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.