ISSN:
1432-2048
Keywords:
Carbon recycling
;
Nitrite reduction
;
Oxidative pentose phosphate pathway
;
Pisum
;
Plastids
;
Roots
Source:
Springer Online Journal Archives 1860-2000
Topics:
Biology
Notes:
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.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00196656
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