PLANT PHYSIOLOGY , Vol 109, Issue 4 1421-1426, Copyright © 1995 by American Society of Plant Biologists
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BIOCHEMISTRY AND ENZYMOLOGY |
Oxidation of Imported or Endogenous Carbohydrates by Isolated Chloroplasts from Green Pepper Fruits
E. Thom and H. E. Neuhaus
Pflanzenphysiologie, FB 5 Biologie/Chemie, Universitat Osnabruck, D-49069 Osnabruck, Germany
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.
