Background: Glutamate decarboxylase (GAD) is a useful enzyme whose main function is to catalyse the irreversible alpha-decarboxylation of L-glutamate to produce gamma-aminobutyric acid. The cheap and abundant rice-processing by-product rice bran contains a high amount of GAD, the purification and characterisation of which have not yet been reported. In this study, research on rice bran GAD was initiated.
Results: Rice bran GAD was purified to homogeneity via a combined purification protocol of ammonium sulfate fractionation, ion exchange chromatography and two gel filtrations, with a purification fold of 128.6 and an activity recovery of 21.3%. The enzyme was active at pH 5.5 and 40 degrees C and retained 80% of its original activity in the pH range 5-9 and the temperature range 30-50 degrees C. GAD activity was significantly enhanced in the presence of Ca2+ but strongly inhibited by Ag+, Hg2+, sodium dodecyl sulfate and CH3COOH. Kinetic determination of the apparent K(m) for L-glutamate and pyridoxal 5'-phosphate gave values of 27.4 mmol L(-1) and 1.16 micromol L(-1) respectively.
Conclusion: Considering that rice bran is cheap and commercially available and that rice bran GAD is relatively stable, the development of cost-effective rice bran GAD-related functional foods would seem to be feasible.