Light-dependent reduction of pyruvate by pea chloroplasts in the presence of glutamate dehydrogenase and C5-dicarboxylic acids

Illuminated pea chloroplasts supported (glutamine plus α-oxoglutarate (α-OG)) and (NH₃ plus α-OG)-dependent O₂ evolution. The properties of these reactions were consistent with light-coupled glutamate synthase and glutamine synthetase activities. In the presence of a glutamate-oxidizing system (component C) comprised of NAD-specific glutamate dehydrogenase (NAD-GDH), lactate dehydrogenase (LDH), 4 mM pyruvate and 0.2 mM NAD, illuminated chloroplasts supported O₂ evolution in the presence of glutamine. The reaction did not proceed in the absence of any one of the constituents of component C and the properties of O₂ evolution were consistent with light-coupled glutamate synthase activity. In the presence of component C, chloroplasts also catalysed O₂ evolution in the presence of catalytic concentrations of glutamate. Studies of O₂ evolution and metabolism of [¹⁴C]-glutamate in the presence of the inhibitors methionine sulphoximine (MSO) and azaserine suggest that O₂ evolution was dependent on the synthesis of glutamine from the products of glutamate oxidation. This was supported by polarographic studies using α-OG and NH₃ instead of glutamate.The results are consistent with a C₅-dicarboxylic acid shuttlemechanism for the export of reducing equivalents from illuminated chloroplasts (glutamate) and recycling of the oxidation products (α-OG and NH₃).