PURIFICATION AND CHARACTERIZATION OF A NOVEL BETA-GLUCOSIDASE FROM CLAVIBACTER MICHIGANENSE THAT HYDROLYZES GLUCOSYL ESTER LINKAGE IN STEVIOL GLYCOSIDES

Clavibacter michiganense was identified as a microorganism that hydrol yzed the glucosyl ester linkages at site 19 of steviol glycosides. An enzyme that catalyzes the hydrolysis was purified from the cell-free e xtract using streptomycin treatment, ammonium sulfate fractionation, Q Sepharose anion exchange chromatography, Sephacryl S-100 gel filtrati on, and Ether Toyopearl hydrophobic chromatography. The purified enzym e migrated as a single protein band in polyacrylamide gel electrophore sis in the presence and absence of sodium dodecyl sulfate, and isoelec tric focusing. The molecular mass was estimated to be approximately 65 kDa, both by gel filtration and sodium dodecyl sulfate/polyacrylamide gel electrophoresis. An isoelectric point, pi, of 4.6, was obtained u sing isoelectric focusing. The enzyme was most active at around pH 7.5 and at 45 degrees C, and was stable between pH 6-10 and below 40 degr ees C. Both Hg2+ and p-chloromercuric benzoate inhibited activity. The enzyme hydrolyzed glucosyl ester linkages at site 19 of rebaudioside A, stevioside, rubusoside, and steviol monoglucosyl ester, although it did not cleave 13-O-linked glucosyl residue of rubusoside and steviol monoside. A transglucosylation product having a cellobiosyl residue a t site 19 was formed when rubusoside was used as a glucosyl donor and acceptor. The enzyme hydrolyzed glucosidic linkages in p-nitrophenyl b eta-glucoside faster than glucosyl ester linkages in the steviol glyco sides. It also acted on phenyl beta-glucoside and salicin, and faintly on sophorobiose and cellobiose. These results indicate that the enzym e is a novel beta-glucosidase that hydrolyzes ester linkages.