SUBSTRATE-SPECIFICITY OF HUMAN LIVER NEUTRAL ALPHA-MANNOSIDASE

The digestion of radiolabelled natural oligosaccharide substrates by human liver neutral alpha-mannosidase has been studied by h.p.l.c. and h.p.t.l.c. The high-mannose oligosaccharides Man9GlcNAc and Man8GlcNAc are hydrolysed by the enzyme by two distinct non-random routes to a common product of composition Man6GlcNAc, which is then slowly converted into a unique Man5GlcNAc oligosaccharide, Man-alpha(1 --> 2)Man-alpha(1 --> 2)Man-alpha(1 --> 3)[Man-alpha(1 --> 6)] Man-beta(1 --> 4)GlcNAc. These pathways are different from the processing and lysosomal catabolic pathways for these structures. In particular, the alpha(1 --> 2)-linked mannose residues attached to the core alpha(1 --> 3)-linked mannose residue are resistant to hydrolysis. The key processing intermediate, Man-alpha(1 --> 3)[Man-alpha(1 --> 6)]Man-alpha(1 --> 6)[Man-alpha(1 --> 3)] Man-beta(1 --> 4)GlcNAc, is not produced in the digestion of high-mannose glycans by the neutral alpha-mannosidase, but it is hydrolysed by the enzyme by a non-random route to Man-beta(1 --> 4)GlcNAc via the core structure Man-alpha(1 --> 3)[Man-alpha(1 --> 6)]Man-beta(1 --> 4)GlcNAc. In contrast with its ready hydrolysis by lysosomal alpha-mannosidase, the core alpha(1 --> 3)-mannosidic linkage is quite resistant to hydrolysis by neutral alpha-mannosidase. The precise specificity of neutral alpha-mannosidase towards high-mannose oligosaccharides suggests that it has a role in the modification of such structures in the cytosol.