Ribonucleotide phosphohydrolase

Nucleotidase (5 -ribonucleotide phosphohydrolase, EC 3.1.3.5) is widely distributed in nature and a voluminous literature has appeared in the past decade on the enzyme from vertebrate tissues, seminal fluid, snake venoms, yeasts, and bacteria. Studies regarding the discovery and early investigations of the enzyme have been reviewed by Heppel (1) and... 

The gradient pattern is highly reproducible. We have also demonstrated that UDP-galactose GM2 galactosyltransferase is similarly located in this gradient peak (48). The peak galacto-syltransferase activities also corresponded to peaks of UMP-ase ( -ribonucleotide phosphohydrolase, EC 3.1.3.5) and UDP-phospho-hydrolase (nucleosidediphosphate phosphohydrolase, EC 3.6.16) activities which are marker enzymes for the endoplasmic reticulum. 

One of the best examples of an enzymatic dephosphorylation for a synthetic purpose is shown in the entry 5 ofTable 13-6. A 5 -ribonucleotide phosphohydrolase was used in the synthesis of (-)-aristeromycin, a carbocyclic analog of adenosine. The (-)-enantiomer of aristeromycin shows some cytostatic and antiviral activity, while the (+)-enantiomer is inactive. The racemate ( )-5 -phosphorylated aristeromycin was resolved by selective hydrolysis of the (-)-enantiomer with the hydrolase. The (-)-alcohol and the (+)-5 -phosphate derivative were separated easily on a silica gel column. Hydrolysis of the (-t-)-enantiomer with calf intestinal phosphatase yielded pure (+)-alcohol. 

The more diffuse peak which spread throughout the 0.8 M-sucrose band had the characteristics of plasma membranes in that there was a high acetylcholinesterase and 5 -nucleotidase (5 -ribonucleotide phosphohydrolase, EC 3.1.3.5) activity. Using the intact P2 fraction, complete with myelin, as starting material and especially tailored gradients, the myelin and plasma membranes can be separated from each other and from the lighter synaptosomal peak. 

Figure 6.63 Isoprenoid cytokinin biosynthesis pathway in Arabidopsis. IPT, phosphate-isopentenyl transferase AK, adenosine kinase 1, phosphatase 2,5 -ribonucleotide phosphohydrolase 3, adenosine nucleosidase 4, purine nucleoside phosphorylase 5, zeatin cis-trans isomerase and 6, zeatin reductase.

Mercaptonurine - 5-Formamide-l-B-ribofuranosylthioimidazole-4-carboxamide 2, 3, 5 -triformate, is comparable with 6-mercaptopurine (6-MP) for the treatment of leukemias.33 it is cross-resistant with 6-MP. Conversion of 6-MP ribonucleoside to 6-MeMP ribonucleotide has been shown in L-1210 leukemia cells, adenocarcinoma 755 cells, H. Ep. No. 2 cells and other systems. 6-MeMP ribonucleotide is responsible for the inhibition of purine synthesis produced by 6-MP.3 Metabolism of 6-MeMP in Ehrlich ascites cells has been studied.Breakdown of the active nucleotide form of the 6-thlopurines by alkaline phosphohydrolase is probably responsible... 

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