Light dependent reduction of nitrate by pea chloroplasts in the presence of nitrate reductase and C4-dicarboxylic acids

In the presence of purified nitrate reductase (NR) and 1 mM NADH, illuminated pea chloroplasts catalysed reduction of NO₃^? to NH₃ with the concomitant evolution of O₂. The rates were slightly less than those for reduction of NO₂^? to NH₃ and O₂, evolution by chloroplasts in the absence of NR and NADH (ca 6 μg atoms N/mg Chl/hr). Illuminated chloroplasts quantitatively reduced 0.2 mM oxaloacetate (OAA) to malate. In the presence of an extrachloroplast malate-oxidizing system comprised of NAD-specific malate dehydrogenase (NAD-MDH), NAD, NR and NO₃^?, illuminated chloroplasts supported OAA-dependent reduction of NO₃^? to NH₃ with the evolution of O₂. The reaction did not proceed in the absence of any of these supplements or in the dark but malate could replace OAA. The results are consistent with the reduction of NO₃^?by reducing equivalents from H₂O involving a malate/OAA shuttle. The ratios for O₂, evolved: C₄-acid supplied and N reduced: C₄-acid supplied in certain experiments imply recycling of the C₄-acids.