The acid metabolism of Annona fruit during ripening

Annona fruit ripening is associated with a high rate of reassimilation of respired CO2, and a large increase in titrable acidity mainly due to malic acid accumulation. However, little is known about the mechanisms involved in acidity changes. In this work, the kinetic properties and activities of phosphoenolpyruvate carboxylase, malate dehydrogenase and malic enzyme, implicated in the regulation of malate metabolism and by extension in cytoplasmic pH, have been analysed. Kinetic studies revealed that the Km values of NADP-malic enzyme for malate were higher than those of malate dehydrogenase for oxaloacetate (1.44 +/- 0.03 mM v 0.78 +/- 0.02 mM). These results together with the higher V-max value of malate dehydrogenase (more than 26-fold) indicated more efficiency in the reaction of malate synthesis v decarboxylation. The in vitro properties of the enzymes and the fact that the activity of malate dehydrogenase is much higher than the activity of NADP-malic enzyme account for the particular high titratable acidity in cherimoyas during ripening. Our results further indicate that the balance between malic acid synthesis and consumption seems to be effectively regulated and consistent with a controlled cytoplasmic pH value, measured by (31)Phosphorus nuclear magnetic resonance spectroscopy.