Changes in enzymes involved in photosynthesis and other metabolic processes in the fruit of Opuntia ficus-indica during growth and ripening

The aim of this study was to investigate changes in the abundance of a number of enzymes in the peel, core and seeds of fruits of Opuntia ficus-indica (L.) Miller during development. The enzymes studied were phosphoenolpyruvate carboxylase (PEPC; EC: 4.1.1.31), ribulose-1,5-bisphosphate carboxylase/oxygenase (RUBISCO; EC: 4.1.1.39), aldolase (EC: 4.1.2.13), pyruvate, orthophosphate dikinase (PPDK; EC: 2.7.9.1), phosphoenolpyruvate carboxykinase (PEPCK; EC: 4.1.1.49) and aspartate aminotransferase (AspAT; EC: 2.6.1.1). To detect these enzymes, antibodies specific for each enzyme were used to probe Western blots of sodium dodecyl sulphate polyacrylamide gels. Fruit weight increased throughout development, and during ripening there was both an accumulation of total soluble solids and a decrease in both total titratable acidity and chlorophyll content. In the early stages of its growth the polypeptide pattern and enzyme composition of the fruit chlorenchyma was similar to that of the cladode. In the peel of the fruit PEPC, RUBISCO, plastidic aldolase and PPDK decreased to undetectable amounts as ripening progressed. PEPCK was not detected in either the chlorenchyma of the fruit or cladode. This showed that O. ficus-indica is not a PEPCK type CAM plant and that in this fruit PEPCK is not involved in the metabolism of organic acids. Cytosolic aldolase and AspAT were present in both the peel and core throughout development. The seeds accumulated storage proteins before the fruit ripened, and the abundance of all the enzymes studied declined once the accumulation of storage proteins was complete.