benzyl. Gratifyingly, 22 enzymes displayed greater than wild type (WT) activity. Glycosylation at positions 62, in the S2 pocket, resulted in five glycosylated forms of SBL that were 1.3- to 1.9-fold more active than WT. At position 217, in the S1' pocket, all glycosylations increased kcat/KM up to a remarkable 8.4-fold greater than WT for the glucosylated enzyme L217C-S-beta-Glc(Ac)3. Furthermore, the ratio of amidase to esterase activity, (kcat/KM)esterase/(kcat/KM)amidase (E/A), is increased relative to wild type for all 48 glycosylated forms of SBL. Again, the most dramatic changes are observed at positions 62 and 217 and L217C-S-beta-Glc(Ac)3 has an E/A that is 17.2-fold greater than WT. The tailored specificity and high activity of this glycoform can be rationalized by molecular modeling analysis, which suggests that the carbohydrate moiety occupies the S1' leaving group pocket and enhances the rate of deacylation of the acyl-enzyme intermediate. These glycosylated enzymes are ideal candidates for use as catalysts in peptide synthesis as they have greatly increased (kcat,KM)esterase and severely reduced (kcat/KM)amidase and will favor the formation of the amide bond over hydrolysis.

Various stages of schistosoma express Lewis(x), LacdiNAc, GalNAcbeta1-4 (Fucalpha1-3)GlcNAc and GalNAcbeta1-4(Fucalpha1-2Fucalpha1-3)GlcNAc carbohydrate epitopes: detection with monoclonal antibodies that are characterized by enzymatically synthesized neoglycoproteins

We report here that fucosylated epitopes such as Lewis(x), LacdiNAc, fucosylated LacdiNAc (LDN-F) and GalNAcbeta1-4(Fucalpha1-2Fucalpha1-3)GlcNAc (LDN-DF) are expressed by schistosomes throughout their life cycle. These four epitopes were enzymatically synthesized and coupled to bovine serum albumin to yield neoglycoproteins. Subsequently these neoglycoproteins were used to probe a panel of 188 monoclonal antibodies obtained from infected or immunized mice, in ELISA and surface plasmon resonance analysis. Of these antibodies, 25 recognized one of the fucosylated structures synthesized, indicating that these structures are immunogenic during infection. The MAbs identified could be subdivided in four different groups based on the recognition of either the Lewis(x)-, the LacdiNAc-, the LDN-DF-, or both the LDN-F- and LDN-DF epitope. These monoclonal antibodies were then used to investigate the localization of the fucosylated epitopes in various stages of Schistosoma mansoni using indirect immunofluorescence. Lewis(x)epitopes were mainly found in the gut and on the tegument of adult worms, on egg shells, and on the oral sucker of cercariae. The LacdiNAc epitope was expressed on the tegument of adult worms, on miracidia, and on the oral sucker of cercariae. In contrast, LDN-DF epitopes were mainly present in the excretory system of adult worms, on miracidia and on whole cercariae. These also stained positive with the LDN-F/LDN-DF epitope antibodies, while whole parenchyma reacted characteristically only with the latter antibodies. The identification of different carbohydrate structures in various stages of schistosomes may lead to a better understanding of the function of glycans in the immune response during infection.

Designing stable blood-brain barrier-permeable prosaptide peptides for treatment of central nervous system neurodegeneration

Prosaposin-derived peptides have been proposed as potential therapeutics for neurodegenerative diseases. Previously, we reported that the minimal length for bioactivity was 12 amino acids, and key amino acids were described based on interspecies conservation. In this article, we have further investigated the sequence requirements for bioactive Prosaptide (Myelos Corporation) peptides in terms of length and amino acid identity. The use of Prosaptide peptides for treatment of central nervous system (CNS) disorders requires that they are stable in vivo. Although robust effects of our prototypical peptide Prosaptide TX14(A) have been shown in the peripheral nervous system, minimal success has been achieved when treating the CNS in rats and this may be due to instability of Prosaptide TX14(A) in brain. Herein, we demonstrate that, indeed, Prosaptide TX14(A) is rapidly degraded in the brain and we have attempted to design prosaptides with increased CNS stability. One peptide, Prosaptide TX15-2, shows increased stability in brain and may be of use in the treatment of CNS disorders. With the aim of designing Prosaptide peptides that may be systemically administered for CNS treatment, we have investigated the blood-brain barrier permeability of Prosaptide TX14(A) and TX15-2. Both of these peptides cross the blood-brain barrier via a nonspecific mechanism.

Synthesis of neoglycoenzymes with homogeneous N-linked oligosaccharides using immobilized endo-beta-N-acetylglucosaminidase A

A procedure for the enzymatic synthesis of neoglycoenzymes is described. The gene encoding endo-beta-N-acetylglucosaminidase from Arthrobacter protophormiae (Endo-A) was overexpressed in Escherichia coli as a fusion protein linked to glutathione S-transferase (GST). GST-Endo-A fusion was extracted as a soluble protein. The fusion protein was purified to homogeneity with glutathione-Sepharose 4B and showed transglycosylation activity toward high-mannose-type glycopeptides without removing the GST moiety. The GST-Endo-A immobilized on glutathione-Sepharose 4B retained its transglycosylation activity. The immobilized enzyme could transfer (Man)(6)GlcNAc en bloc to partially deglycosylated ribonuclease B without damaging its enzyme activity. The immobilized GST-Endo-A should be very useful for synthesizing active neoglycoenzymes attached with homogeneous N-linked oligosaccharides.

Chemical synthesis and receptor binding of catfish somatostatin: a disulfide-bridged beta-D-Galp-(1-->3)-alpha-D-GalpNAc O-glycopeptide

The glycopeptide hormone catfish somatostatin (somatostatin-22) has the amino acid sequence H-Asp-Asn-Thr-Val-Thr-Ser-Lys-Pro-Leu-Asn-Cys-Met-Asn-Tyr-Phe-Trp-Lys-Se r-Arg-Thr-Ala-Cys-OH; it includes a cyclic disulfide connecting the two Cys residues, and the major naturally occurring glycoform contains D-GalNAc and D-Gal O-glycosidically linked to Thr5. The linear sequence was assembled smoothly starting with an Fmoc-Cys(Trt)-PAC-PEG-PS support, using stepwise Fmoc solid-phase chemistry. In addition to the nonglycosylated peptide, two glycosylated forms of somatostatin-22 were accessed by incorporating as building blocks, respectively, Nalpha-Fmoc-Thr(Ac3-alpha-D-GalNAc)-OH and Nalpha-Fmoc-Thr(Ac4-beta-D-Gal-(1-->3)-Ac2-alpha-D-GalNAc)-O H. Acidolytic deprotection/cleavage of these peptidyl-resins with trifluoroacetic acid/scavenger cocktails gave the corresponding acetyl-protected glycopeptides with free sulfhydryl functions. Deacetylation, by methanolysis in the presence of catalytic sodium methoxide, was followed by mild oxidation at pH 7, mediated by Nalpha-dithiasuccinoyl (Dts)-glycine, to provide the desired monomeric cyclic disulfides. The purified peptides were tested for binding affinities to a panel of cloned human somatostatin receptor subtypes; in several cases, presence of the disaccharide moiety resulted in 2-fold tighter binding.

Extensive characterization of the immunophenotype and pattern of cytokine production by distinct subpopulations of normal human peripheral blood MHC II+/lineage- cells

Dendritic cells (DC) represent the most powerful professional antigen-presenting cells (APC) in the immune system. The aim of the present study was to analyse, on a single-cell basis by multiparametric flow cytometry with simultaneous four-colour staining and a two-step acquisition procedure, the immunophenotypic profile and cytokine production of DC from 67 normal whole peripheral blood (PB) samples. Two clearly different subsets of HLA-II+/lineage- were identified on the basis of their distinct phenotypic characteristics: one DC subset was CD33strong+ and CD123dim+ (0.16 +/- 0.06% of the PB nucleated cells and 55.9 +/- 11. 9% of all PB DC) and the other, CD33dim+ and CD123strong+ (0.12 +/- 0.04% of PB nucleated cells and 44.53 +/- 11.5% of all PB DC). Moreover, the former DC subpopulation clearly showed higher expression of the CD13 myeloid-associated antigen, the CD29 and CD58 adhesion molecules, the CD2, CD5 and CD86 costimulatory molecules, the CD32 IgG receptor and the CD11c complement receptor. In addition, these cells showed stronger HLA-DR and HLA-DQ expression and a higher reactivity for the IL-6 receptor alpha-chain (CD126) and for CD38. In contrast, the CD123strong+/CD33dim+ DC showed a stronger reactivity for the CD4 and CD45RA molecules, whereas they did not express the CD58, CD5, CD11c and CD13 antigens. Regarding cytokine production, our results show that while the CD33strong+/CD123dim+ DC are able to produce significant amounts of inflammatory cytokines, such as IL-1beta (97 +/- 5% of positive cells), IL-6 (96 +/- 1.1% of positive cells), IL-12 (81.5 +/- 15.5% of positive cells) and tumour necrosis factor-alpha (TNF-alpha) (84 +/- 22.1% of positive cells) as well as chemokines such as IL-8 (99 +/- 1% of positive cells), the functional ability of the CD123strong+/CD33dim+ DC subset to produce cytokines under the same conditions was almost null. Our results therefore clearly show the presence of two distinct subsets of DC in normal human PB, which differ not only in their immunophenotype but also in their functionality, as regards cytokine production.

Interaction between lectins and neoglycoproteins containing new sialylated glycosynthons

Neoglycoconjugates are useful tools to study carbohydrate/protein interactions. In order to discover new lectins, to define their fine specificity or to study their intracellular trafficking, there is a need for neoglycoconjugates containing complex oligosaccharides. We recently set up a simple way to transform native oligosaccharides into glycosynthons. The present paper describes i) the synthesis of such glycosynthons starting with sialylated oligosides, ii) the preparation of sialylated neoglycoproteins and iii) their binding to sialic acid-specific lectins assessed by surface plasmon resonance experiments.

Chlamydial lipopolysaccharide

Chlamydiae are obligatory intracellular parasites which are responsible for various acute and chronic diseases in animals and humans. The outer membrane of the chlamydial cell wall contains a truncated lipopolysaccharide (LPS) antigen, which harbors a group-specific epitope being composed of a trisaccharide of 3-deoxy-D-manno-oct-2-ulosonic (Kdo) residues of the sequence alpha-Kdo-(2-->8)-alpha-Kdo-(2-->4)-alpha-Kdo. The chemical structure was established using LPS of recombinant Escherichia coli and Salmonella enterica strains after transformation with a plasmid carrying the gene encoding the multifunctional chlamydial Kdo transferase. Oligosaccharides containing the Kdo region attached to the glucosamine backbone of the lipid A domain have been isolated or prepared by chemical synthesis, converted into neoglycoproteins and their antigenic properties with respect to the definition of cross-reactive and chlamydia-specific epitopes have been determined. The low endotoxic activity of chlamydial LPS is related to the unique structural features of the lipid A, which is highly hydrophobic due to the presence of unusual, long-chain fatty acids.

High level of aspartic acid-bond isomerization during the synthesis of an N-linked tau glycopeptide

An increased degree of utilization of the potential N-glycosylation site in the fourth repeat unit of the human tau protein may be involved in the inability of tau to bind to the corresponding tubulin sequence(s) and in the subsequent development of the paired helical filaments of Alzheimer's disease. To model these processes, we synthesized the octadecapeptide spanning this region without sugar, and with the addition of an N-acetyl-glucosamine moiety. The carbohydrate-protected, glycosylated asparagine was incorporated as a building block during conventional Fmoc-solid phase peptide synthesis. While the crude non-glycosylated analog was obtained as a single peptide, two peptides with the identical, expected masses, in approximately equal amounts, were detected after the cleavage of the peracetylated glycopeptide. Surprisingly, the two glycopeptides switched positions on the reversed-phase high performance liquid chromatogram after removal of the sugar-protecting acetyl groups. Nuclear magnetic resonance spectroscopy and peptide sequencing identified the more hydrophobic deprotected peak as the target peptide, and the more hydrophilic deprotected peak as a peptide analog in which the aspartic acid-bond just preceding the glycosylated asparagine residue was isomerized resulting in the formation of a beta-peptide. The anomalous chromatographic behavior of the acetylated beta-isomer could be explained on the basis of the generation of an extended hydrophobic surface which is not present in any of the other three glycopeptide variants. Repetition of the syntheses, with altered conditions and reagents, revealed reproducibly high levels of aspartic acid-bond isomerization of the glycopeptide as well as lack of isomerization for the non-glycosylated parent analog. If similar increased aspartic acid-bond isomerization occurs in vivo, a protein modification well known to take place for both the amyloid deposits and the neurofibrillary tangles in Alzheimer's disease, this process may explain the aggregation of glycosylated tau into the paired helical filaments in the affected brains.

Synthesis of glycosylated human interleukin-1alpha, neoglyco IL-1alpha, coupled with N-acetylneuraminic acid

In order to develop glycosylated cytokine, recombinant human IL-1alpha was chemically modified with N-acetylneuraminic acid (NANA). NANA with C9 spacer, 8-(hydrazinocarbonyl)octyl 5-acetamido-3, 5-dideoxy-D-glycero-alpha-D-galacto-2-nonulo-pyranosidonic acid potassium salt (6), was synthesized by glycosylation of C9 spacer, 8-[2-N-(benzyloxycarbonyl)hydrazinocarbonyl]octanol, with methylthio glycoside derivatives of NANA in the presence of molecular sieves 3A and methyl (methylthio)sulfonium trifrate in propionitrile, followed by separation of a and beta anomers with a column chromatography and deprotection. Compound 6 was coupled to IL-1alpha by the acyl azide method. The glycosylated IL-1 was purified by anion-exchange chromatography, and NANA coupled to IL-1 was confirmed by oxidation with NaIO4. Based on the molecular weight average number of carbohydrate molecules introduced per molecule of IL-1alpha was estimated to be 2.9.

Rabbit corneal and conjunctival permeability of the novel aldose reductase inhibitors: N-[[4-(benzoylamino)phenyl] sulphonyl]glycines and N-benzoyl-N-phenylglycines

Corneal and conjunctival permeability has been investigated for novel aldose reductase inhibitors (ARIs) of the N{[4-(benzoylamino)phenyl]sulphonyl}glycine (benzoylaminophenylsulphonylglycine) and N-benzoyl-N-phenylglycine (benzoylphenylglycine) series, compounds developed for prevention of cataract formation in diabetic subjects. Six benzoylaminophenylsulphonylglycines were synthesized with modifications either of the phenyl group or of the glycine structure and three benzoylphenylglycines were synthesized with modification in the phenyl group of the benzoyl moiety. Transport of ARIs in the mucosal to serosal direction was evaluated across rabbit cornea and conjunctiva bathed in glutathione-bicarbonate Ringer's solution maintained at pH 7.4 and 37 degrees C. The permeability coefficients of the novel ARIs across cornea and conjunctiva ranged from 1.87 to 8.95 x 10(-6) cm s(-1) and from 4.6 to 19.15 x 10(-6) cm s(-1), respectively. The ratio of corneal to conjunctival permeability ranged from 0.12 to 0.79. The calculated log partition coefficient (log P) values for the ARIs were in the range 0.84 to 2.78. The log distribution coefficients (log D) were in the range -2.87 to -0.89. There was no apparent relationship between log P or log D and the permeability coefficients of the ARIs for either tissue. Cornea was more resistant to ARI transport than was conjunctiva. Substitution of a phenyl group for hydrogen in the glycine methylene group reduced the permeability coefficient. Permeability coefficients were different for different stereoisomers. Compared with the permeability coefficient of benzoylaminophenylsulphonylglycine, that of 4-fluorobenzoylaminophenylsulphonylglycine was lower in the cornea but similar in the conjunctiva. In both tissues, the permeability coefficient of 2-nitrobenzoylaminophenylsulphonylglycine was less than that of 4-nitrobenzoylaminophenylsulphonylglycine. There was no significant difference between the permeability coefficients of 3-nitro- and 4-nitrobenzoylphenylglycines through either tissue and the permeability coefficients of these compounds were greater than that of the more lipophilic 4-methylbenzoylphenylglycine. The lack of dependence of the permeability coefficients on log P or log D and the different permeabilities of stereoisomers imply the existence of specialized transport processes for the ARIs tested in this study.

Artificial antifreeze polypeptides: alpha-helical peptides with KAAK motifs have antifreeze and ice crystal morphology modifying properties

Antifreeze polypeptides from fish are generally thought to inhibit ice crystal growth by specific adsorption onto ice surfaces and preventing addition of water molecules to the ice lattice. Recent studies have suggested that this adsorption results from hydrogen bonding through the side chains of polar amino acids as well as hydrophobic interactions between the non-polar domains on the ice-binding side of antifreeze polypeptides and the clathrate-like surfaces of ice. In order to better understand the activity of one of the antifreeze polypeptide families, namely the alpha-helical type I antifreeze polypeptides, four alpha-helical peptides having sequences not directly analogous to those of known antifreeze polypeptides and containing only positively charged and non-polar side chains were synthesized. Two peptides with regularly spaced lysine residues, GAAKAAKAAAAAAAKAAKAAAAAAAKAAKAAGGY-NH2 and GAALKAAKAAAAAALKAAKAAAAAALKAAKAAGGY-NH2, showed antifreeze activity, albeit weaker than seen in natural antifreeze polypeptides, by the criteria of freezing point depression (thermal hysteresis) and ice crystal modification to a hexagonal trapezohedron. Peptides with irregular spacing of Lys residues were completely inactive. Up to now, lysine residues have not been generally associated with antifreeze activity, though they have been implicated in some antifreeze polypeptides. This work also shows that lysine residues in themselves, when properly positioned on an alpha-helical polyalanine scaffold, have all the requisite properties needed for such an activity.

Synthesis of cyclic Herpes simplex virus peptides containing 281-284 epitope of glycoprotein D-1 in endo- or exo-position

We have prepared two types of cyclopeptides containing the 281DPVG284 sequence from the 276-284 region of glycoprotein gD-1 of the Herpes simplex virus (HSV). The syntheses were performed by solid phase methodology using MBHA or BHA resin and orthogonal protection schemes. Head-to-side-chain cyclization included the N-terminal part of the epitope, while side-chain-to-side-chain lactam bridge formation resulted in a peptide containing a C-terminal cycle. Peptides elongated by Cys at the N-terminal of the sequence were also prepared. Boc chemistry using Fmoc and OFm orthogonal protection was applied for on-resin cyclization. Based on the orthogonality of Bzl and cHex esters under a 1 M TMSOTf-thioanisole/TFA cleavage condition, a new approach for the cyclization on BHA-resin has also been developed. Preliminary studies on solution conformation of the cyclic peptides by CD spectroscopy indicated the importance of the location and the size of the cycle within the epitope sequence.

Chemoenzymatic synthesis of spacer-linked oligosaccharides for the preparation of neoglycoproteins

In the present work, the combination of chemical and enzymatic methods to obtain neoglycoproteins is described. Three bovine serum albumin (BSA)-conjugates, BSA-[GalNAc alpha-], BSA-[Gal(beta 1-3)GalNAc(alpha-], and BSA-[Neu5Ac(alpha 2-3)Gal(beta 1-3)GalNAc(alpha-], were prepared. alpha GalNAc derivatives were galactosylated employing crude beta-galactosidase from bovine testes. The use of oversaturated donor solutions (pNP beta Gal) enhanced the yields up to 60%. This method was verified using divalent structures as acceptors, that rendered di- and tri-galactosylated products. Further treatment of the disaccharides with CMP-Neu5Ac and alpha 2-3 sialyltransferase from pork liver led to formation of trisaccharides. Finally, mono-, di-, and trisaccharides were coupled to BSA employing a thiolic group introduced into the protein for Michael addition to a maleinimide group in the spacer-arm of the saccharide components. The results were monitored by HPLC and MALDI-TOF.

Specific renal delivery of sugar-modified low-molecular-weight peptides

To develop a novel delivery system for peptides involving sugar modification, Arg-vasopressin (AVP) was modified by linking it to a variety of sugars via an octamethylene group and the subsequent tissue uptake by rats was then monitored after administration by i.v. injection. The glucosyl, mannosyl, and 2-deoxyglucosyl derivatives of AVP exhibited selective renal uptake. These derivatives were found to be distributed in the proximal tubules of the renal cortex. In addition, they exhibited specific binding to the kidney microsomal fraction in vitro (Kd = approximately 60 nM), suggesting that they are taken up by a specific recognition mechanism located in the kidneys. From the results of the uptake study of glucosyl derivatives, the following points are clear: 1) renal uptake in vivo becomes saturated with increasing dose, and the Km from the uptake study is almost the same as the Kd obtained in the binding assay in vitro and 2) because the renal first-pass uptake extraction is about 70% at a low dose (10 nmol/kg), there is an effective mechanism for uptake from blood. Furthermore, glucosyl and mannosyl derivatives of oxytocin, a neutral peptide, unlike AVP that is basic, also have high renal uptake clearances. Thus, the renal uptake may not be dependent on derivatives having a cationic nature. We conclude that there is a novel transport mechanism in the kidneys that can be used for the specific renal delivery of glycosylated peptides.

Chemoselective ligation reactions with proteins, oligosaccharides and cells

Traditional chemical synthesis does not lend itself to the easy, rapid construction of even moderately sized biomolecules, because it requires elaborate protection schemes. Furthermore, many biological studies would be aided by the ability to assemble biomolecules under physiological conditions. These challenges have motivated the development of chemoselective ligation, the selective covalent coupling of mutually and uniquely reactive functional groups under mild, aqueous conditions. This technique has attracted significant attention recently for the synthesis of biological macromolecules of defined homogeneous composition, the design of self-assembling drugs and the chemical remodeling of cell surfaces.

Improved synthesis of an aldobiouronic acid related to hardwood xylans, and preparation of a derivative thereof suitable for linking to proteins

Treatment of 1,3,4-tri-O-acetyl-alpha-D-xylopyranose with methyl 2,3-di-O-benzyl-l-chloro-l-deoxy-4-O-methyl-alpha, beta-D-glucopyranuronate in the presence of silver trifluoromethanesulfonate was highly stereoselective to give the alpha-linked aldobiouronic acid derivative (4) in 86% yield, after hydrogenolysis of the crude product of the coupling and chromatography. Compound 4 was acetylated and the fully protected substance was converted to the corresponding glycosyl chloride. Reaction of the latter with p-nitrophenol under phase-transfer catalysis afforded, after deacetylation, p-nitrophenyl 2-O-(methyl 4-O-methyl-alpha-D-glucopyranosyluronate)-beta-D-xylopyranoside.

Ga3+ as a functional substitute for Fe3+: preparation and characterization of the Ga3+Fe2+ and Ga3+Zn2+ forms of bovine spleen purple acid phosphatase

A general method has been developed that allows the specific substitution of both iron atoms in the enzyme bovine spleen purple acid phosphatase (BSPAP), which possesses a dinuclear iron center at the active site. The approach is demonstrated by the preparation and characterization (atomic absorption spectrometry, enzyme kinetics, optical spectroscopy, and electron paramagnetic resonance spectroscopy) of two metal-substituted forms in which the ferric iron has been replaced by Ga3+: Ga3+Fe2+-BSPAP and Ga3+Zn2+-BSPAP. Both forms are colorless but exhibit enzymatic activity comparable to that of the native Fe3+Fe2+-BSPAP. Small but consistent changes in kinetics parameters and pH profiles were detected both upon substitution of Fe3+ by Ga3+ and upon substitution of Fe2+ by Zn2+. These results constitute the first evidence that the diamagnetic Ga3+ ion can serve as a functional analogue of Fe3+ in an enzyme, and suggest a novel approach for the study of the role of Fe3+ in other iron enzymes.

Synthesis of a neoglycoprotein containing the Lewis X analogous trisaccharide beta-D-GalpNAc-(1-->4)[alpha-L-Fucp-(1-->3)]-beta-D-GlcpNAc

The trisaccharide allyl glycoside 36 and related disaccharide part structures have been prepared using the 2-trichloroacetamido-2-deoxy-alpha-D-galactopyranosyl trichloroacetimidate derivative 9 as glycosyl donor under promotion with TMSOTf or Sn(OTf)2, respectively, to produce the beta-(1-->4) linkage to suitably protected glucosamine derivatives in fair yields. Fucosylation was effected employing the ethyl 1-thio glycosyl donor 20 in the presence of IDCP. Deprotection of the intermediates afforded the disaccharide allyl glycosides beta-D-GalpNAc-(1-->4)- beta-D-GlcpNAc 13, beta-D-GalpNClAc-(1-->4)-beta-D-GlcpNAc 14, alpha-L-Fucp-(1-->3)-beta-D-GlcpNAc 24, alpha-L-Fucp-(1-->4)-beta-D- GlcpNAc 31 and the branched trisaccharide allyl glycoside beta-D-GalpNAc-(1-->4)[alpha-L-Fucp-(1-->3)]-beta-D-GlcpNAc 36. The trisaccharide which corresponds to a structural motif occurring in N-glycoprotein glycans from human urokinase, human recombinant protein C, phospholipase A2 as well as O-glycans, was converted into a neoglycoprotein following introduction of a cysteamine-derived spacer group and subsequent activation with thiophosgene.

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

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