Properties of glutamate dehydrogenase from developing maize endosperm

Glutamate dehydrogenase (EC 1.4.1.3) activity was assayed in homogenates of maize ( Zea mays L. inbred lines Oh43 and Oh43o₂) endosperm during development. During the period 20–35 days after pollination anabolic (aminative) activities were higher than catabolic (deaminating) ones. In order to study the regulation of GDH activity, glutamine or glutamate were injected into the ear peduncle before sample harvesting. The amination and deamination reactions showed similar behaviour with different nitrogen sources: glutamine increased, whereas glutamate decreased, both aminative and deaminative reactions. Partially purified enzyme was active with NADH and NADPH in a ratio 9:1. In Tris-HCl buffer a broad optimum at pH 7.6–8.9 and pH 6.8–8.9 was observed with NADH and NADPH, respectively, NADH activity was activated by Ca²⁺. Saturation curves for (NH₄)₂SO₄ and NADH showed normal Michaelis-Menten kinetics in the presence of 1 m M Ca²⁺, but substrate inhibition occurred without Ca²⁺. The enzyme was inactivated by EDTA. The effect of EDTA was reversed by Ca²⁺ and Mn²⁺, but not by Cu₂₊ and Mg²⁺.