Oxygen evolution by a reconstituted spinach chloroplast system in the presence ofL-glutamine and 2-oxoglutarate.

Intact chloroplasts prepared from summer-grown spinach plants supported (aspartate plus 2-oxoglutarate)-dependent O2 evolution but not (glutamine plus 2-oxoglutarate)-dependent O2 evolution. The former activity, which was sensitive to amino oxyacetate, was attributed to transaminase activity and reduction of the resulting oxalo-acetate to malate using H2O as eventual electron donor. A reconstituted chloroplast system which included chloroplast stroma, thylakoid membranes, ferredoxin and NADP(H) supported O2 evolution in the presence ofL-glutamine and 2-oxoglutarate at rates of 15-22 μmol mg(-1) chlorophyll h(-1) although lower rates were obtained with material from winter-grown plants. Activity was not observed in the absence of ferredoxin and omission of NADP(H) decreased activity by 40%. The reaction was associated with the production of 0.49 mol O2 mol(-1) 2-oxoglutarate consumed and up to 0.46 mol O2 mol(-1) glutamine supplied. The reaction, which was inhibited by azaserine but not by methionine sulphoximine or amino oxyacetate, was attributed to light-coupled glutamate synthase (EC 1.4.1.13) with H2O serving as eventual electron donor. Activity was not affected significantly byL-malate. The reconstituted system also supported O2 evolution in the presence of nitrite, oxaloacetate, (aspartate plus 2-oxoglutarate) and oxidised glutathione.