Characterization of the main NADPH-generating enzymes from apple fruits in model experiments

摘要

Effects of the medium composition, reducing agents, and EDTA on the activity of two NADPH-generating enzymes-malic enzyme (EC 1.1.1.40) and glucose-6-phosphate dehydrogenase (G6PDH, EC 1.1.1.49), the key enzymes of malic acid metabolism and the pentose phosphate pathway, respectively-were studied in model experiments on disks of apple fruit subepidermal tissue. Sucrose solutions appeared the most favorable medium for studying these enzymes. Both enzymes were appreciably inhibited in distilled water. Even stronger inhibition was observed in mannitol solutions. Malic enzyme was stimulated in phosphate buffer. Ascorbic acid, 2-mercaptoethanol, Na_2S_2O_5, and EDTA added to sucrose solution prevented tissue browning and inhibited both NADPH-generating enzymes.