D-(-)-Fructose → Psicose

The activity of the ketose 3-epimerase enzyme before purification may be measured by measuring the amount of D-psicose produced with a D-fructose substrate. Specifically, an enzyme reaction mixture composition containing 50 mM tris-HCl buffer (pH 7.0; 200 ml), 0.2 M D-fructose (375 μl), enzyme liquid (100 μl), and 10 mM manganese chloride (75 μl) is allowed to react at 55° C. for 30 min, and the reaction is quenched by boiling the mixture for 2 min. The resulting liquid composition is then measured by HPLC. One unit of enzyme activity is defined as the amount of enzyme that epimerizes D-fructose and produces 1 μmol of D-psicose per minute under the foregoing conditions. The enzyme unit for the epimerization of D-psicose to D-fructose in the reverse reaction is defined in the same manner through measurement performed under the same conditions.