Hydrolytic properties of a beta-mannosidase purified from Aspergillus niger

A beta-mannosidase was purified to homogeneity from the culture filtrate of Aspergillus niger. A specific activity of 500 nkat mg-1 and a 53-fold purification was achieved using ammonium sulfate precipitation, anion-exchange chromatography, and gel filtration. The isolated enzyme has an isoelectric point of 5.0 and appears to be a dimer composed of two 135-kDa subunits. It is a glycoprotein and contains 17% N-linked carbohydrate by weight. Maximal activity was observed at pH 2.4 5.0 and at 70 degrees C. The beta-mannosidase hydrolyzed beta-1,4-linked manno-oligosaccharides of degree of polymerization (DP) 2-6 and also released mannose from polymeric ivory nut mannan and galactomannan. The Km and Vmax values for p-nitrophenyl-beta-D-mannopyranoside were 0.30 mM and 500 nkat mg-1, respectively. Hydrolysis of D-galactose substituted manno-oligosaccharides showed that the beta-mannosidase was able to cleave up to, but not beyond, a side group. An internal peptide sequence of 15 amino acids was highly similar to that of an Aspergillus aculeatus beta-mannosidase belonging to family 2 of glycosyl hydrolases.