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Published on April 1, 2009; 10.1104/pp.108.135129

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Received January 6, 2009
Accepted March 26, 2009

VTC4 Is A Bifunctional Enzyme That Impacts myo-Inositol and Ascorbate Biosynthesis In Plants

Javad Torabinejad , Janet L. Donahue , Bhadra N. Gunesekera , Matthew J. Allen-Daniels , and Glenda E. Gillaspy *

Department of Biochemistry, Virginia Tech, Blacksburg, VA 24061

* Corresponding author; email: gillaspy{at}vt.edu.

myo-Inositol synthesis and catabolism are crucial in many multiceullar eukaryotes for production of phosphatidylinositol signaling molecules, glycerophosphoinositide membrane anchors, cell wall pectic non-cellulosic polysaccharides, and several other molecules including ascorbate. myo-Inositol monophosphatase (IMP) is a major enzyme required for the synthesis of myo-inositol and breakdown of myo-inositol (1,4,5)-trisphosphate, a potent second messenger involved in many biological activities. It has been shown that the VTC4 enzyme from kiwifruit has similarity to IMP and can hydrolyze L-galactose 1-phosphate (L-Gal 1-P), suggesting that this enzyme may be bifunctional and linked with two potential pathways of plant ascorbate synthesis. We describe here the kinetic comparison of the Arabidopis recombinant VTC4 with D-myo-inositol 3-phosphate (D-Ins 3-P) and L-galactose 1-phosphate (L-Gal 1-P). Purified VTC4 has only a small difference in the Vmax/Km for L-Gal 1-P as compared to D-Ins 3-P, and can utilize other related substrates. Inhibition by either Ca2+ or Li+, known to disrupt cell signaling, was the same with both L-Gal 1-P and D-Ins 3-P. To determine whether the VTC4 gene impacts myo-inositol synthesis in Arabidopsis, we isolated T-DNA knockout lines of VTC4 that exhibit small perturbations in ABA, salt and cold responses. Analysis of metabolite levels in vtc4 mutants showed that less myo-inositol and ascorbate accumulate in these mutants. Therefore, VTC4 is a bifunctional enzyme that impacts both myo-inositol and ascorbate synthesis pathways.






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