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

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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 Km values for glutamine, .alpha.-ketoglutatate and NADH were 1.35 mM, 0.57 mM and 7 .mu.M, 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 nmol of glutamate formed/min per endosperm compared with a rate of N accumulation of 9.5 nmol/min. Apparently 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.