Regulation of apple leaf aldose-6-phosphate reductase activity by inorganic phosphate and divalent cations

Aldose-6-phosphate reductase (A6PR), a key enzyme in sorbitol biosynthesis, has been purified to apparent homogeneity from fully developed apple (Malus domestica Borkh. cv. Gala) leaves. Inorganic phosphate inhibited A6PR by decreasing the maximum velocity of the enzyme and by increasing the K_m for the substrate, glucose-6-phosphate (Glc6P). Divalent cations including Ca²⁺, Mg²⁺, Zn²⁺ and Cu²⁺ altered A6PR activity. Effects of Ca²⁺ and Mg²⁺ on A6PR activity were dependent upon both the metal ion concentration and the concentration of Glc6P. The activity of A6PR was increased by 0.5-5 mM Ca²⁺ or Mg²⁺ when Glc6P concentration was below 10 mM. However, these same metal ions decreased A6PR activity at greater Glc6P concentrations or in the presence of higher metal ion concentrations. A6PR displayed Michaelis-Menten kinetics either in the presence or absence of 2.5 mM MgCl₂, but the apparent K_m for Glc6P decreased from 11.3 mM for the control to 5.1 mM in the presence of 2.5 mM MgCl₂ in the assay mixture. By contrast, Zn²⁺ and Cu²⁺ dramatically inactivated A6PR activity. A6PR activity was decreased approximately 50 and 70%, respectively, when the enzyme was pre-incubated with 2 mM Zn²⁺ or Cu²⁺ for 60 min at room temperature. This inactivation 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 A6PR from inactivation by Zn²⁺. The above results suggest that both Mg²⁺ and Ca²⁺ were mixed-type, non-essential activators of A6PR that decreased the K_m for sugar-phosphates but lowered the overall V_max. The physiological significance of these findings is also discussed.