Arabinose enzymic phosphorylation

Deoxy-3-fluoro-D-glucose (see Section 11,2), a weak substrate for yeast hexokinase, is phosphorylated enzymically - to give the 6-phosphate 588, which is transformed into 2-deoxy-2-fluoro-D-arabinose 5-phos-phate (589) by lead tetraacetate oxidation. 

The data in Table III indicate that the enzyme KD0-8-phos-phate synthase is more difficult to inhibit with phosphorylated substrate analogues than is D-arabinose-5-phosphate isomerase. Non-phosphorylated substrate analogues (Tables II) were also tested as inhibitors or substrates of KDO-8-phosphate synthase but as with D-arabinose-5-phosphate isomerase, these analogues were... 

The third enzyme in the pathway, KD0-8-phosphate phosphatase, has been purified to homogeneity (26). Because of its abosolute specificity, it should be a focal point for chemotherapeutic studies. jThe apparent for KD0-8-phosp te was+ etermined to be 5.8 x 10 M in the presence of 1.0 mM Co or Mg. This specific KD0-8-phosphate phosphatase was separated from enzymes, present in crude extracts, having phosphatase activity on other phosphorylated compounds by column chromatography on DGAE-Sephadex (26). Three distinct peaks of activity were detected. Fractions from each peak were pooled and the rates for the hydrolysis of five compounds were measured. Peak A possessed phosphatase activity for D-glucose-6-phosphate, D-arabinose-5-phosphate, D-ribose-5-phosphate and j-nitrophenylphosphate Peak B dephosphorylated D-arabinose-5-phosphate, D-ribose-5-phosphate and D-glucose-6-phos-phate. Peak C, which was well separated from the other two peaks, could only utilize KD0-8-phosphate as a substrate. KD0-8-phos-phate was not hydrolyzed by the phosphatases present in peaks A and B. 

The isolation and purification of the enzymes involved in KDO biosynthesis was necessary for the study of potential inhibitors of the individual reactions. A number of phosphorylated substrate analogues were synthesized and tested as inhibitors of D-arabinose-5-phosphate isomerase since this enzyme is the first direct reaction involved in KDO biosynthesis. (We have found no evidence that D-arabinose-5-phosphate is required in any other biosynthetic reaction in E. coli and Salmonella typhimurium it... 

Cytarabine (ara-C) is an arabinose analog of cytosine. Cytarabine was originally isolated from sponges, but is now produced synthetically. Ara-C is phosphorylated to its active triphosphate form (ara-CTP) within tumor cells. Ara-CTP inhibits DNA polymerase, an enzyme responsible for strand elongation. It is also incorporated directly into DNA, where it inhibits the replication of DNA and acts as a chain terminator to prevent DNA elongation. Activation of ara-C is opposed by deaminase enzymes, particularly cytidine deaminase, which degrades ara-C to an inactive form, ara-U. " " ... 

The substrate specificity of this enzyme has not been thoroughly examined for synthetic application. In the example given below the expensive arabinose S-phosphate was generated from arabinose by hexoki-nase-catalyzed phosphorylation (Scheme 9). 

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