Purification and characterization of an alpha-L-rhamnosidase from Aspergillus niger

An enzyme with alpha-L-rhamnosidase activity was purified by anion exchange chromatography from an Aspergillus niger commercial preparation. The alpha-L-rhamnosidase was shown to be N-glycosylated, and had a molecular mass of 85 kD on sodium dodecylsulfate-polyacrylamide gel electrophoresis of which approximately 12% was contributed by carbohydrate. The enzyme was optimally active at pH 4.5 and 65 degrees C. When tested towards p-nitrophenyl-alpha-L-rhamnopyranoside it showed Km and Vmax values of 2.9 mM and 20.6 U mg-1, respectively whereas it was inhibited competitively by L-rhamnose (Ki 3.5 mM). Substrate specificity studies showed alpha-L-rhamnosidase to be active both on alpha-1,2 and alpha-1,6 linkages to beta-D-glucose. Moreover, the enzyme was able to release L-rhamnose from geranyl-beta-D-rutinoside and 2-phenylethyl-beta-D-rutinoside.