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Glutamate Synthase: A Possible Role in Nitrogen Metabolism of the Developing Maize Endosperm ¹

^a Departments of Plant Physiology and Genetics, Instituto de Biologia, Universidade Estadual de Campinas, 13100 Campinas, SP. Brazil

Glutamate synthase activity was demonstrated in the developing endosperm of maize (Zea mays L.). The enzyme shows specificity for glutamine and -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, -ketoglutarate, and NADH were 1.35 mM, 0.57 mM, and 7 µM, 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.

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