In this paper, a colorimetric method with high sensitivity, reproducibility and low cost for determining glutamate decarboxylase (GAD) activity was developed based on Berthelot reaction. The optimum substrate system is 0.2 mol/L MacIlvaine buffer pH4.7, containing 0.1 mmol/L pyridoxal phosphate (PLP) and 10 mmol/L L-monosodium glutamate (L-MSG). The reaction mixture consisted of 200 μL substrate solution and 1~100 μL enzyme preparation from lactic acid bacteria and was incubated at 30℃. The enzymatic reacti...