References

172 A. Fait, A. Yellin, H. Fromm
Fig.12.1. Overview of the γ-aminobutyric acid (GABA) shunt and relevant metabolic
branches in plants and other organisms. GABA is metabolized via a short pathway known
as the GABA shunt, which bypasses two steps of the tricarboxylic acid cycle (TCA), and
is composed of three enzymes: the cytosolic-localized Ca 2+ -calmodulin (CaM)-regulated
glutamate decarboxylase (GAD), and the mitochondrial-localized GABA trans-aminase
(GABA-T) and succinic semialdehyde dehydrogenase (SSADH). The SSADH substrate
succinic semialdehyde (SSA) isalsocatabolizedtoγ-hydroxybutyrate (GHB) byGHBdehydrogenase
(GHBDH), which is cytosolic in mammals. GHB catabolism occurs in mammals
either in the reverse direction by GHBDH/semialdehyde reductase (see text) or by
the mitochondrial-localized L-glucuronate reductase (GR; EC 1.1.1.19). In plants, GHB
catabolism has not been characterized. Chemical structures are given for GABA-shunt
substrates, intermediates, and products. Unknown functions and transport systems are
depicted as dotted arrows and question marks. Glu glutamate, α-KG α-ketoglutarate, GABA-
TK/P α-ketoglutaric acid/pyruvate-dependent GABA-T
in plants. We also address the occurrence, in plants, of γ-hydroxybutyrate
(GHB), a by-product of the GABA shunt (Fig. 12.1) and a neurotransmitter,
which was only recently discovered in plants (Allan et al. 2003; Breitkreuz et
al. 2003; Fait et al. 2005). For further details on the different roles of GABA
in plants the readers are referred to two recent reviews (Bouché et al. 2003;
Bouché and Fromm 2004) and to earlier reviews by Shelp et al. (1999) and
Kinnersley and Turano (2000).