Cytosolic glycosidases: do they exist?

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Abstract
The substrate specificity of the α-D-mannosidases of rat liver lysosome and cytosol was examined using oligosaccharides of the oligomannosidic type. The hydrolysis products were characterized by 400 MHz 1H-NMR spectroscopy. Both catabolic pathways occur in ordered ways, but are quite different. In fact, the lysosomal pathway is a two-step process: the first step involves a Zn2+-independent α-1, 2-mannosidase activity, whereas the second involves a Zn2+-dependent α-1, 3- and α-1, 6-mannosidase activity. The final product is the disaccharide Man(β1–4)GlcNAc. In contrast, the cytosolic pathway leads, in one step, to a unique hexasaccharide (Man5GlcNAc) which has the same structure as the polyprenolic intermediate synthesized on the cytosolic face of the rough endoplasmic reticulum during the biosynthesis of N-glycosylprotein glycans: Man(α1–2)-Man(α1–2)Man(α1–3)[Man(α1–6)] Man(β1–4)GlcNAc(β1–4)-GlcNAc(α)P-P-Dol. In addition, the enzymatic parameters of lysosome, endoplasmic reticulum and cytosol α-D-mannosidases are quite different. These results lead to the conclusion that the cytosol contains specific α-D-mannosidases which do not originate from lysosomes nor from endoplasmic reticulum. The discovery of cytosolic endo-N-acetyl β-D-glucosaminidase active on ‘immature complex glycans’ (glycopeptides of the oligomannosidic type and of the desialylated N-acetyllactosaminic type) as well as on the glycosyl-dolichol pyrophosphate intermediates allows us to hypothesize that these enzymes belong to a control system of N-glycosylprotein biosynthesis, their role being to destroy unfinished glycans. The fate of the formed oligosaccharide structures is discussed: are they destroyed by cytosolic or lysosomal exoglycosidases, or do they carry an ‘oligo-saccharin-like activity’?

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