Abstract:
Addition of L-glutamate caused alkalinization of the
medium surrounding Asparagus spreng.ri mesophyll cells. This
suggests a H+/L-glutmate symport uptake system for L-glutamate.
However stoichiometries of H+/L-glutamate symport into Asparagus
cells were much higher than those in other plant systems. Medium
alkalinization may also result from a metabolic decarboxylation
process. Since L-glutmate is decarboxylated to r-amino butyric
acid (SABA) in this system, the origin of medium alkalinization
was reconsidered.
Suspensions of mechanically isolated and
photosyntheically competent Asparagus sprengeri mesophyll cells
were used to investigate the H+/L-glutamate symport system, SABA
production, GABA transport, and the origin of L-glutamate
dependent medium alkalinization.
The major results obtained are summarized as follows:
1. L-Glutamate and GABA were the second or third most abundant
amino acids in these cells. Cellular concentrations of
L-glutamate were 1.09 mM and 1.31 mM in the light and dark,
respectively. Those of SABA were 1.23 mM and 1.17 mM in the
light and dark, respectively.
2. Asparagine was the most abundant amino acid in xylem sap and
comprised 54 to 68 1. of the amino acid pool on a molar basis.
GABA was the second most abundant amino acid and represented
10 to 11 1. of the amino acid pool. L-Slutamate was a minor component.
3. A 10 minute incubation with 1 mM L-glutamate increased the
production of GABA in the medium by 2,743 7. and 2,241 7. in
the light and dark, respectively.
4. L-Glutamate entered the cells prior to decarboxylation.
5. There was no evidence for a H+/GABA symport process •
6. GABA was produced by loss of carbon-1 of L-glutamate.
7. The specific activity of newly synthesized labeled GABA
suggests that it is not equilibrated with a storage pool of
GABA.
8. The mechanism of GABA efflux appears to be a passive process.
9. The evidence indicates that the origin of L-glutamate
dependent medium alkalinization is a H+/L-glutamate symport
not an extracellular decarboxylation.
The possible role of GABA production in regulating
cytoplasmic pH and L-glutamate levels during rapid electrogenic
H+/L-glutamate symport is discussed.
