BACKGROUND: γ-Amino butyric acid (GABA) is a major inhibitory neurotransmitter of the mammalian central nervous system that plays a vital role in regulating vital neurological functions. The enzyme responsible for producing GABA is glutamate decarboxylase (GAD), an intracellular enzyme that both food and pharmaceutical industries are currently using as the major catalyst in trial biotransformation process of GABA. We have successfully isolated a novel strain of Aspergillus oryzae NSK that possesses a relatively high GABA biosynthesizing capability compared to other reported GABA-producing fungal strains, indicating the presence of an active GAD. This finding has prompted us to explore an effective method to recover maximum amount of GAD for further studies on the GAD's biochemical and kinetic properties. The extraction techniques examined were enzymatic lysis, chemical permeabilization, and mechanical disruption. Under the GAD activity assay used, one unit of GAD activity is expressed as 1 μmol of GABA produced per min per ml enzyme extract (U/ml) while the specific activity was expressed as U/mg protein.RESULTS: Mechanical disruption by sonication, which yielded 1.99 U/mg of GAD, was by far the most effective cell disintegration method compared with the other extraction procedures examined. In contrast, the second most effective method, freeze grinding followed by 10% v/v toluene permeabilization at 25°C for 120 min, yielded only 1.17 U/mg of GAD, which is 170% lower than the sonication method. Optimized enzymatic lysis with 3 mg/ml Yatalase® at 60°C for 30 min was the least effective. It yielded only 0.70 U/mg of GAD. Extraction using sonication was further optimized using a one-variable-at-a-time approach (OVAT). Results obtained show that the yield of GAD increased 176% from 1.99 U/mg to 3.50 U/mg.
CONCLUSION: Of the techniques used to extract GAD from A. oryzae NSK, sonication was found to be the best. Under optimized conditions, about 176% of GAD was recovered compared to recovery under non optimized conditions. The high production level of GAD in this strain offers an opportunity to conduct further studies on GABA production at a larger scale.