Abstract
In higher plants, ascorbic acid (AsA) is mainly synthesized from hexose phosphates via D-mannose/L-galactose pathway. Phosphomannose isomerase (PMI) catalyses the reversible isomerization of D-fructose 6-phosphate and D-mannose 6-phosphate (M6P), as the first step of AsA biosynthesis. Accordingly, PMI may act as a determinant of the carbon partitioning from major carbon metabolisms to AsA biosynthesis. Two genes of PMI (PMI-1 and PMI-2) existed in Arabidopsis. To analyze the enzymological properties of Arabidopsis PMIs, recombinant proteins were expressed from each cDNA in Escherichia coli and purified. The Vmax and Km value for M6P of the recombinant PMI-2 were 21.3 μmol/min/mg protein and 328 μM, respectively. PMI-2 was inhibited by zinc and cadmium ions. Next, we obtained T-DNA insertion Arabidopsis line in the PMI-2 gene. However, the lack of PMI-2 had no effect on the cellular AsA level. Now, we are analyzing enzymological property of PMI-1 and isolating PMI-1 knockout mutants.
