Amino acid metabolism in developing soybeans (Glycine max): glutamate synthase in the cotyledons

Abstract

Extracts of developing soybean (Glycine max (L.) Merr.) cotyledons contained glutamate synthase activity (EC 2.6.1.53). The enzyme apparently was soluble and could be separated from glutamate dehydrogenase and NADH oxidase. Optimal synthase activity was at pH 7.8 in phosphate buffer. The enzyme was specific for L-glutamine as nitrogen donor and α-ketoglutarate as nitrogen acceptor. Activity was much greater with NADH than NADPH as reductant and evidence was obtained which indicated that the latter activity was an artifact in the in vitro assay owing to the presence of pyridine nucleotide phosphatase activity. Synthase activity was not reversible but was influenced by the addition of NAD+ to the reaction mixture. The synthesis of [14C] glutamate by crude extracts was also detected with ferredoxin as electron donor. The role of glutamate synthase in providing α-amino N for amino acid synthesis in the developing legume seed was further clarified.Glutamate dehydrogenase (EC 1.4.1.3) activity was also measured in the cotyledons and its possible role in NH4+ metabolism was discussed.