Oxidation of Imported or Endogenous Carbohydrates by Isolated Chloroplasts from Green Pepper Fruits

Recently, we demonstrated that intact chloroplasts isolated from green pepper (Capsicum annum L.) fruits use exogenous glucose-6-phosphate (Glc-6-P) as the most efficient precursor for starch biosynthesis (O. Batz, R. Scheibe, H.E. Neuhaus [1995] Planta 196: 50-57). Here we demonstrate that these chloroplasts transport this hexose phosphate in counter-exchange for orthophosphate. By measuring the release of 14CO2 from [1-14C]Glc-6-P, we show that isolated fruit chloroplasts also use exogenous Glc-6-P as a substrate for the oxidative pentose-phosphate pathway. The rate of decarboxylation appears to be linear with time and is significantly reduced in the presence of Triton X-100, indicating that the reaction is dependent on plastid integrity. Pyruvate has been identified as a positive effector for flux through the oxidative pentose-phosphate pathway. However, the highest rates of Glc-6-P-driven oxidative pentosephosphate pathway activity are achieved in the presence of nitrite, 2-oxoglutarate, and glutamine, indicating a strong interaction between nitrogen metabolism and this pathway. In addition, we show that carbohydrates liberated during starch mobilization are used as substrates for the oxidative pentose-phosphate pathway. Orthophosphate was found to act as an activator for the observed 14CO2 release from carbohydrates formerly bound as starch. In this context, we demonstrate that exogenous Glc-6-P competes with endogenous carbohydrates. A possible interaction between exogenous and endogenous carbohydrates is discussed with respect to altered levels of carbohydrates during fruit development.