Characterization of Dextran Glucosidase (1,6-a-D-Glucan Glucohydrolase) of Streptococcus mitis

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Abstract
Dextran glucosidase (EC 3.2.1.70) isolated from cell extracts of Streptococcus mitis ATCC 903 was purified 925-fold by DEAE-Sephadex, Sephadex G-100 and hydroxylapatite chromatography. The enzyme showed normal Michaelis-Menten kinetics when tested with p-nitrophenyl-α-D-glucopyranoside (pNPG), dextran (MW 9,400) or isomaltose as substrates. The apparent Km values were 1.60, 4.95 and 15.7 mM, respectively. The molecular weight of the enzyme was estimated to 54,000 and the pi was 4.45. Among the metal ions tested (1 mM solutions), Hg2+ and Zn2+ inhibited the enzyme activity completely. In spite of an inhibiting effect of EDTA, no cationic metallic cofactor was found. Glucose exerted competitive inhibition of enzyme activity (KI = 2.5 mM). Optimal enzyme activity was found at pH 7.2 in the temperature range of 37–40 °C and at a low salt concentration (I = 0.06).