Nucleosides enzymic

Thymidine phosphorylase can also use deoxyuridine as substrate [161-163], and the purine nucleoside enzyme can use either the ribonu-cleoside or the deoxyribonucleoside forms of adenine or guanine [115,164], Uridine phosphorylase (EC 2.4.2.3) is a separate entity and will not be considered here, since its regulation is not clearly understood. The four enzymes under consideration are interrelated in function and operate in concert in the regulation of nucleoside catabolism. The mechanisms of their regulation evolved from a number of independent and seemingly devious observations and events, the essence of which may be summarized as follows. 

Adenosine. Adenosine [58-61-7] (Ado), (29), a purine nucleoside, is an intracellular constituent acting as both an enzyme cofactor... 

Basically, AZT is anabohcaHy phosphorylated to AZT mono-, di-, and tri-phosphates by various enzymes (kinases) of a target ceU (159). AZT-triphosphate competes with other phosphorylated pyrimidine nucleosides for incorporation into HIV DNA by the viral reverse transcriptase. Incorporation of the AZT-triphosphate into reverse transcriptase results in viral DNA chain termination. Reverse transcriptase is essential in the repHcative cycle of HIV. 

Another dideoxypyrimidine nucleoside active against human immunodeficiency vims is 3 -azido-2/3 -dideoxyuridine [84472-85-5] (AZDU or CS-87, 64) C H N O. Since its synthesis, (167) CS-87 has been identified as a promising antiHIV agent (168) and is currentiy undergoing phase I clinical trials in patients with AIDS and AIDS-related complex. It appears to be less potent than AZT against HIV in a peripheral blood mononuclear (PBM) cell screening system and in MT-4 cell lines. This lower activity in PBM cells appears to be related to a lower affinity of CS-87 for the enzyme responsible for its initial phosphorylation (169). However, CS-87 has significantly lower toxicity on bone marrow cells than AZT (170) and penetration of the CNS as a 5 -dihydropyridine derivative. 

The search for new antivkal agents is ongoing and extensive not all vimses have been included here, and new vimses pathogenic to humans will continue to be identified. Novel nucleosides as weU as nonnucleosidic compounds which possess greater potency and enzymic specificity for the future treatment of both acute and persistent human vkal infection will continue to be discovered. 

FIGURE 23.6 The PEP carboxykinase reaction. GTP formed in this reaction can be converted to ATP by nucleoside diphosphate kinase, althongh liver cells in some species may not contain this enzyme. 

Sugar nucleotides are formed from sugar-l-phosphates and nucleoside triphosphates by specific pyrophosphorylase enzymes (Figure 23.18). For example, UDP-glucose pyrophosphorylase catalyzes the formation of UDP-glucose from glucose-l-phosphate and uridine 5 -triphosphate ... 

The bromide (2a) reacted smoothly with 2,4-diethoxy-5-methylpyrimi-dine to give, after de-ethylation and deacylation, l-(2-deoxy-/ -D-arabino-hexopyranosyl)thymine (3) (15). The new nucleoside (3) is the first truly competitive inhibitor of a pyrimidine phosphorylase (7), that is, it inhibits the phosphorylase, yet is not a substrate for the enzyme. It was recently shown that 3 enhances the incorporation of 2 -deoxy-5-iodouridine in vivo in cats (8). 

The nucleic acids DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are biological polymers that act as chemical carriers of an organism s genetic information. Enzyme-catalyzed hydrolysis of nucleic acids yields nucleotides, the monomer units from which RNA and DNA are constructed. Further enzyme-catalyzed hydrolysis of the nucleotides yields nucleosides plus phosphate. Nucleosides, in turn, consist of a purine or pyrimidine base linked to Cl of an aldopentose sugar—ribose in RNA and 2-deoxyribose in DNA. The nucleotides are joined by phosphate links between the 5 phosphate of one nucleotide and the 3 hydroxyl on the sugar of another nucleotide. 

Enzymes a) citrate synthase b) aconitase c) isocitrate dehydrogenase d) a-oxoglutarate dehydrogenase e) succiny CoA synthetase f) succinate dehydrogenase g) fumarase h) malate dehydrogenase i) nucleoside diphosphokinase. 

Decitabine (5-aza-deoxycytosine) is an analog of the nucleoside 2 -deoxycytidine. It is believed to exert its antineoplastic effects after phosphorylation and direct incorporation into DNA and by inhibition of the enzyme DNA methyltransferase, causing hypomethylation of DNA and cellular differentiation or apoptosis. DNA hypomethylation is achieved at concentrations below those required to significantly inhibit DNA synthesis, which may promote restoration of function to genes associated with control of cellular differentiation and proliferation. Cytotoxicity in rapidly dividing cells may also result from covalent adducts between DNA methyltransferase and decitabine. 

HBV infection remains a major worldwide public health problem. The World Health Organization estimates that there are still 350 million chronic carriers of the vims, who are at risk of developing chronic hepatitis, liver cirrhosis, and hepatocellular carcinoma. The success of IFN-a treatment - mainly performed as combined treatment with adenine-arabinoside - has been measured by the normalization of liver enzymes, loss of HBe antigen and of detectable viral DNA in the serum of patients. It has been estimated from several clinical trials that as many as 40% of treated HBV patients would respond to therapy with IFN-a or combined treatment with nucleoside analogues and IFN-a. 

All NRTIs, as exemplified for AZT (Fig. 7), act in a similar fashion following their uptake by the cells, they are phosphorylated successively to their 5 -monophosphate, 5 -diphosphate, and 5 -triphosphate form (De Clercq 2002). Unlike the first phosphorylation step in the metabolic pathway of the acyclic guanosine analogues (see above), which is carried out by a virus-encoded enzyme (thymidine kinase), the first as well as the subsequent phosphorylations of the 2, 3 -dideoxynucleosides are carried out by cellular enzymes, that is, a 2 -deoxynucleoside (e.g., dThd) kinase, a 2 -deoxynucleotide (e.g., dTMP) kinase, and a (2 -deoxy)nucleoside 5 -diphosphate (NDP) kinase. 

The molyhdopterin cofactor, as found in different enzymes, may be present either as the nucleoside monophosphate or in the dinucleotide form. In some cases the molybdenum atom binds one single cofactor molecule, while in others, two pterin cofactors coordinate the metal. Molyhdopterin cytosine dinucleotide (MCD) is found in AORs from sulfate reducers, and molyhdopterin adenine dinucleotide and molyb-dopterin hypoxanthine dinucleotide were reported for other enzymes (205). The first structural evidence for binding of the dithiolene group of the pterin tricyclic system to molybdenum was shown for the AOR from Pyrococcus furiosus and D. gigas (199). In the latter, one molyb-dopterin cytosine dinucleotide (MCD) is used for molybdenum ligation. Two molecules of MGD are present in the formate dehydrogenase and nitrate reductase. 

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