Glutamate decarboxylase (GAD) is an important enzyme in biological metabolisms acting on catalyzing the irreversible α-decarboxylation of L-glutamic acid to γ-aminobutyric acid (GABA) and CO2, which was focused in this study. Three rice varieties different in color were germinated at different times and used for crude GAD extraction. Crude GADs with an optimal germination time from germinated black (GBR), red (GRR), and white (GWR) rice were evaluated for enzymatic properties, including the effect of pHs, temperatures, and concentrations of both L-glutamic acid and pyridoxal 5'-phosphate (PLP). Crude GAD with optimum enzymatic properties was selected to be partially purified using ammonium sulfate (AMS) precipitation. The obtained GAD was supplemented to soymilk and determined for GABA content. All crude GADs from germinated rice at 10 germination days presented the highest enzyme activity. For enzymatic properties, crude GADs showed the highest activity at pH in a range of 5.6-6.0 at 60ºC. The Km values of crude GADs were in the range of 7.68-8.06 mM for L-glutamic acid and 0.15-0.20 μM for PLP and were the lowest in crude GAD from GBR. GAD from GBR presented the highest enzyme activity in the fraction with 50% saturation (v/v) after AMS precipitation and it was purified for 14.61 folds. The addition of this GAD (1.0%, v/v) resulted in the increasing of GABA content in soymilk to 53.79 mg/100 mL, accounted for 1.23 times compared with control.
Keywords: enzymatic properties; germinated rice; glutamate decarboxylase; soymilk; γ-aminobutyric acid.