For specific activity measurement, 10 μL of the GAD-immobilized resin slurry (consisting of 50% v/v resin and binding buffer) was transferred to 2 mL screw top microtubes containing 1 mL of reaction buffer; 0.1 M GA or MSG, 0.6 mM CaCl2 and 0.15 mM pyridoxal 5'-phosphate pre-incubated at 37 °C and pH 4.0.