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In Vivo and In Vitro Studies on -Aminobutyric Acid Metabolism with the Radish Plant (Raphanus sativus, L.)

^a United States Plant, Soil and Nutrition Laboratory, Soil and Water Conservation Research Division, Agricultural Research Service, United States Department of Agriculture, and Section of Genetics, Development, and Physiology, Division of Biological Sciences, Cornell University, Ithaca, New York 14850

Labeled glutamate was rapidly converted to -aminobutyrate in intact, excised radish (Raphanus sativus L., var. Champion) leaves. Labeled -aminobutyrate was metabolized via succinate and the Krebs cycle and was not carboxylated to form glutamate. Administration of carbon-14 and tritium-labeled succinate indicated that less than 10% of the -aminobutyrate formation occurs by amination of succinic semialdehyde. Therefore, most -aminobutyrate formation must be via glutamate decarboxylation.

Radish leaf extracts were more active in catalyzing transamination between -aminobutyrate and pyruvate than that between -aminobutyrate and -ketoglutarate. Glutamate decarboxylase was approximately 20 times more active than -aminobutyrate: pyruvate transaminase. Succinic semialdehyde dehydrogenase was found in the extracts, and NAD was much more active as a hydrogen acceptor than NADP. No reduction of succinate to succinic semialdehyde by the NAD-linked dehydrogenase could be demonstrated. The following pH optima were determined: glutamate decarboxylase, 5.9; -aminobutyrate: pyruvate transaminase, 8.9; succinic semialdehyde: NAD dehydrogenase, about 9.0.

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