Abstract
Activities and roles of the sorbitol-related enzymes, NAD+-dependent Sorbitol dehydrogenase, sorbitol oxidase, NADP+-dependent sorbitol dehydrogenase and sorbitol-6-P dehydrogenase, and acid invertase were studied in Japanese pear (Pyrus serotina Reder var. culta Rehder) fruit. NAD+-dependent sorbitol dehydrogenase that converts sorbitol to fructose had the highest activity of the 4 enzymes throughout the developing season. The activity rose in June, decreased with fruit expansion, and increased again with fruit maturation. Fluctuation in the enzyme activity correlated with the accumulation of fructose in immature fruit. This enzyme was suggested to be important for sorbitolmetabolism and sugar accumulation in Japanese pear fruit.Sorbitol oxidase activity, which was about one-tenth that of NAD+-dependent sorbitol dehydrogenase, had high activity in immature fruit, decreased with fruit expansion and increased again with fruit maturation. Sorbitol-6-P dehydrogenase and NADP+-dependent sorbitol dehydrogenase activities were detected a little. The differences in sorbitol metabolism among Rosaceae plants were discussed, based on the seasonal fluctuations of these enzyme activities. Acid invertase activity was distinctly higher than that of sorbitol-related enzymes in fruit, and its role in sugar translocation and metabolism was discussed.
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