Cytidine nucleotides synthesis

It is an enzyme used in the early stages of cytidine nucleotide synthesis. 

A series of other barbiturates (phenobarbital, barbital, thiopental, pentobarbital at 1 mmol l i concentration inhibit the orotate uptake system without affecting the incorporation of uracil into cellular pyrimidines [287]. While barbituric acid and hexobarbital are less active, phenylethylhydan-toin, chlorpromazine and phenethyl alcohol are extremely active. Phenobarbital also depresses the utilization of orotic acid for the synthesis of cytidine nucleotides in the liver [288]. a-Hexachlorocyclohexane, an inhibitor of the phenobarbital type, was even more effective in depressing de novo cytidine nucleotide synthesis from orotic acid [289]. 

While mammahan cells reutilize few free pyrimidines, salvage reactions convert the ribonucleosides uridine and cytidine and the deoxyribonucleosides thymidine and deoxycytidine to their respective nucleotides. ATP-dependent phosphoryltransferases (kinases) catalyze the phosphorylation of the nucleoside diphosphates 2 "-de-oxycytidine, 2 -deoxyguanosine, and 2 -deoxyadenosine to their corresponding nucleoside triphosphates. In addition, orotate phosphoribosyltransferase (reaction 5, Figure 34-7), an enzyme of pyrimidine nucleotide synthesis, salvages orotic acid by converting it to orotidine monophosphate (OMP). 

Figure 20.9 The positions in the pathway for de novo pyrimidine nucleotide synthesis where GLUCOSE provides the ribose molecule and GLUTAMINE provides nitrogen atoms. Glucose forms ribose 5-phosphate, via the pentose phosphate pathway (see chapter 6), which enters the pathway, after phosphorylation, as 5-phospho-ribosyl 1-pyrophosphate. Glutamine provides the nitrogen atom to synthesise carbamoylphos-phate (with formation of glutamate), and also to form cytidine triphosphate (CTP) from uridine triphosphate (UTP), catalysed by the enzyme CTP synthetase. It is the amide nitrogen of glutamine that is the nitrogen atom that is provided in these reactions.

A classic description of the role of cytidine nucleotides in phospholipid synthesis. 

The common pyrimidine ribonucleotides are cytidine 5 -monophosphate (CMP cytidylate) and uridine 5 -monophosphate (UMP uridylate), which contain the pyrimidines cytosine and uracil. De novo pyrimidine nucleotide biosynthesis (Fig. 22-36) proceeds in a somewhat different manner from purine nucleotide synthesis the six-membered pyrimidine ring is made first and then attached to ribose 5-phosphate. Required in this process is carbamoyl phosphate, also an intermediate in the urea cycle (see Fig. 18-10). However, as we noted... 

The structures of these cytidine nucleotides were confirmed by synthesis. Condensation of 1,2-0-isopropylidene-L-glyceritol 3-phosphate with cytidine 5-phosphate in the presence of dicyclohexylcarbodiimide, followed by careful acid hydrolysis of the isopropylidene group, yielded cytidine glyceritol pyrophosphate identical with the natural product. 

Cidofovir (Figure 24.4) is an antiviral cytidine nucleotide analog with inhibitory activity against HCMV and other herpes viruses. Cidofovir is first converted to an active diphosphate form by cellular enzymes. Antiviral effects of cidofovir are due to inhibition of viral DNA polymerase by the diphosphate metabolite (Neyts and De Clercq, 1994 Plosker and Noble, 1999 Scholar and Pratt, 2000). The diphosphate probably interacts with DNA polymerase either as an alternate substrate (incorporation at the 3 end or within the interior of the DNA chain) or as a competitive inhibitor (with respect to the normal substrate dCTP). Cidofovir inhibits HCMV DNA synthesis at intracellular concentrations 1000-fold lower than are required to inhibit cellular DNA synthesis (Neyts and De Clercq, 1994). For HSV-1 and HSV-2 corresponding concentrations are at least 50-fold lower. 

Cidofovir is an antiviral agent that inhibits viral DNA synthesis by interfering with viral DNA polymerase. It is indicated in the treatment of Cytomegalovirus retinitis in patients with AIDS. Cidofovir (l-[(5)-3-hydroxy-2-(phosphonomethoxy)-propyljcytosine dihydrate) is a cytidine nucleotide analog with inhibitory activity against human herpes, papilloma, polyoma, pox, and adenoviruses. 

The synthesis of deoxyuridine, cytidine, deoxycytidine and thymidine nucleotides from UMP (Fig. 6.13) involves three reactions CTP synthetase, ribonucleotide reductase, and thymidylate synthase (80). The first enzyme converts UTP into CTP and the second catalyzes the conversion of CDP, UDP, ADP and GDP into their respective deoxyribonucleotides. The last enzyme, thymidylate synthase, catalyzes the reductive methylation of deoxyUMP at the C-5 position giving deoxyTMP. The human enzyme has been extensively studied as it is a target enzyme in cancer chemotherapy. Besides these three enzymes, two other enzymes are involved in pyrimidine nucleotide synthesis and interconversion. DeoxyCMP deaminase converts deoxyCMP into deoxyUMP and deoxyUTP triphosphatase converts deoxyUTP into deoxyUMP. Giardia lamblia, and Trichomonas vaginalis lack both ribonucleotide reductase and thymidylate synthase and... 

Radiolabeled cytidine is recovered in both cytidine and uridine nucleotides but not from DNA. Uracil and uridine are used predominantly for uridine nucleotide synthesis. 

Seifert, J. and Vacha, J. Decreased utilization of orotic acid for the synthesis of cytidine nucleotides in rat hver after administration of a-hexychlorocyclohexane. Toxicology, 7, 155-161 (1977)... 

DeoxyribOM phosphates phosphorylated derivatives of the deoxypentose, l-deoxy-n-rihose. They are biosynthesized by reduction of ribose phosphates in the course of nucleotide synthesis There are two enzymes in E. coli which reduce cytidine diphosphate to deoxycytidine diphosphate. Deoxytibose 5- and 1-phosphate form an equilibrium mixture in the presence of phosphopentomutase (EC 2.7.5.6). 

The overall capacity of pyrimidine nucleotide de novo synthesis appears to be higher in rat liver than in rat brain. This can also be concluded from the higher activities of carbamyl phosphate synthetase II and aspartate transcarbamylase in liver (2,4). The liver primarily depends on the de novo pathway for nucleotide synthesis. With liver slices pyrimidine nucleotides are predominantly derived from OA uridine is mainly catabolyzed to uracil and 3-alanine (16) in agreement with high activity of uridine phosphorylase. With brain slices uridine was superior to CO2 or OA in labelling RNA (8). This concords with the relatively high activity of uridine kinase. In vivo, however, cytidine appears to be a more important substrate for nucleotide synthesis (17), since uridine in predominantly catabolyzed by various tissues, including liver. 

The further conversion of UMP to cytidine nucleotides has not been demonstrated at the mononucleotide level. Apparently, uridine triphosphate was aminated by ammonia and ATP in E. coU to form cytidine triphosphate (CTP) no other nitrogen donors were able to perform the amination step (382, 383) (Fig. 22). Since the presence of glutamine was necessary for optimal synthesis of cytidine nucleotides from UMP in a homogenate of a hepatoma (384), it is possible that a different amination reaction exists in mammalian tissues. 

The synthesis of phosphatidylcholine (Ptd-choline) in animal tissues is carried out chiefly by the cytidine nucleotide pathway, although base-exchange reaction and stepwise methylation of preexisting phosphatidylethanolamine (Ptd-ethanolamine) also contribute to its formation "7 The N-methylation pathway, first demonstrated in liver by Bremer and Greenberg and successively described in this tissue by several authors, has not been however unequivocally demonstrated in brain, and conflicting data have been produced in this c onnection, ... 

In this situation, two different polypeptide chains interact to form a new specific complex, whose biologic activity can be modified by ligands that bind to the precursor subunits. This type of regulation has been extensively studied for the aspartate transcarbamylase of E. coli (Gerhart, 1970). This enzyme catalyzed the initial step in the synthesis of cytidine nucleotides it is allosterically inhibited by CTP and shows positive cooperativity for substrate. It may be dissociated by mercurials into catalytic subunits, which are insensitive to CTP and which exhibit hyperbolic kinetics for substrate, and into regulatory subunits which bind CTP. 

The intermediate phosphite employed in this synthesis was prepared by condensation of duly protected sialic acid with the nucleosidyl phospho-roamidite in the presence of N-PhIMT. Oxidation by TBHP and deprotection according to standard procedures gave the cytidine-S -monophospho-iST-ace-tylneuraminic acid. This synthetic route is claimed to have advantages over procedures published earlier [26]. The same group demonstrated the importance of 3 A and 4 A molecular sieves as moisture scavengers in the reaction of nucleoside phosphoroamidite with a nucleotide. This approach should be likely to find application in the synthesis of biophosphates outside nucleotide chemistry. 

During the past 15 years data from experiments with different types of animal tissues and micro-organisms, using intact cells, crude extracts or purified enzymes, have firmly established the general occurrence of nucleotide reductases and have stressed their importance for DNA synthesis in essentially all types of rapidly growing cells [54]. It has been proposed that ribonucleotide diphosphates lose a hydroxide ion from C-2 to form a carbonium ion which is then stero-specifically reduced by a hydride ion derived from thioredoxin [54]. Adenosine diphosphate and guanosine diphosphate (as well as uridine and cytidine diphosphates) are reduced in this manner. 

The enzyme is also responsible for converting cytidine diphosphate (CDP) to 2 -dCDP and uridine diphosphate (UDP) to 2 -dUDP for use in making nucleotides for DNA synthesis. 

Choline and ethanolamine are activated in much the same way as are sugars. For example, choline can be phosphorylated using ATP (Eq. 17-58, step a) and the phosphocholine formed can be further converted (Eq. 17-58, step b) to cytidine diphosphate choline. Phosphocholine is transferred from the latter onto a suitable acceptor to form the final product (Eq. 17-58, step c). Tire polymerization pattern differs from that for polysaccharide synthesis. When the sugar nucleotides react, the entire nucleoside diphosphate is eliminated (Eq. 17-56), but CDP-choline and CDP-ethanolamine react with elimination of CMP (Eq. 

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