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Plant and Cell Physiology, 1982, Vol. 23, No. 5 891-899
© 1982

Article

Localization of Sorbitol Oxidase in Vacuoles and Other Subcellular Organelles in Apple Cotyledons1

Shohei Yamaki

Fruit Tree Research Station, Ministry of Agriculture, Forestry and Fisheries Yatabe, Ibaraki 305, Japan

To investigate the function and subcellular localization of sorbitol oxidase, free cells, protoplasts and isolated vacuoles of apple cotyledons (Malus pumila Mill. var. domestica Schneid.) were examined by differential and sucrose density gradient centrifugation. Twenty percent of the activity of sorbitol oxidase in the whole tissue was contained in the subcellular fraction (d=1.06) which corresponded closely to the main peaks of activity and protein after the recentrifugation of the 150,000×g pellet of ruptured vacuoles with a linear sucrose density gradient. The enzyme thus appears to be derived from the tonoplast membrane. This tonoplast membrane-bound sorbitol oxidase may play an important role in the transport of vacuolar sorbitol into the cytoplasm, rather than in the transport of sorbitol into the vacuole. About 10% of the enzyme activity also occurred in the subcellular fraction having a density of 1.12–1.16, which coincided with the peaks of acid phosphatase and ATPase activities. Therefore sorbitol oxidase may also be associated with the plasma membrane. Furthermore, 30–40% of its activity was located in the interspace between the cell wall and the plasma membrane, or perhaps attached weakly to them. These results suggest that sorbitol is transported into the cytoplasm by being converted to glucose by sorbitol oxidase.

1 This paper is contribution A-138 of the Fruit Tree Research Station.

(Received January 20, 1982; Accepted May 18, 1982)
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