Two extracellular β-xylosidases, xylosidase I and II, were isolated from the ruminal fungus Neocallimastix patriciarum 27 after growth in a barley straw medium. Xylosidase I was purified 88-fold to apparent homogeneity by ion-exchange, affinity, and gel filtration chromatography. The purified xylosidase I had an isoelectric point (pI) of 4.7 and was a monomelic protein with a molecular mass of 39.5 kDa as estimated by both SDS-PAGE and gel filtration. Xylosidase II was partially purified to approximately 95% purity. Xylosidase II had the same pI (4.7) as xylosidase I, and appeared to be a dimeric enzyme composed of two polypeptides with molecular masses of 85 and 45 kDa, respectively, on SDS-PAGE. Peptide mapping of the three proteins suggested that xylosidase I was a truncated product originating from xylosidase II. Xylosidases I and II had similar pH optima of 6.0, but different temperature optima of 50 and 40 °C, respectively. The Km and Vmax for xylosidase I were 0.59 mM of p-nitrophenyl-β-D-xylopyranoside and 38.04 U∙mg protein−1, respectively, and those for xylosidase II were 0.13 mM and 8.9 U∙mg protein−1, respectively. Both enzymes hydrolysed pNPX and xylobiose with the production of xylose, but only xylosidase I exhibited activity toward p-nitrophenyl-α-L-arabinofuranoside.Key words: xylosidase, Neocallimastix, patriciarum, glycosidase.
Solubilization of lignin by the ruminal anaerobic fungus Neocallimastix patriciarum.
