Cell-free extracts of L-arabinose- and D-xylose-grown cells of the mesophilic anaerobic bacterium Bacteroides xylanolyticus X5-1 contained high activities [2 units ((U)/mg] of an alpha-L-arabinofuranosidase (EC 3.2.1.55). The enzyme was also produced during growth on xylan, but not during growth on glucose or cellobiose. The enzyme was mainly extracellularly attached to the cell when the organism was grown on xylan and was not released into the medium. The enzyme was purified 41-fold to apparent homogeneity. The native enzyme had an apparent molecular mass of 364 kDa and was composed of six polypeptide subunits of 61 kDa. The enzyme displayed a pH optimum of 5.5-6.0, and a pH stability of 5.5-9.0. The temperature optimum was 50 degrees C and the enzyme was stable up to 50 degrees C. Thiol groups were essential for activity, but the enzyme activity was not dependent on divalent cations. The Michaelisconstant (K-m) and maximal reaction velocity (V-max for p-nitrophenyl-alpha-L-arabinofuranoside were 0.5 mM and 155 U/mg protein, respectively. The enzyme was specific for the alpha-linked arabinoside in the furanoside configuration. The enzyme displayed activity with ara binose-containing xylo-oligosaccharides with a polymerization degree of 2-5, but not with the polymeric substrates oat-spelt xylan or arabinogalactan. The enzyme belongs to the Streptomyces purpurascens-type of alpha-L-arabinofuranosidase.