Enzymatic Characteristics of Pumpkin Glutamate Decarboxylase and Accumulation Conditions of γ-Aminobutyric Acid

Several enzymatic characteristics of pumpkin glutamate decarboxylase(GAD)including optimal reaction temperature,optimal reaction pH,heat stability,freezing stability were investigated. γ-Aminobutyric acid(GABA)accumulation was carried out utilizing glutamate decarboxylase in fresh pumpkin(Cucurbita moschata)and the effects of conditions(reaction time,variety of pumpkin,buffer system,pumpkin and MSG concentrations,the ratio of material to water)on GABA accumulation were investigated. The results showed that,the optimal temperature and pH of pumpkin GAD were 30~35 ℃ and pH5.8 respectively. Pumpkin GAD was sensitive to heat,after incubation for 30 minutes at 50 ℃,20% loss of GAD activity,while incubation for 30 minutes at a temperature higher than 70 ℃,totally inactivation of GAD activity occurred. Freezing had little effect on GAD activity,while long-term freezing could still cause a loss of GAD activity,for example,36% GAD activity lost after eight weeks freezing. GABA concentration and conversion rate were 7633.2 mg/L and 93.3% respectively under the optimum conditions of pumpkin:disodium hydrogen phosphate-citric acid buffer(pH5.8,0.2 mol/L)ratio 1:3,pumpkin concentration 25%(w/w),MSG concentration 1.5%(w/w),temperature 30 ℃ and reaction time 18 h. Besides,under the optimum conditions,GABA yield reached 30.5 mg/g,132 times higher than before.