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.
al Daher
S
S
Division of Biochemistry and Metabolism, Institute of Child Health (University of London), U.K.
De Gasperi
R
R
Daniel
P
P
Hirani
S
S
Warren
C
C
Winchester
B
B
D002236:Carbohydrate Conformation; D002240:Carbohydrate Sequence; D002851:Chromatography, High Pressure Liquid; D006801:Humans; D008099:Liver / Q000201:enzymology*; D008361:Mannosidases / Q000302:isolation & purification / Q000378:metabolism*; D008969:Molecular Sequence Data; D009844:Oligosaccharides / Q000302:isolation & purification / Q000378:metabolism*; D013379:Substrate Specificity; D043323:alpha-Mannosidase
D009844:Oligosaccharides; D008361:Mannosidases; D043323:alpha-Mannosidase