PURIFICATION AND CHARACTERIZATION OF AN ACETYL XYLAN ESTERASE FROM ASPERGILLUS-NIGER

An acetyl esterase has been purified from an Aspergillus niger culture filtrate based on its activity towards p-nitrophenyl acetate. Its molecular weight determined by SDS-PAGE, was 30 480; its iso-electric point was around 3.0-3.2. When tested on steamed birchwood xylan, acetylated apple or sugar beet pectin, only in the case of steamed birchwood xylan, acetic acid could be released by the acetyl esterase. The specific activity of acetyl esterase on steamed birchwood xylan was 32.3 U mg-1, the optimum pH and optimum temperature were 5.5-6.0 and 50-degrees-C, respectively. The pH stability was very high in the range 3.0-8.0. The activity decreased strongly within 2 h between 50-degrees-C and 55-degrees-C. Three types of endo-xylanase and one beta-xylosidase could not degrade steamed birchwood xylan. In combination with acetyl esterase, however, they released xylose and xylooligomers like X2, X3 and X4. These oligomers were not released until most of the acetyl groups had been removed by the acetyl esterase. Prolonged incubation with acetyl esterase released most of the acetyl groups from the steamed birchwood xylan.