Bioconverion of Glutamic Acid to Gamma-Aminobutyric Acid in Soybean by High Pressure with Precursor Feeding

Abstract

Soybean (Glycine max L.) is rich in nutrients and bioactive substances such as γ-aminobutyric acid (GABA), which has physiological functions. In general, soybean is often soaked in water prior to cooking. During water sorption into soybean matrix, component in soybean could be changed by internal enzymes responsible for proteolysis, amino acid metabolisms, and other biological synthetic reactions. In this study, we focused on the enzymatic reaction of glutamic acid (Glu) to GABA catalyzed by glutamate decarboxylase (GAD). We applied high pressure (HP) on water-soaked and Glu (0.01, 0.05 g/ml)-soaked soybean cotyledon as a cellular biological material, and investigate the effects of HP on bioconversion of Glu to GABA from the viewpoints of the cell structure and enzymatic reaction system. Damage to cell structure was evaluated by measuring dielectric properties using the Cole-Cole arc, the radius of which decreased as pressure level increased. Results suggested that cell structure was damaged by HP. After preservation, concentrations of most amino acids increased in pressurized soybeans after soaking in water compared with unpressurized ones. HP caused a slight increase in GABA concentration in water-soaked samples, but would be predicted to cause a greater increase in Glu-soaked samples, since Glu is a precursor of GABA. The highest GABA concentration was 4.20μmol/g (12.35μmol/g dry basis, d.b.) in Glu-soaked (0.05 g/ml) pressurized soybean at 2-d, while that in water-soaked at 0-d was 0.90μmol/g. GABA concentration eventually increased 5-fold during preservation. We concluded that HP with precursor feeding could lead to high accumulation of target compound. These events can be considered “high-pressure induced transformation (Hi-Pit)” of soybean.