β-xylosidases and a nonspecific wall-bound β-glucosidase of the yeast cryptococcus albidus

Cryptococcus albidus grown on wood xylans possesses a soluble intracellular β-xylosidase (EC 3.2.1.37) as an additional constituent of the xylan-degrading enzyme system of this yeast. The enzyme attacks linear 1,4-β-xylooligosaccharides in an exo-fashion, liberating xylose from the non-reducing ends. The activity of the enzyme increases in the cells during growth on xylan and incubation with xylobiose or methyl β-D-xylopyranoside which are the best inducers of extracellular β-xylanase (EC 3.2.1.8). Various alkyl-, alkyl-1-thio- and aryl β-D-xylopyranosides were excellent of a different β-xylosidase of Cryptococcus albidus. This enzyme is localized outside the plasma membrane and is principally associated with cell walls. Unlike the soluble intracellular β-xylosidase, the wall-bound enzyme does not hydrolyze xylooligosaccharides. Evidence has been obtained that β-xylosidase activity in the cell walls is not due to the presence of a specific aryl β-xylosidase, but is exhibited by a nonspecific β-glucosidase (EC 3.2.1.21) inducible by β-D-xylopyranosides. The ratio of β-glucosidase and β-xylosidase activity in the cells and isolated cell walls from yeast induced by various β-xylopyranosides and β-glucopyranosides was very similar. Both wall-bound activities were inhibited in a similar pattern by inhibitors of β-glucosidases, 1,5-gluconolactone and nojirimycin. This bifunctional enzyme does not bear any relationship to the utilization of xylans in Cryptococcus albidus.