We have previously reported the substrate specificity of the cytosolic α-D-mannosidase purified from rat liver using Man9GlcNAc, i.e. Manα1-2Manα1-3(Manα1-2Manα1-6)Man α1-6(Manα1-2Manα1-2Manα1-3)Manβ1-4GlcNAc, as substrate [Grard, Saint-Pol, Haeuw, Alonso, Wieruszeski, Strecker and Michalski (1994) Eur. J. Biochem. 223, 99–106]. Man9GlcNAc is hydrolysed giving Man5GlcNAc, i.e. Manα1-2Manα1-2Manα1-3(Manα1-6)Manβ1-4GlcNAc, possessing the same structure as the oligosaccharide of the dolichol pathway formed in the cytosolic compartment during the biosynthesis of N-glycosylprotein glycans. We study here the activity of the purified cytosolic α-D-mannosidase towards the oligosaccharide-diphosphodolichol intermediates formed during the biosynthesis of N-glycans, and also towards soluble oligosaccharides released from the endoplasmic reticulum which are glucosylated or not and possessing at their reducing end either a single N-acetylglucosamine residue or a di-N-acetylchitobiose sequence. We demonstrate that (1) dolichol pyrophosphate oligosaccharide substrates are poorly hydrolysed by the cytosolic α-D-mannosidase; (2) oligosaccharides with a terminal reducing di-N-acetylchitobiose sequence are not hydrolysed at all; (3) soluble oligosaccharides bearing a single reducing N-acetylglucosamine are the real substrates for the enzyme. These results suggest a role for α-D-mannosidase in the catabolism of glycans released from the endoplasmic reticulum rather than in the regulation of the biosynthesis of asparagine-linked oligosaccharides.

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