Regulation by intracellular and intercellular transport
Separation of GABA synthesis from GABA catabolism by subcellular
compartmentation is another potential mechanism for
regulating GABA levels (Fig. 2). A recent study investigated the subcellular
localization of GABA shunt enzymes in protoplasts prepared
from developing soybean cotyledons23. Protoplast lysate was
fractionated by differential and continuous percoll-gradient centrifugation
to separate the organelle fractions. GAD is located exclusively
in the cytosol, whereas GABA-T and SSADH are associated
exclusively with the mitochondrial fractions. However, these results
might be complicated by the marked instability of a-ketoglutaratedependent
GABA-T activity21. Mitochondrial fractions also catabolize
[U-14C]GABA to labeled succinate23. These results provide
convincing evidence for the transport of GABA from the cytosol
across the mitochondrial membranes into the matrix. An explanation
for the presence of both pyruvate- dependent and a-ketoglutaratedependent
GABA-T activities in mitochondria21 is not obvious