The reaction mixture contained 250 lL 100 mM sodium citrate (pH 6.0) and 200 lL 4% sucrose solution. Sucrose isomerization was initiated by adding 50 lL enzyme, and the reaction was allowed to run for 10 min. The reaction was terminated by adding 500 lL DNS solution and inactivated by boiling the reaction mixture. The absorbance of reducing sugar was measured to be 575 nm using a spectrophotometer (DU730, Beckman, Fullerton, CA, USA). One unit of SIase activity was defined as the amount of enzyme that released 1 lmol sucrose isomers during 1 min of the reaction.