Quantitation and Isomeric Structure Analysis of Free Oligosaccharides Present in the Cytosol Fraction of Mouse Liver: Detection of a Free Disialobiantennary Oligosaccharide and Glucosylated Oligomannosides

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Abstract
The amounts and isomeric structures of free oligosaccharides derived from N-linked sugar chains present in the cytosol fraction of perfused mouse liver were analyzed by tagging the reducing end with 2-aminopyridine followed by 2-dimensional HPLC mapping with standard sugar chains. Sixteen pyridylaminated (PA-) oligomannosides terminating with a PA-GlcNAc residue (GNl-type), three glucose-containing oligomannosides, and four oligomannosides terminating with a PA-di-N-acetylchitobiose (GN2-type) were detected. The total contents of the GN1- and GN2-type oligomannosides were 3.4 and 0.5 nmol, respectively, per gram of wet tissue. Maltooligosaccharides (dimer to pentamer) were also detected, the total content of which was 13 nmol per gram of wet tissue. Besides these oligosaccharides, a PA-disialobiantennary sugar chain—the sole complex-type sugar chain—was also detected. All the oligomannosides identified had partial structures of GlC3Man9GlNAc2-p-p-dolichol, revealing that they were metabolic degradation products. Manal-2Manα1-2Manα1-3(Manα1-6)Manβ1-4GlcNAc (MSB′) was the major oligomanno-side, suggesting that cytosolic endo-β-N-acetylglucosaminidase and neutral α-manno-sidase participate in the degradation, because these enzymes have suitable substrate specificities for the production of M5B′. Degradation by these enzymes seems to be the main pathway by which oligomannosides are degraded in mouse cytosol; however, small amounts of Manα1–6(Manα1-3)Manα1-6(Manα1-3)Manα1-4(GlcNAc)1-2 and related oligomannosides together with parts of their structures were also detected, suggesting that there is another minor route by which cytosolic free oligomannosides are produced.

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