RECYCLING OF CARBON IN THE OXIDATIVE PENTOSE-PHOSPHATE PATHWAY IN NONPHOTOSYNTHETIC PLASTIDS

Recycling of carbon in the oxidative pentose phosphate pathway (OPPP) of intact pea root plastids has been studied. The synthesis of dihydro xyacetone phosphate (DHAP) and evolution of CO2 was followed in relati on to nitrite reduction. A close coupling was observed between all thr ee measured fluxes which were linear for up to 60 min and dependent up on the integrity of the plastids. However, the quantitative relationsh ip between 1-(CO2)-C-14 evolution from glucose 6-phosphate and nitrite reduction varied with available hexose phosphate concentration. When 10 mM glucose 6-phosphate was supplied to intact plastids a stoichiome try of 1.35 was observed between (CO2)-C-14 evolution and nitrite redu ction. As exogenous glucose 6-phosphate was decreased this value fell, becoming 0.47 in the presence of 0.2 mM glucose 6-phosphate, indicati ve of considerable recycling of carbon. This conclusion was reinforced when using [2-C-14]glucose-6-phosphate. The measured release of 2-(CO 2)-C-14 was consistent with the data for 1-(CO2)-C-14, suggesting comp lete recycling of carbon in the OPPP. Ribose 5-phosphate was also able to support nitrite reduction and DHAP production. A stoichiometry of 2 NO2- reduced: 1 DHAP synthesised was observed at concentrations of 1 mM ribose 5-phosphate or less. At concentrations of ribose 5-phosphat e greater than 1 mM this stoichiometry was lost as a result of enhance d DHAP synthesis without further increase in nitrite reduction. It is suggested that this decoupling from nitrite reduction is a function of excess substrate entering directly into the non-oxidative reactions o f the OPPP, and may be useful when the demand for OPPP products is not linked to the demand for reductant. The significance of recycling in the OPPP is discussed in relation to the coordination of nitrate assim ilation with carbohydrate oxidation in roots and with the utilisation of carbohydrate by other pathways within plastids.