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. B atz, R. Scheibe, H.E. Neuhaus [1995] Planta 196: 50-57). Here we demon strate that these chloroplasts transport this hexose phosphate in coun ter-exchange for orthophosphate. By measuring the release of (CO2)-C-1 4 from [1-C-14]Glc-6-P, we show that isolated fruit chloroplasts also use exogenous Glc-6-P as a substrate for the oxidative pentose-phospha te pathway. The rate of decarboxylation appears to be linear with time and is significantly reduced in the presence of Triton X-100, indicat ing 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-drive n oxidative pentose-phosphate pathway activity are achieved in the pre sence of nitrite, 2-oxoglutarate, and glutamine, indicating a strong i nteraction between nitrogen metabolism and this pathway. In addition, we show that carbohydrates liberated during starch mobilization are us ed as substrates for the oxidative pentose-phosphate pathway. Orthopho sphate was found to act as an activator for the observed (CO2)-C-14 re lease from carbohydrates formerly bound as starch. In this context, we demonstrate that exogenous Glc-6-P competes with endogenous carbohydr ates. A possible interaction between exogenous and endogenous carbohyd rates is discussed with respect to altered levels of carbohydrates dur ing fruit development.