Abstract
Flavobacterium johnsonae was isolated as a microorganism that produced a β-glucosidase with hydrolytic activity of β-glucosyl ester linkages in steviol glycosides. The enzyme was purified to homogeneity from a cell-free extract by streptomycin treatment, ammonium sulfate fractionation, and column chromatographies on S-Sepharose and phenyl-Toyopearl. The molecular mass of the purified enzyme was about 72 kDa by SDS-PAGE. An isoelectric point of pI 8.8 was estimated by isoelectric focusing. The enzyme was most active at pH 7.0, and was stable between pH 3.0 and 9.0. The optimum temperature was 45°C, and the enzyme was stable below 35°C. The enzyme hydrolyzed glucosyl ester linkages at site 19 of rebaudioside A, stevioside, and rubusoside, although it could not degrad β-glucosidic linkages at site 13 of rebaudioside B or steviol bioside. The enzyme acted on aryl β-glucosides such as p-nitrophenyl β-glucoside, phenyl β-glucoside, and salicin, and glucobioses such as sophorose and laminaribiose. The enzyme activity on Rub was inactivated completely by Hg2+, and reduced by Fe3+, Cu2+, p-chloromercuric benzoate, and phenylmethylsulfonyl fluoride (residual activity; 67.9-84.8%). The pNPG hydrolysis was also inactivated to almost the same degrees. Kinetic behaviors in the mixed substrate reactions of rebaudioside A and steviol monoside, and of steviol monoglucosyl ester and phenyl β-glucoside suggested the glucosidic and glucosyl ester linkages were hydrolyzed at a single active site of the enzyme.
