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

Abstract

Aldose-6-phosphate reductase (alditol-6-phosphate:NADP 1-oxidoreductase) was isolated and characterized from mature apple leaves (M. domestica cv. Sarkrimson). The enzyme was purified 79-fold. The enzyme catalyzed the following reversible reaction: D-glucose 6-phosphate + NADPH + H+ .dblarw. 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 Km values for D-sorbitol 6-phosphate oxidation and D-glucose 6-phosphate reduction were 3.9 and 20 mM, respectively. AgNO3 (0.1 mM) and p-chloromercuribenzoate (1.0 mM) completely inhibited the enzyme. Aldose-6-phosphate reductase activity was also detected in mature leaves from Golden Delicious and Antonovka apples (M. domestica), Conference and Bartlet pears (Pyrus communis), Redhaven peach (Prunus persica), and Perfection apricot (Prunus armeniaca). The enzyme may have a wide distribution and may play an important role in sorbitol synthesis.