Intact preparations of plastids from pea (Pisum sativumL.) roots have been used to investigate the metabolism of glucose-6-phosphate and reduction of inorganic nitrite within these organelles. The ability of hexose-phosphates to support nitrite reduction was dependent on the integrity of the preparation and was barely measurable in broken organelles. In intact plastids, nitrite was reduced most effectively in the presence of glucose-6-phosphate (Glc6P), fructose-6-phosphate and ribose-5-phosphate and to a lesser extent glucose-1-phosphate. The Km(Glc6P) of plastid-located Glc6P dehydrogenase (EC 1.1.1.49) and Glc6P-dependent nitrite reduction were virtually identical (0.68 and 0.66 mM respectively) and a similar relationship was observed between fructose-6-phosphate, hexose-phosphate isomerase (EC 5.3.1.9) and nitrite reduction. The pattern of release of CO2from different carbon atoms of Glc6P supplied to root plastids, indicates the operation of both glycolysis and the oxidative pentose-phosphate pathway with some recycling in the latter. During nitrite reduction the evolution of CO2from carbon atom 1 of Glc6P was stimulated but not from carbon atoms 2, 3, 4, or 6. The importance of these results with regard to the regulation of the pathways of carbohydrate oxidation and nitrogen assimilation within root plastids is discussed.
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