Characterization and Partial Purification of Aldose-6-phosphate Reductase (Alditol-6-Phosphate:NADP 1-Oxidoreductase) from Apple Leaves.

Aldose-6-phosphate reductase (alditol 6-phosphate:NADP 1-oxidoreductase) was isolated and characterized from mature apple leaves (Malus domestica cv. Starkrimson). The enzyme was purified 79-fold. The enzyme catalyzed the following reversible reaction: d-glucose 6-phosphate + NADPH + H(+) right arrow over left arrow d-sorbitol 6-phosphate + NADP(+). No activity was detected when NAD(+) was substituted for NADP(+) or when NADH was substituted for NADPH. The enzyme reduced d-galactose 6-phosphate at a higher rate than d-glucose 6-phosphate. d-Mannose 6-phosphate and 2-deoxy-d-glucose 6-phosphate were reduced at low rates. d-Glucose 1-phosphate, d-fructose 6-phosphate, d-ribose 5-phosphate, d-glucose, and sorbitol did not serve as substrates. The pH optimum for both d-sorbitol 6-phosphate oxidation and d-glucose 6-phosphate reduction was 9.5. The K(m) values for d-sorbitol 6-phosphate oxidation and d-glucose 6-phosphate reduction were 3.9 and 20 millimolar, respectively. AgNO(3) (0.1 millimolar) and p-chloromercuribenzoate (1.0 millimolar) completely inhibited the enzyme.Aldose-6-phosphate reductase activity was also detected in mature leaves from Golden Delicious and Antonovka apples (Malus domestica), Conference and Bartlett pears (Pyrus communis), Redhaven peach (Prunus persica), and Perfection apricot (Prunus armeniaca). This suggests that the enzyme has a wide distribution and plays an important role in sorbitol synthesis.