Recycling of carbon in the oxidative pentose phosphate pathway in non-photosynthetic plastids

Abstract

Recycling of carbon in the oxidative pentose phosphate pathway (OPPP) of intact pea root plastids has been studied. The synthesis of dihydroxyacetone phosphate (DHAP) and evolution of CO₂ was followed in relation to nitrite reduction. A close coupling was observed between all three measured fluxes which were linear for up to 60 min and dependent upon the integrity of the plastids. However, the quantitative relationship between 1-¹⁴CO₂ 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 stoichiometry of 1.35 was observed between ¹⁴CO₂ evolution and nitrite reduction. As exogenous glucose 6-phosphate was decreased this value fell, becoming 0.47 in the presence of 0.2 mM glucose 6-phosphate, indicative of considerable recycling of carbon. This conclusion was reinforced when using [2-¹⁴C]glucose-6-phosphate. The measured release of 2-¹⁴CO₂ was consistent with the data for 1-¹⁴CO₂, suggesting complete recycling of carbon in the OPPP. Ribose 5-phosphate was also able to support nitrite reduction and DHAP production. A stoichiometry of 2 NO ⁻₂ reduced: 1 DHAP synthesised was observed at concentrations of 1 mM ribose 5-phosphate or less. At concentrations of ribose 5-phosphate greater than 1 mM this stoichiometry was lost as a result of enhanced 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 of 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 assimilation with carbohydrate oxidation in roots and with the utilisation of carbohydrate by other pathways within plastids.