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

Functional Plant Biology : FPB
Rui ZhouBruno Quebedeaux

Abstract

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 Km 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 Km 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 in...Continue Reading

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