Glutamate Synthase: A Possible Role in Nitrogen Metabolism of the Developing Maize Endosperm

Abstract

Glutamate synthase activity was demonstrated in the developing endosperm of maize (Zea mays L.). The enzyme shows specificity for glutamine and alpha-ketoglutarate as amino donor and acceptor, respectively. Both NADH and NADPH function as electron donors although lower activities were often, but not always, obtained with NADPH. The apparent K(m) values for glutamine, alpha-ketoglutarate, and NADH were 1.35 mm, 0.57 mm, and 7 mum, respectively.The pattern of activity during endosperm development revealed a well defined peak coinciding with the period of most active N accumulation. Activity in general was related to the rates of accumulation throughout development. Maximum glutamate synthase activity was the order of 56 nmoles of glutamate formed per minute per endosperm compared with a rate of N accumulation of 9.5 nmoles per minute.It is suggested that glutamate synthase plays a key role in the N nutrition of the maize endosperm providing a means whereby N transported in the form of glutamine is made available for the synthesis of other seed protein amino acids via transaminase reactions. Transaminase activity involving glutamate, the product of the glutamate synthase reaction, was also demonstrated.

Acetolactate Synthase Activity in Developing Maize (Zea mays L.) Kernels

“…3), similar to the patterns found for other enzymes of amino acid metabolism during endosperm development, including aspartate kinase (5), diaminopimelate decarboxylase (18), glutamine synthetase (4,8,11), glutamate synthase (11,19), threonine deaminase, and dihydrodipicolinic acid synthase (MJ Muhitch, unpublished data). These patterns suggest a coordinated regulation of gene expression for the enzymes of nitrogen assimilation and amino acid biosynthesis during maize endosperm development.…”

Variations in β‐aspartate kinase activity during the development of maize endosperm

“…To meet the greater demand for amino acids, the developing endosperm might be expected to exhibit an increased capacity to synthesize these amino acids from the amide nitrogen and various carbon sources. A sharp increase in glutamate synthase activity was observed [9] in developing maize endosperm from lo-30 days after pollination, followed by a decline in activity to low levels at endosperm maturity. The peak they observed in glutamate synthase activity coincided with the period of most active nitrogen accumulation in the endosperm.…”

“…A sharp increase in maize endosperm dry weight and total protein begins -10 days after pollination and continues for -4 weeks ( fig.1) [9,27]. During this active period of storage protein deposition and protein synthesis the greatest demand for amino acids would presumably exist.…”

Glutamate Synthase Isoforms in Rice

“…In contrast, both NADH and NADPH are efficient electron donors for enzymes in rice etiolated leaf and root tissues at a pyridine nucleotide concentration of0.22 mm. The lack ofpyridine nucleotide specificity has been found for enzymes in cultured cells (5), roots (6,17,19,27), and endosperms (19,26). In these tissues, the ratio of NADPH-to NADH-dependent activities ranges between 0.5 and 1.0.…”