Characterization and Role of Glucose-6-phosphate Dehydrogenase of Populus suaveolens in Induction of Freezing Resistance

Glucose-6-phosphate dehydrogenase (G6PDH, EC 1.1.1.49) was purified from the leaves of 8-week-old Populus suaveolens cuttings. The enzyme activity in the absence and presence of reduced dithiothreitol (DTT_red) was determined. The results show that the G6PDH activity is not inactivated by pre-incubation with DTT_red, indicating that the purified enzyme probably presented in cytosol of P. suaveolens. The catalytic characteristics and kinetic parameters of cytosolic G6PDH purified from P. suaveolens cuttings were also studied. The results show that G6PDH is characterized by K_m value of 360 mmol·L^-1 for G6P and 16 mmol·L^-1 for NADP, a pH range of 7.3-8.9, and the maximum activity around pH 8.2. The enzyme activity is inhibited by various metabolites such as NADPH, NADH, GTP, UTP, ATP, AMP, ADP, CoA, acetyl CoA, fructose-6-phosphate (F6P), erythrose-4-phosphate (E4P), ribose-5-phosphate (R5P) and 3-phosphoglycerate (3-PG) (all at 1 mmol·L^-1 except for NADPH and NADH) to different extents. NADPH is the most effective inhibitor of enzyme activity, with an inhibition of 72.0%. The addition of metal ions such as MgCl₂, CaCl₂ and KCl (all 1.0 mmol·L^-1) to the standard reaction mixture has no remarkable influence on the cytosolic G6PDH activity. However, CdCl₂ (1.0 mmol·L^-1) causes high inhibitory effect on the enzyme activity. To explore the role of G6PDH on the enhancement of freezing resistance induced by freezing acclimation, the changes in the cytosolic G6PDH activity and freezing resistance (expressed as LT₅₀) of P. suaveolens cuttings during freezing acclimation at -20℃ were investigated. The results reveal that freezing acclimation decreases LT₅₀ of cuttings, and increases the activity of cytosolic G6PDH compared with control ones, while 2 d of de-acclimation at 25℃ result in a decrease in cytosolic G6PDH activity, and caused an increase in LT₅₀. Furthermore, the change in cytosolic G6PDH activity is found to be closely correlated to the degree of freezing resistance of cuttings during freezing acclimation. It is suggested that cytosolic G6PDH may be involved in the induction of freezing resistance of cuttings.