<em>In vitro</em> hydrolysis of oligomannosyl oligosaccharides by the lysosomal α-D-mannosidases.

In vitro incubation of the oligomannosyl oligosaccharides Man₉GlcNAc and Man₅GlcNAc with isolated disrupted lysosomes yields different oligosaccharide isomers resulting from mannosidase hydrolysis. These isomers were isolated by HPLC and characterized by ¹H-NMR spectroscopy. The first steps of the degradation involve an (α1–2)mannosidase activity and lead to the formation of one Man₈GlcNAc, one Man₇GlcNAc, two Man₆GlcNAc and two Man₅GlcNAc isomers. These reactions do not require Zn²⁺ as activator. On the other hand, the following steps, which lead to the formation of Man₃GlcNAc and Man₂GlcNAc, are Zn²⁺-dependent. This process is characterized by the preferential action of an (α1–3)mannosidase activity, and the formation of Man(α1–6)Man(αl–6)Man(βl–4)(GlcNAc and Man(α1–6)Man(β1–4)GlcNAc. Therefore, the digestion of Man₉GlcNAc inside the lysosome appears to follow a very specific pathway, since only nine intermediate compounds can be identified instead of the 38 possible isomers. Our results are consistent both with the existence of several specific enzymes for α1–2, α1–3 and α1–6 linkages, and with the presence of a unique enzyme whose specificity would be dependent either on Zn²⁺ or on the spatial conformation of the glycan. Nevertheless, previous work on the structural analysis of oligosaccharides excreted in the urine of patients suffering from mannosidosis, demonstrates the absence of the core α1-6-1inked mannosyl residue in the major storage product derived from oligomannosyl oligosaccharides. This observation indicates the presence of a specific (α1–6)mannosidase form, unaffected in mannosidosis. [ABSTRACT FROM AUTHOR]