Description:
<jats:p>Man<jats:sub>9</jats:sub>‐mannosidase, an α1,2‐specific enzyme located in the endoplasmic reticulum and involved in N‐linked‐oligosaccharide processing, has been isolated from crude pig‐liver microsomes and its substrate specificity studied using a variety of free and peptide‐bound high‐mannose oligosaccharide derivatives. The purified enzyme displays no activity towards synthetic α‐mannosides, but removes three α1,2‐mannose residues from the natural Man<jats:sub>9</jats:sub>‐(GlcNAc)<jats:sub>2</jats:sub> substrate (M<jats:sub>9</jats:sub>). The α1,2‐mannosidic linkage remaining in the M<jats:sub>6</jats:sub> intermediate is cleaved about 40‐fold more slowly. Similar kinetics of hydrolysis were determined with Man<jats:sub>9</jats:sub>‐(GlcNAc)<jats:sub>2</jats:sub> N‐glycosidically attached to the hexapeptide Tyr‐Asn‐Lys‐Thr‐Ser‐Val (GP‐M<jats:sub>9</jats:sub>), indicating that the specificity of the enzyme is not influenced by the peptide moiety of the substrate. The α1,2‐mannose residue which is largely resistant to hydrolysis, was found to be attached in both the M<jats:sub>6</jats:sub> and GP‐M<jats:sub>6</jats:sub> intermediate to the α1,3‐mannose of the peripheral α1,3/α1,6‐branch of the glycan chain. Studies with glycopeptides varying in the size and branching pattern of the sugar chains, revealed that the relative rates at which the various α1,2‐mannosidic linkages were cleaved, differed depending on their structural complexity. This suggests that distinct sugar residues in the aglycon moiety may be functional in substrate recognition and binding. Reduction or removal of the terminal GlcNAc residue of the chitobiose unit in M<jats:sub>9</jats:sub> increased the hydrolytic susceptibility of the fourth (previously resistant) α1,2‐mannosidic linkage significantly. We conclude from this observation that, in addition to peripheral mannose residues, the intact chitobiose core represents a structural element affecting Man<jats:sub>9</jats:sub>‐mannosidase specificity. A possible biological role of the enzyme during N‐linked‐oligosaccharide processing is discussed.</jats:p>