In situ [C-14]glutamate metabolism by developing soybean cotyledons .2. The importance of glutamate decarboxylation

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.