Synthesis of 13 C-labelled sulfated N-acetyl-d-lactosamines to aid in the diagnosis of mucopolysaccharidosis diseases

Morquio A syndrome is an autosomal mucopolysaccharide storage disorder that leads to accumulation of keratan sulfate. Diagnosis of this disease can be aided by measuring the levels of keratan sulfate in the urine. This requires the liquid chromatography tandem mass spectrometry (LCMS/MS) measurement of sulfated N-acetyl-d-lactosamines in the urine after cleavage of the keratan sulfate with keratanase II. Quantification requires isotopically-labelled internal standards. The synthesis of these ¹³ C₆ -labelled standards from ¹³ C₆ -galactose and N-acetylglucosamine is described. The required protected disaccharide is prepared utilising a regioselective, high yielding β-galactosylation of a partially protected glucosamine acceptor and an inverse addition protocol. Subsequent synthesis of the ¹³ C₆ -labelled mono and disulfated N-acetyllactosamines was achieved in five and eight steps, respectively, from this intermediate to provide internal standards for the LCMS/MS quantification of keratan sulfate in urine.

Synthesis of a biotinylated keratan sulfate tetrasaccharide composed of dimeric Galβ1-4GlcNAc6Sβ

We successfully synthesized the biotinylated keratan sulfate tetrasaccharide, Galβ1-4GlcNAc6Sβ1-3Galβ1-4GlcNAc6Sβ in a stereocontrolled manner. The suitably protected Galβ1-4GlcNPhth unit was converted to the corresponding donor and acceptor. Optimization in 2 + 2 coupling using AgOTf, CuBr₂, and n-Bu₄NBr in CH₃NO₂ at a low temperature afforded the desired tetrasaccharide that suppressed glycal formation. The subsequent chemoselective removal of the protecting group at O-6 of two GlcNAcs, sulfation, and deprotection procedures as well as biotinylation gave the target compound.

Synthesis of biotinylated keratan sulfate repeating disaccharides

We synthesized four types of keratan and keratan sulfate repeating disaccharides containing non-sulfate, Galβ1-4GlcNAcβ, and three types of sulfates, Gal6Sβ1-4GlcNAcβ, Galβ1-4GlcNAc6Sβ, and Gal6Sβ1-4GlcNAc6Sβ in an efficient and stereo-controlled manner. These disaccharides were conjugated with biotin via a hydrophilic linker at the reducing terminal.

Solid-phase synthesis of alpha-glucosamine sulfoforms with fragmentation analysis by tandem mass spectrometry

Sulfated epitopes of alpha-glucosamine (GlcN sulfoforms) were prepared by solid-phase synthesis as models of internal glucosamines within heparan sulfate. An orthogonally protected 2'-hydroxyethyl GlcN derivative was immobilized on a trityl resin support and subjected to regioselective deprotection and sulfonation conditions, which were optimized with the aid of on-resin infrared or Raman analysis. The sulfoforms were cleaved from the resin under mild Lewis acid conditions without affecting the O- or N-sulfate groups and purified by reversed-phase high-performance liquid chromatography (HPLC). The alpha-GlcN sulfoforms and their 4- O-benzyl ethers were examined by electrospray ionization tandem mass spectrometry (ESI-MS/MS), with product ion spectra produced by collision-induced dissociation (CID). ESI-MS/MS revealed significant differences in parent ion stabilities and fragmentation rates as a function of sulfate position. Ion fragmentation by CID resulted in characteristic mass losses with strong correlation to the positions of both free hydroxyl groups and sulfate ions. Most of these fragmentation patterns are consonant with elimination pathways, and suggest possible strategies for elucidating the structures of glucosamine-derived sulfoforms with identical m/ z ratios. In particular, fragmentation analysis can easily distinguish GlcN sulfoforms bearing the relatively rare 3- O-sulfate from isomers with the more common 6- O-sulfate.

Keratanase II-Catalyzed Synthesis of Keratan Sulfate Oligomers by Using Sugar Oxazolines as Transition-State Analogue Substrate Monomers: A Novel Insight into the Enzymatic Catalysis Mechanism

Keratan sulfate (KS) oligomers with well-defined structures were synthesized by keratanase II (KSase II)-catalyzed transglycosylation. N-Acetyllactosamine [Galbeta(1-->4)GlcNAc; LacNAc] oxazoline derivatives with sulfate groups at the C-6 (1 a) and both the C-6 and the C-6' (1 b) were prepared as transition-state analogue substrate monomers for KSase II. Monomer 1 a was effectively oligomerized by the enzyme under weak alkaline conditions, to give alternating 6-sulfated KS oligomers (2 a) in good yields, and with total control of regioselectivity and stereochemistry. KSase II also recognized 1 b, which provided fully 6-sulfated KS oligomers (2 b) in good yields under similar conditions. Nonsulfated LacNAc oxazoline was difficult to oligomerize enzymatically. These results imply that the catalysis mechanism of KSase II involves a sugar oxazolinium ion that requires the 6-sulfate group in the GlcNAc residue not only in hydrolysis of KS chains, but also in oligomerization of oxazoline monomers. This is the first report of KSase II-catalyzed transglycosylation to form beta(1-->3)-glycosidic bond through a substrate-assisted mechanism.