Developing cotyledons of soybean (Glycine max (L.) Merrill) metabolize glutamate in situ via multiple routes including direct decarboxylation, deamination, and transmination reactions. The production of [C-14]4-aminobutyrate (GABA) from [U-C-14]glutamate, but not from [1-C-14]glutamate, confirmed that alpha-decarboxylation is responsible for the production of GABA. [U-C-14]GABA was rapidly metabolized to [C-14]succinate and [C-14]malate, consistent with the entry of glutamate carbon into the Krebs cycle via GABA and succinic semialdehyde. Aminooxyacetate, at a concentration of 100 mM, reduced the in situ metabolism of [U-C-14]glutamate and almost fully inhibited [U-C-14]GABA synthesis. These data suggested that the GABA shunt (glutamate --> GABA --> succinic semialdehyde --> succinate) may be important in the glutamate metabolism of a developing soybean seed. Although the GABA shunt bypasses the 2-oxoglutarate dehydrogenase reaction of the Krebs cycle, metabolism of [1-C-14]acetate or [U-C-14]2-oxoglutarate indicated that this reaction is not restricted under our experimental conditions. The in situ glutamate flux through glutamate decarboxylase, as estimated from the changing specific activity of [C-14] GABA during the early metabolism of [U-C-14] glutamate, indicated that glutamate flux through the GABA shunt is comparable to direct incorporation of glutamate into protein.
