Submitted to: Functional Plant Biology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/14/2002
Publication Date: 3/1/2003
Citation: Zhou, R., Sicher Jr, R.C., Cheng, L., Quebedeaux, B. 2003. Regulation of apple leaf aldose-6-phosphate reductase activity by inorganic phosphate and divalent cations. Functional Plant Biology. 30:1037-1043. Interpretive Summary: One solution to rising atmospheric concentrations of carbon dioxide is to promote photosynthesis by trees. Trees are capable of removing large amounts of carbon dioxide from the air. However, photosynthesis can create excess levels of various sugars in leaves that inhibit the fixation of carbon dioxide. Many economically important fruit-bearing tree species, including apple, pear, peach and apricot, are unusual in that sorbitol is the main product of photosynthesis. Sorbitol is not a sugar so apple trees may be resistant to inhibition of photosynthesis. The present study investigated biochemical properties of sorbitol dehydrogenase, the enzyme thought to be responsible for sorbitol synthesis in apple leaves. Results showed that sorbitol dehydrogenase was activated by certain metals, calcium, magnesium and manganese, but was inhibited by others, namely zinc and copper. However, compounds similar to sorbitol protected sorbitol dehydrogenase from zinc inactivation. This may mean that the enzyme has a metal binding site near the active center. These results should be of interest to enzymologists, horticulturists and plant physiologists working in the area of photosynthesis and global climate change.
Technical Abstract: Aldose-6-phosphate reductase (A6PR) has been purified to apparent homogeneity from apple (Malus domestica Borhk. cv. Gala) source leaves. A6PR activity was increased by 0.5 to 5 mM Ca2+, Mg2+, or Mn2+ but these same metal ions inhibited enzyme activity at higher concentrations. Increased enzyme activity by divalent metal ions also was dependent on glucose-6-phosphate (Glc6-P). A6PR displayed Michaelis- Menten kinetics and the apparent Km for Glc6-P decreased from 12.2 to 2.8 mM when 2.5 mM MgCl2 was added to the assay. However, the Vmax of A6PR in assays using 50 mM Glc6-P was decreased by 6-10% in assays with 2.5 mM MgCl2. Also, 2.5 mM MgCl2 lowered the Km for sorbitol 6-phosphate from 3.1 to 1.3 mM when A6PR activity was measured in the oxidative direction. In contrast to the above, enzyme activity was decreased about 50 and 70%, respectively, when enzyme preparations were preincubated with 2 mM Zn2+ and Cu2+ for 60 min at room temperature. The inactivation of A6PR activity by 2 mM Zn2+ was partially reversed by dialysis or by chelation with 20 mM EDTA. NADPH and NADP, which are substrates for A6PR in the oxidative and reductive directions, respectively, partially protected the enzyme from inactivation by Zn2+. The above findings suggested that Mg2+ and Ca2+ were mixed-type nonessential activators of A6PR that decreased the Km for sugar-phosphates and lowered the overall Vmax.