Something about γ-aminobutyric acid

Source and application

The content of GABA in plant tissues is extremely low, usually between 0.3 and 32.5 μmol/g. It has been reported in the literature that GABA enrichment in plants is related to the stress response experienced by plants, and GABA will accumulate rapidly when subjected to stresses such as hypoxia, heat shock, cold shock, mechanical damage, and salt stress. After the plant food raw materials are treated with a certain stress method, or through microbial fermentation to increase the GABA content in the body, the processing of this raw material into GABA-rich functional products has become a research hotspot. As a new type of functional factor, GABA has been widely used in the food industry. Foods developed using raw materials such as GABA-rich germinated brown rice, soybeans, and broad beans are already on the market.

Allowed dosage

Although the European Food Safety Agency (EFSA) allows GABA to be added to food and stipulates the upper limit of the dietary intake of GABA is 550mg/d, its main functional properties still need to be supported by the results of strict population trials. The U.S. Food and Drug Administration (FDA) pointed out that it is safe to add GABA to food based on the results of toxicology experiments. The scope of use includes beverages, coffee, tea and chewing gum, but it is not allowed to be added to baby food, meat products or meat-containing products. . The Ministry of Health of China announced No. 12 in 2009 that the intake of GABA should not exceed 500mg/d, and the scope of use is beverages, cocoa products, chocolate and beverages, candies, baked foods and puffed foods, but it cannot be added to baby food.

biological functions

GABA has many discoveries in animals, plants and microorganisms. Among them, it was first discovered in potato tubers in 1949, and in the central system of mammals in 1950. It is also considered to be mammals, insects or some The nerve inhibitors in the nervous system of these parasitic worms have an important influence on the excitability of neurons.

Metabolic pathways in plants

There are two GABA synthesis and transformation pathways in plants: one is glutamic acid decarboxylase catalyzed by glutamic acid decarboxylase (GAD) to synthesize GABA, which is called GABA shunt; the other is GABA shunt; One is the conversion of polyamine degradation products to form GABA, which is called polyamine degradation pathway.

GABA branch

In higher plants, the metabolism of GABA is mainly completed by three enzymes. First, under the action of GAD, L-glutamic acid (Glu) undergoes irreversible decarboxylation at the α-position to produce GABA, and then GABA transaminase ( Catalyzed by GABA transaminase (GABA-T), GABA reacts with pyruvate and α-ketoglutarate to form succinic semialdehyde, which is finally catalyzed by succinic semialdehyde dehydrogenase (SSADH) to produce succinic semialdehyde. Oxidative dehydrogenation forms succinic acid and finally enters the Krebs circle. This metabolic pathway constitutes a branch of the TCA cycle, called the GABA branch.

In plants, GAD in the cytoplasm and GABA-T and SSADH in the mitochondria regulate the GABA branch metabolism, and GAD is the rate-limiting enzyme for the synthesis of GABA. Plant GAD contains a calmodulin (CaM) binding domain. GAD activity is not only regulated by the concentration of Ca2+ and H+, but also by the concentration of the GAD coenzyme-pyridoxal phosphate (PLP) and the substrate glutamate. This dual regulation mechanism connects the cellular accumulation of GABA with the nature and severity of environmental stress. Cold shock, heat shock, osmotic stress and mechanical damage will increase the concentration of Ca2+ in the cell sap. Ca2+ combines with CaM to form a Ca2+/CaM complex, which can stimulate GAD gene expression and increase GAD activity under normal physiological pH conditions; while acid pH The appearance of stimulating GAD is due to stress lowering the pH of the cells and slowing down the acid damage of the cells. The GABA branch in plants is considered to be the main way to synthesize GABA. At present, most researches focus on how to improve GAD activity to achieve GABA enrichment.

Polyamine degradation pathway

Polyamines (PAs) include putrescine (Put), spermine (Spermine, Spm) and spermidine (Spermidine, Spd), among which putrescine is the central substance in the biological metabolism of polyamines. Polyamine degradation pathway means that diamines or polyamines (PAs) are catalyzed by diamine oxidase (DAO) and polyamine oxidase (PAO) to produce 4-aminobutyraldehyde, and then 4-aminobutyraldehyde is catalyzed by diamine oxidase (DAO) and polyamine oxidase (PAO). In the process of 4-amino aldehyde dehydrogenase (AMADH) dehydrogenation to GABA, the polyamine degradation pathway finally merges with the GABA branch and participates in the TCA cycle metabolism. Among them, diamine oxidase and polyamine oxidase are the key enzymes that catalyze the degradation of Put, Spd, and Spm, respectively. During the germination period of broad bean, anaerobic stress can induce the increase of the key enzyme activity of polyamine synthesis and promote the accumulation of polyamines. At the same time, the activity of polyamine oxidase also increases. It promotes the synthesis and accumulation of GABA through the degradation of polyamines. Improved the resistance to adversity of broad beans. Studies have shown that the content of free polyamines in soybean roots increases under salt stress, DAO activity increases, and GABA enrichment increases 11-17 times. Although the polyamine degradation pathway is considered to be another important way to synthesize GABA, its ability to synthesize GABA in monocotyledonous plants is much lower than that of the GABA branch.

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