Purification and biochemical characterisation of a novel glutamate decarboxylase from rice bran

Abstract

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. 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. 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.