Ribonucleosides 2 , 5 -diphosphates

REDUCTION OF RIBONUCLEOSIDE DIPHOSPHATES FORMS DEOXYRIBONUCLEOSIDE DIPHOSPHATES... 

Figure 34-5. Reduction of ribonucleoside diphosphates to 2 -deoxyribonucleoside diphosphates.

Inhibition of ribonucleoside diphosphate reductase by hydroxyurea. Cancer Res 1968 28 1559-1565. 

In view of the difficulty of hydrolyzing the pyrimidine nucleosidic linkages, ribonucleic acids have been hydrolyzed to a mixture of purine bases and pyrimidine nucleotides which is then separated by paper chromatography.132, 163 164 This method has been employed extensively for the analysis of ribonucleic acids, and gives reproducible results,166 but it has not been used to any great extent for deoxyribonucleic acids, probably because, under these conditions of hydrolysis, they yield some pyrimidine deoxy-ribonucleoside diphosphates.166... 

Hydrolysis of ribonucleic acids by snake venom was found to yield inorganic phosphate, nucleosides, and pyrimidine ribonucleoside diphosphates.197 These diphosphates were shown by their behavior toward various enzymes to be mixtures of 2,5- and 3,5-diphosphates, and it therefore seems likely that they were formed through intermediate, cyclic phosphates. Thus, although this evidence confirms the existence of 2(or 3) — 5 linkages, it does not distinguish between the 2- and 3-positions. 

These enzymes use DNA as a template and the ribonucleotide substrates must be present in the nucleus, i.e. ATP, GTP, CTP and UTP. Similarly, for the synthesis of DNA, the deoxyribonucleotides dATP, dGTP, dCTP and dTTP must be present in the nucleus. In addition, since the ribonucleoside diphosphates are required for synthesis of deoxyribonucleotides, these diphosphates must also be present. The concentrations of these various nucleotides have not been measured in the nucleus but it may be assumed that the concentrations of the ribonucleotides will be similar in the nucleus to those in the cytosol. 

The deoxyribonucleotides, except for deoxythymidine nucleotide, are formed from the ribonucleotides by the action of an enzyme complex, which comprises two enzymes, ribonucleoside diphosphate reductase and thioredoxin reductase (Figure 20.11). The removal of a hydroxyl group in the ribose part of the molecule is a reduction reaction, which requires NADPH. This is generated in the pentose phosphate pathway. (Note, this pathway is important in proliferating cells not only for generation... 

Deoxyrihonucleotides are generally formed by reduction of ribonucleoside diphosphates. This involves a series of redox reactions in which NADP+ and FAD play a role (see Section 15.1.1), with a subsequent electron transport chain. DNA contains thymine rather than uracil, so thymidine triphosphate (dTTP) is a requirement. Methylation of dUMP to dTMP is a major route to thymine nucleotides, and is dependent upon N, A °-methylenetetrahydrofolate as the source of the methyl group (see Box 11.13). 

Ribonucleotide reductase (diphosphate) [EC 1.17.4.1], also known as ribonucleoside-diphosphate reductase, catalyzes the reaction of a 2 -deoxyribonucleoside diphosphate with oxidized thioredoxin and water to produce a ribonucleoside diphosphate and reduced thioredoxin. This system requires the presence of iron ions and ATP. Ribonucleotide reductase (triphosphate) [EC... 

C. Formation of deoxyribonudeotides by reduction of the 2 -hydroxyl group of the ribose sugars on the ribonucleoside diphosphates ADP and GDP is catalyzed by ribonucleotide reductase (Figure 10-3). 

The primary action is inhibition of enzyme ribonucleoside diphosphate reductase. The drug is specific for S phase of the cell cycle and causes cell to arrest at the Gj-S interface. 

Cory, J. G. (1989). Role of ribonucleotide reductase in cell division. In Inhibitors of Ribonucleoside Diphosphate Reductase Activity (J. G. Cory and A. H. Cory, eds.), pp. 1-16. Pergamon, New York. 

The nucleotides described thus far in this chapter all contain ribose (ribonucleotides). The nucleotides required for DNA synthesis, however, are 2 -deoxyribonucleotides, which are produced from ribonucleoside diphosphates by the enzyme ribonucleotide reductase. 

Ribonucleotide reductase ribonucleoside diphosphate reductase) is a multisubunit enzyme (two identical B1 subunits and two identical B2 subunits) that is specific for the reduction of nucleoside diphosphates (ADP, GDP, CDP, and UDP) to their deoxy-forms (dADP, dGDP, dCDP, and dUDP). The immediate donors of the hydrogen atoms needed for the reduction of the 2-hydroxyl group are two sulfhydryl groups on the enzyme itself, which, during the reaction, form a disulfide bond (Figure 22.12). 

Deoxyribonucleotides. A chain involving NADPH, a flavoprotein, thioredoxin, and ribonucleotide reductase converts either the ribonucleoside diphosphates or triphosphates to the corresponding 2-deoxy forms (step j, Fig. 25-14) as indicated in Eq. 25-16. 

Formylpurine thiosemicarbazone is a potent inhibitor of ribonucleoside diphosphate reductase and is active against herpes simplex and cytomegalovirus132), but is nephrotoxic. Attempts, so far unsuccessful, have been made to prepare derivatives which possess an antitumour effect but are less toxic133). 

Hydroxyurea suppresses DNA synthesis by inhibiting ribonucleoside diphosphate reductase, which catalyzes the reduction of ribonucleotides to deoxyribonucleotides. Hydroxyurea is used in chronic cases of granulocytic leukemia that are unresponsive to busulfan. In addition, it is used for acute lymphoblastic leukemia. Hydroxyurea may cause bone marrow depression. 

The other thiosemicarbazones are less well studied and as yet the link between antiviral action and chelation is not fully established. It has been proposed that the chelation of iron(II), a cofactor of ribonucleoside diphosphate reductase, could be the principal mode of action of the thiosemicarbazones300. However, other mechanisms are possible. Investigations of the ESR spectra of copper(II) complexes of thiosemicarbazones has been used to follow the intracellular reactions of the complexes - see Antholine et al.301 for a review. In Ehrlich cells the chelate becomes localized in the cell membrane302. This spectroscopic technique could also be used to monitor the antimala-rial action of 2-acetylpyridine thiosemicarbazones303. ... 

Ribonucleotide reductase uses ribonucleoside diphosphates (ADP, GDP, CDP, and UDP) as substrates and reduces the 2 position of ribose. 

Inhibition of DNA synthesis is brought about by the action of dTTP as an allosteric inhibitor of ribonucleotide reductase (Reichard et al., 1961 Moore and Hurlbert, 1966 Brown and Reichard, 1969 Rummer et al., 1978). This enzyme is responsible for reducing all four ribonucleoside diphosphates (NDP) to the corresponding de-oxyribonucleoside diphosphates (dNDP). It is subject to a complex allosteric control which has been most studied with the bacterial enzyme. Most studies with the mammalian enzyme show it to be similar to the bacterial enzyme (Fig.11.7). 

The control of ribonucleotide reductase activity is affected in the classic feedback fashion by cellular nucleotide concentrations. dATP inhibits the reduction of all four ribonucleoside diphosphates. dTTP inhibits the reduction of only CDP and UDP. ATP is the positive effector for the reduction of these two nucleotides, and both dTTP and dGTP stimulate the reduction of GDP and ADP. Hydroxyurea, an antitumor agent, inhibits ribonucleotide reductase, and this depletes the deoxyribonucleotide supply required for tumor DNA biosynthesis. 

The synthesis of DNA is dependent on a ready supply of deoxyribonucleotides. The substrates for these are the ribonucleoside diphosphates ADP, GDP, CDP, and UDP the enzyme responsible for the reduction of these substrates to their corresponding deoxy derivatives is ribonucleotide reductase, which has thioredoxin as a cosubstrate. 

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