Fructose content and fructose-related enzyme activity during the fruit development of apple and Japanese pear

Fructose content and fructose-related enzyme activity based on fresh weight were measured during fruit development to characterize the metabolic properties responsible for fructose accumulation in apple (Malus domestica Borkh.) and Japanese pear (Pyrus pyrifolia Nakai). When fructose content gradually increased from 66 to 92 days after flowering (DAF) in Japanese pear fruit, NAD-dependent sorbitol dehydrogenase (SDH) and fructokinase (FK) activities decreased simultaneously. Subsequently, while fructose content rapidly increased from 92 to 126 DAF, the decrease in FK activity continued, whereas SDH activity increased. In apple fruit, FK activity throughout fruit development was lower than 10% of the maximum activity in Japanese pear fruit. Therefore, It seems that the low FK activity contributes to fructose accumulation while SDH activity increased in apple and Japanese pear fruits. Fructose-6-phosphatase (F6Pase) activity was much lower than FK activity during the fruit development of apple and Japanese pear, which indicates that FK plays a more important role than does F6Pase as a determinant of fructose level in conversion between fructose and fructose-6-phosphate. Phosphoglucose isomerase (PGI) activity was similar or higher compared with FK activity during the fruit development of apple and Japanese pear, suggesting that the isomerization by PGI is not a rate-limiting step from fructose to glucose-6-phosphate. Thus, it is not likely that PGI is responsible for the regulation of fructose levels.