The metabolism and functions of gamma-aminobutyric acid
The metabolism and functions of gamma-aminobutyric acid
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DOI：
10.1104/pp.115.1.1
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发表时间：
1997-09-01
期刊：
影响因子：
7.4
通讯作者：
Shelp, BJ
中科院分区：
文献类型：
--
作者：
Bown, AW;Shelp, BJ
GABA是一种四碳非蛋白质氨基酸，作为游离氨基酸库的重要组成部分，几乎存在于所有原核和真核生物中。在高等植物中，GABA主要通过L-谷氨酸（L-Glu）在LG-11脱羧酶（EC 4.1）催化的反应（L-Glu+ H+-+ GABA+ CO）中消耗H+的α-脱羧来合成。1.15）。它通过GABA转氨酶（EC 2.6）催化的可逆转氨作用（GABA+丙酮酸=琥珀酸半醛+ Ala）代谢。1.19）。转氨作用的产物琥珀酸半醛在琥珀酸半醛脱氢酶（EC-1.2）催化的不可逆反应中被氧化成琥珀酸（琥珀酸半醛+ NAD+ H，O-+琥珀酸+ NADH）。1.16）。这三个反应构成了一个称为GABA分流的途径（图1）。高等植物中GABA代谢的这些和其他特征以前已经讨论过（Bown和Shelp，1989; Satyanaryan和Nair，1990，以及参考文献）。其中）。有大量文献表明，响应于许多不同的刺激，GABA快速和大量积累。例如，响应于冷休克或机械刺激，大豆（Glycine max）叶中的GABA水平在5分钟内上升20至40倍至1至2pmol g-* 鲜重（Wallace等人，1984年）。LG~ u浓度相应降低。氨基酸浓度的这些变化在60分钟内被逆转。各种组织中的其他报道表明，对机械刺激、机械损伤、冷休克、热休克、缺氧、细胞溶质酸化、水分胁迫和植物激素的反应增加（参考文献，见表I）。没有证据表明快速的、应激诱导的GABA合成涉及GAD的耗时的从头合成，并且增加的合成被归因于GAD活性的快速的、体内刺激（Wallace et al.，1984; Tsushida和Murai，1987）。这些数据提出了两个重要的问题：激活GAD的体内机制是什么，以及GABA积累的作用是什么？最近的报道表明，GAD是通过增加H+或Ca 2+的胞质水平激活的，并且GABA积累在pH调节中是重要的，作为GAD的替代途径。
GABA is a four-C, nonprotein amino acid found in virtually all prokaryotic and eukaryotic organisms as a significant component of the free amino acid pool. In higher plants GABA is synthesized primarily through the H+-consuming a-decarboxylation of L-glutamate (L-Glu) in a reaction (L-Glu+ H+-+ GABA+ CO,) catalyzed by LG~ u decarboxylase (EC 4.1. 1.15). It is metabolized through a reversible transamination (GABA+ pyruvate= succinic semialdehyde+ Ala) catalyzed by GABA transaminase (EC 2.6. 1.19). The product of transamination, succinic semialdehyde, is oxidized to succinate (succinic semialdehyde+ NAD+ H, O-+ succinate+ NADH) in an irreversible reaction catalyzed by succinate semialdehyde dehydrogenase (EC-1.2. 1.16). These three reactions constitute a pathway known as the GABA shunt (Fig. 1). These and other features of GABA metabolism in higher plants have been discussed previously (Bown and Shelp, 1989; Satyanaryan and Nair, 1990, and refs. therein). There is considerable literature demonstrating rapid and large accumulations of GABA in response to many diverse stimuli. For example, in response to cold shock or mechanical stimulation, GABA levels in soybean (Glycine max) leaves rise 20-to 40-fold within 5 min to 1 to 2 pmol g-* fresh weight (Wallace et al., 1984). There is a corresponding decrease in LG~ u concentration. These changes in amino acid concentration are reversed within 60 min. Other reports in various tissues indicate increases in response to mechanical stimulation, mechanical damage, cold shock, heat shock, hypoxia, cytosolic acidification, water stress, and phytohormones (for refs., see Table I). There is no evidence that rapid, stress-induced GABA synthesis involves time-consuming de novo synthesis of GAD, and increased synthesis has been ascribed to a rapid, in vivo stimulation of GAD activity (Wallace et al., 1984; Tsushida and Murai, 1987). The data raise two significant questions: what are the in vivo mechanisms activating GAD, and what are the roles of GABA accumulation? Recent reports indicate that GAD is activated by increases in the cytosolic levels of H+ or Ca2+ and that GABA accumulation is important in pH regulation, as an alternative pathway for