Abstract: A colorimetric method of determining gama-aminobutyric acid (GABA) in glutamate decarboxylase (GAD) activity assay with high sensitivity, accuracy and reproducibility is proposed, and the standard procedure leading to the establishment of the method is described. The reaction mixture (0.4ml) of GAD and L-Glu is mixed with 1mol/L Na2CO3 (0.1ml), 0.2mol/L borate buffer (0.5ml) and 6% phenol (1.0ml), then added 1ml of 5.2% sodium hypochlorite solution at 20℃ within 5min. After being kept at 20℃ for 4～8min, the mixture is heated by boiling water bath for 10min and promptly cooled in ice bath for 20min, then added 2ml of 60% ethanol, and kept at 20℃ for 20～40min.The optical density of the mixture is measured by the absorbance at 640nm. The accuracy and sensitivity of the determination method are also discussed, indicating that the relative error is 4. 91% in comparison with HPLC method.

Key words: GAD, GABA, colorimetry, enzymatic activity assay