In apple fruit, three types of sorbitol enzymes were certified by column chromatography using Sephadex G-200 gel. The first type was sorbitol-6-phosphate dehydrogenase catalyzing the conversion between sorbitol-6-phosphate and glucose-6-phosphate, which was active in the alkaline region (optimum pH 9.8) and a Km value of 7 mM for sorbitol-6-phosphate. The second type was sorbitol dehydrogenase interconverting sorbitol and fructose, having optimum activity at pH 9.8 and a Km value of 125 mM for

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... orbitol. The third was sorbitol oxidase converting sorbitol to glucose, which had a Km value of 70 mM for sorbitol and was active in the acid region (optimum pH 4.0). The above three enzymes were also found in the apple cotyledon. Fifty-five and 66% of the sorbitol-6-phosphate dehydrogenase and sorbitol oxidase activities respectively were distributed in the subcellular fraction of 100-2, 000 xg. The majority of the sorbitol dehydrogenase activity was contained in the supernatant. In the fruit flesh sorbitol dehydrogenase showed the highest activity of the three enzymes. In the cotyledon, sorbitol-6-phosphate dehydrogenase activity was highest among the activities of the three enzymes. Sorbitol oxidase showed the weakest activity of the three enzymes in both tissues. The roles of these three enzymes in the metabolism of sorbitol were discussed in relation to their distributions in subcellular fractions.