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BIOCHEMICAL PROCESSES AND MACROMOLECULAR STRUCTURES

VTC4 Is a Bifunctional Enzyme That Affects Myoinositol and Ascorbate Biosynthesis in Plants^1,2,[W],[OA]

Department of Biochemistry, Virginia Tech, Blacksburg, Virginia 24061

Myoinositol synthesis and catabolism are crucial in many multiceullar eukaryotes for the production of phosphatidylinositol signaling molecules, glycerophosphoinositide membrane anchors, cell wall pectic noncellulosic polysaccharides, and several other molecules including ascorbate. Myoinositol monophosphatase (IMP) is a major enzyme required for the synthesis of myoinositol and the breakdown of myoinositol (1,4,5)trisphosphate, a potent second messenger involved in many biological activities. It has been shown that the VTC4 enzyme from kiwifruit (Actinidia deliciosa) 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 Arabidopsis (Arabidopsis thaliana) recombinant VTC4 with D-myoinositol 3-phosphate (D-Ins 3-P) and L-Gal 1-P. Purified VTC4 has only a small difference in the V_max/K_m for L-Gal 1-P as compared with 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 myoinositol synthesis in Arabidopsis, we isolated T-DNA knockout lines of VTC4 that exhibit small perturbations in abscisic acid, salt, and cold responses. Analysis of metabolite levels in vtc4 mutants showed that less myoinositol and ascorbate accumulate in these mutants. Therefore, VTC4 is a bifunctional enzyme that impacts both myoinositol and ascorbate synthesis pathways.

² This paper is dedicated to the memory of Jocelyne Couture-Nowak, a lover of plants.

The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantphysiol.org) is: Glenda E. Gillaspy (gillaspy{at}vt.edu).

^[W] The online version of this article contains Web-only data.

^[OA] Open Access articles can be viewed online without a subscription.

www.plantphysiol.org/cgi/doi/10.1104/pp.108.135129

^* Corresponding author; e-mail gillaspy{at}vt.edu.

Received January 6, 2009; accepted March 26, 2009; published April 1, 2009.