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 pu rification was achieved using ammonium sulfate precipitation, anion-exchang e chromatography, and gel filtration. The isolated enzyme has an isoelectri c 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. Maxi mal activity was observed at pH 2.4-5.0 and at 70 degrees C. The beta-manno sidase hydrolyzed beta-1,Clinked manno-oligosaccharides of degree of polyme rization (DP) 2-6 and also released mannose from polymeric ivory nut mannan and galactomannan. The K-m and V-max Values for p-nitrophenyl-beta-D-manno pyranoside were 0.30 mM and 500 nkat mg(-1), respectively. Hydrolysis of D- galactose substituted manno-oligosaccharides showed that the beta-mannosida se was able to cleave up to, but not beyond, a side group. An internal pept ide sequence of 15 amino acids was highly similar to that of an Aspergillus aculeatus beta-mannosidase belonging to family 2 of glycosyl hydrolases. ( C) 1999 Elsevier Science B.V. All rights reserved.