PURIFICATION AND SOME PROPERTIES OF FIVE ENDO-1,4-BETA-D-XYLANASES AND A BETA-D-XYLOSIDASE PRODUCED BY A STRAIN OF ASPERGILLUS-NIGER

Five different xylanases and a beta-D-xylosidase in the culture medium of Aspergillus niger have been purified to homogeneity from 13- to 52-fold by a procedure of gel and hydroxylapatite chromatography. The strain was isolated from soil of the African equatorial forest. Gel chromatography of the purified enzymes indicated that three of the xylanases have molecular weights of 31,000 and the other two xylanases have molecular weights of 50,000. beta-D-Xylosidase has a molecular weight of 78,000. The pH curves of the xylanases were quite diverse and showed pH optima ranging from 4.0 to 6.5. Characteristic action patterns were obtained for each of the purified xylanases by gel chromatography of the xylan digests on Bio-Gel P-2. The enzymes degraded arabinoxylan by an endomechanism, producing L-arabinose, D-xylose, xylobiose, and a mixture of branched arabinose-xylose and D-xylose oligosaccharides. All xylanases seemed to be capable of liberating L-arabinose from either arabinoxylan or the arabinose-xylose oligosaccharides. Branched arabinose-containing D-xylose oligosaccharides were slowly hydrolyzed, so that these sugars accumulate in the digest. Two xylanases showed relatively broad substrate specificity and were able to degrade also crystalline cellulose. beta-D-Xylosidase showed optimal activity at pH 6.7 to 7.0 and at 42 degrees C. The Km for o-nitrophenyl-beta-D-xylopyranoside was 0.22 mM and xylotriose was hydrolyzed more rapidly than xylobiose.