Enzymes of Sugar and Acid Metabolism in Stored ‘Valencia’ Oranges

The activities of key enzymes of sugar and acid metabolism were measured in desalted protein extracts from ‘Valencia’ oranges [ Citrus sinensis (L.) Osb.] at 0, 3, 6, and 9 weeks of storage at 15C to investigate the relationship between the activities of gluconeogenic and glycolitic enzymes with postharvest changes in sugars and organic acids. Fructose-1,6-bisphosphatase (regulatory enzyme of the gluconeogenic pathway) activity was present throughout storage. Activities of the enzymes of acid metabolism (malic enzyme, isocitrate dehydrogenase/aconitase, and alcohol dehydrogenase) either increased during the first 3 weeks (malic enzyme) or remained constant during storage. Activity of enzymes involved in sugar catabolism (hexokinase, sucrose synthase, UDPG pyrophosphorylase, and PPi-dependent phosphofructokinase) increased during storage. These enzymes are necessary for organic acid use and for the subsequent oxidization of sugars in harvested sweet oranges.