Myo-INOSITOL 2-DEHYDROGENASE

Formation of the Initial Cyclitol. The first steps of FOR production are the same as in the biosynthesis of STR in short, the first step in FOR biosynthesis is postulated to be the formation of a myo-inositol monophosphate (D-myo-inositol-3-phosphate or L-myo-inositol-1-phosphate) via the cellular l-myo-inositol-1-phosphate synthase as in the STR pathway (Ca pathway see Section 2.2.1.2). As in the itr-Att-clusters, no gene for this enzyme has been found in the/or-cluster. As a second step in FOR biosynthesis the dephosphorylation of D-myo-inositol-3-phosphate via an inositolmonophosphate phosphatase has to follow. A putative gene product with this activity is that of the ForA protein (cf. Tables 2.17 and 2.18). The cyclitol is postulated to be first converted via two enzymes, a cluster-encoded myo-inositol 3-dehydrogenase (ForG member of the GFO/IDH/MocA oxidoreductase family) and the L-glutamine icy//o-3-inosose 3-aminotransferase (ketocyclitol aminotransferase I ForS), to icy//o-inosamine (3-deoxy-3-amino- cy/to-mositol). 

Eukaryotic peptide chain elongation factor-2 Triosephosphate isomerase Myo-inositol-phosphate synthase Mannose-1-phosphate guanylj transferase Alcohol dehydrogenase Aldehyde reductase Ade5... 

Inositol derived by dephosphorylation of purified inositol phosphates may be quantified. Phosphates are removed from the inositol ring, either by acid hydrolysis or by treatment with alkaline phosphatase, and the free inositol is determined by enzymatic conversion to wyo-inose with myo-inositol dehydrogenase and NAD+. The NADH thus formed is oxidized back to NAD+ with oxoloacetate to form malate, which is measured fluorometrically. This coupled reaction is stoichiometric and is sufficiently sensitive to assay 0.2 to 8 nmol of inositol per sample (Dean and Beaven, 1989 Palmer and Wakelam, 1989). 

M. Szaboles, M. Schablik, Separation of Neurospora crassa myo-inositol-1 -phosphate synthase from glucose-6-phosphate dehydrogenase by affinity chromatography. Prep. Biochem. 12(2) (1982), 137-151. 

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