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Two arabinofuranosidases, termed Ara I and Ara II, from solid-state cultures of Penicillium capsulatum were purified to apparent homogeneity as judged by electrophoresis and isoelectric focusing. Each enzyme is a single subunit glycoprotein, and they have M(r)s and pIs of 64,500 and 4.15 (Ara I) and 62,700 and 4.54 (Ara II), respectively. Ara I is most active at pH 4.0 and 60 degrees C, while Ara II exhibits optimal activity at pH 4.0 and 55 degrees C. Ara I is the more thermostable, with its half-life at 70 degrees C and pH 4.0 being 17.5 min. By contrast, the half-life of Ara II is only 9 min at 60 degrees C and pH 4.0. Ara I has the lower Km and higher catalytic constant values with p-nitrophenyl-alpha-L-arabinofuranoside being used as the substrate. Arabinose, a competitive inhibitor (Ki, 16.4 mM) of Ara II, has no effect on Ara I activity at concentrations of up to 40 mM. Each enzyme catalyzes the release of arabinose from pectin, araban, and certain arabinose-containing xylans. The last activity is enhanced by pretreatment of the relevant substrates with xylanase, ferulic acid esterase, or combinations of these enzymes. Thus, arabinoxylooligosaccharides in which arabinose is the sole side chain substituent appear to be the preferred substrates. On the basis of the evidence cited above, each enzyme has been classified as an alpha-L-arabinofuranoside arabinofuranohydrolase (EC 184.108.40.206).