Structural diversity of cytosolic free oligosaccharides in the human hepatoma

It is thought that free oligosaccharides in the cytosol are an out-come of quality control of glycoproteins by endoplasmic reticu-lum-associated degradation (ERAD). Although considerable amounts of free oligosaccharides accumulate in the cytosol, where they presumably have some function, detailed analyses of their structures have not yet been carried out. We isolated 21 oligosaccharides from the cytosolic fraction of HepG2 cells and analyzed their structures by the two-dimensional high-performance liquid chromatography (HPLC) sugar-mapping method. Sixteen novel oligosaccharides were identified in the cytosol in this study. All had a single N-acetylglucosamine at their reducing-end cores and could be expressed as (Man)n (GlcNAc)1. No free oligosaccharide with N,N-diacetylchitobiose was detected in the cytosolic fraction of HepG2 cells. This suggested that endo-b-N-acetylglucosaminidase was a key enzyme in the production of cytosolic free oligosaccharides. The 21 oligosaccharides were classified into three series— series 1: oligosaccharides processed from Mana1-2Mana1-