PURIFICATION OF CHLOROPLASTS FROM FRUITS OF GREEN-PEPPER (CAPSICUM-ANNUUM L) AND CHARACTERIZATION OF STARCH SYNTHESIS - EVIDENCE FOR A FUNCTIONAL CHLOROPLASTIC HEXOSE-PHOSPHATE TRANSLOCATOR

Abstract

We have developed a method for the purification of chloroplasts from green-pepper fruits. These chloroplasts are characterized by a high degree of intactness and low contamination with other cellular components. The purified chloroplasts perform CO2 fixation and posses a fructose-1,6-bisphosphate phosphatase, necessary for the conversion of CO2 to starch. Besides carbon dioxide, these chloroplasts take up external carbon skeletons as precursors for starch synthesis. From various potential precursors tested, glucose-ti-phosphate (Glc6P) is used with the highest efficiency for starch synthesis. The Glc6P-dependent starch synthesis is strongly enhanced in the presence of light, ATP, and phosphoglyceric acid. Adenosine 5'-monophosphate acts as an inhibitor of ATP-stimulated Glc6P-dependent starch synthesis. The ability to use Glc6P as a precursor for starch synthesis indicates the presence of a functional hexose-phosphate translocator in isolated chloroplasts from green-pepper fruits. The results are discussed with respect to the physiological function of chloroplasts in fruit tissues. We predict that chloroplasts from green-pepper fruits possess a chloroplastic hexose-phosphate translocator which enables these plastids to maintain starch synthesis from cytosolic precursors during both day and night.