Glycosidases of Aspergillus niger

A 100-fold purified preparation of Aspergillus niger α-mannosidase free of α- and β-galactosidases, β-N-acetylglucosaminidase, and 1,2-α-mannosidase has been obtained. Unlike many other mannosidases it is quite stable and hydrolyzes specific intersugar linkages involving nonreducing terminal mannose residues in large as well as small molecules. The enzyme hydrolyzes specifically 1,4-α- and 1,6-αmannosidic bonds to the adjoining mannose or N-acetylglucosamine residues. Therefore, 4-O-α- and 6-O-α-mannobioses and 2-acetamido-2-deoxy-4-O-α- and 6-O-α-glucoses are readily hydrolyzed. The β-galactosidase and β-N-acetylglucosaminidase-treated desialyzed glycoproteins or glycopeptides of α1 acid glycoprotein, fetuin, and human chorionic gonadotropin liberate mannose when digested with the enzyme. The enzyme, unlike other mannosidases, hydrolyzes p-nitrophenyl-α-d-mannopyranoside very slowly. It is inhibited strongly by d-mannono-(1→5)-lactone but does not appear to be Zn++-dependent or inactivated by various chelating agents. The pH optimum for the enzyme is 4.2. It has Km and Vmax values of 1.2 x 10-3 and 8.2 x 10-3 m, and 12.3 and 41.3 µmoles per mg per hour for p-nitrophenylα-d-mannopyranoside and methyl 4-O-α-mannopyranosyl-α-d-mannopyranoside as substrates, respectively.