Abstract
The β- d-mannosidase (β- d-mannoside mannohydrolase, EC 3.2.1.25) from culture filtrate of Aspergillus niger has been purified in large amounts by fractionation with (NH 4) 2SO 4 and DEAE-cellulose chromatography. The removal of traces of α- d-galactosidase was performed on a Sepharose-ϵ-aminocaproylgalactosylamine column. The final enzyme preparation (specific activity 188 units) has no other glycosidase activity and is judged homogeneous. The enzyme has a molecular weight of 130 000 ± 5000 and an isoelectric point of 4.7. The amino acid composition of the enzyme is characterized by high proportion of acidic amino acids and no cysteine residues and a single chain structure of the enzyme is suggested. The enzyme shows maximum activity on p- nitrophenyl-β- d- mannopyranoside at pH 3.5 and 55°C. The presence of 80% of β-sheet structure in the protein and 20.8% of monosaccharides (Gal : 1.3; Man : 7; GlcNAc : 1) could explain this relative high heat stability (up to 2 h at 55°C). Enzyme activity is inhibited by mannose ( K i = 7.85 mM) and the specificity is examined.
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