Nitrogenous Bases and Nucleosides

The purine bases are produced de novo by pathways that use amino acids as precursors and produce nucleotides. Most de novo synthesis occurs in the hver (Fig. 41.2), and the nitrogenous bases and nucleosides are then transported to other tissues by red blood cells. The brain also synthesizes significant amounts of nucleotides. Because the de novo pathway requires six high-energy bonds per purine produced, a salvage pathway, which is used by many cell types, can convert free bases and nucleosides to nucleotides. 

Nucleic Acids. Phosphoms is an essential component of nucleic acids, polymers consisting of chains of nucleosides, a sugar plus a nitrogenous base, and joined by phosphate groups (43,44). In ribonucleic acid (RNA), the sugar is D-ribose in deoxyribonucleic acids (DNA), the sugar is 2-deoxy-D-ribose. 

PCR Incorporated (associated with the Schuman-Tarrant group) manufactures 5-fluorouracil by this process for use in cancer chemotherapy and has had annual sales of over a million dollars for the last several years. This fluorination process, which is selective even above room temperature and may be conducted in H20, CF3C02H, or CF3CH2OH, is a curiosity. The selectivity observed at higher temperatures must involve a strong solvent effect rather than being primarily a feature of the fluorination process itself. A report by Cech and Holz describes the fluorination of nitrogenous bases and associated nucleosides to produce 2 -deoxy-5-fluorouridine derivatives (96). This study requires that a saturated room-temperature solution of molecular fluorine in acetic (or tri-... 

Acid-Base Titration of Nitrogenous Bases, Nucleotides, Nucleosides, and Nucleic Acids... 

Two Nitrogen Atoms (it is self-sub divided into 1,2-Heterocycles, 1,3-Heterocycles Monocyclic Pyrimidines and Hydropyrimidines Except Pyrimidine Nucleoside Bases and Nucleosides, Annulated Pyrimidines Except Purines, Pteridines, and Flavins, Pyrimidine Nucleoside Bases and Purines, Nucleotides and Nucleosides, Nucleic Acids, Pyrazines, and Hydropyrazines). 

As mentioned above, immobilized cells are studied mainly for practical reasons, since they show a number of economic advantages over the use of growing cells or cell suspensions. Production of organic acids is one of the prospective applications of immobilized cells. Another one is related to the release of nitrogenous bases and some nucleosides by immobilized cells. In nitrogen-starved immobilized cells the levels of all metabolites (first of all, nucleotides) are reduced (Leps and Ensign, 1979). It was shown (Ikonnikov et al., 1982) that immobilized cells of propionic acid bacteria, incubated periodically in nitrogen-free medium, released substances of protein and nucleic acid nature, whose quantity decreased with the time of incubation in... 

Excess ultrasonic absorptions due to protolysis and hydrolysis have also been observed for a variety of bases, nucleosides and nucleotides (38a). These proton transfers involve the secondary phosphoric acid function of nucleotides and/or functional groups or aromatic nitrogen atoms in bases and nucleosides. The latter are also observed with nucleic acids (38c). 

Nitrogenous base plus sugar moiety are called nucleosides. Ribonucleic acids (RNA) resemble DNA in that nucleoside monophosphates are joined through phosphodiester bonds. RNAs differ in that the sugars are p-D-ribose units and the pyrimidine uracil is found in place of thymine. Molecular structures and nomenclature for nitrogenous bases, nucleosides, and nucleotides are delineated in Table 2.2. 

Table 2.2 Nitrogenous Bases, Nucleosides, Nucleotides, and Sugars Found in DNA and RNA...

DNA is a polymer composed of monomeric nucleic acids called nucleotides. A nucleotide consists of a nitrogenous base, sugar, and phosphoric acid (whereas a nucleoside consists of only the base and sugar see Fig. A2.4). 

Hi) Phosphoric acid Nucleotides are the phosphoric acid esters of nucleosides (nucleosides contain only pentose sugar and a nitrogen base). 

Nucleotides are the phosphoric acid ester of nucleoside, while nucleosides are compounds in which nitrogenous bases (purines and pyrimidines) are conjugated to the pentose sugar (ribose or deoxyribose) by a b-glycosidic linkage. AMP, ATP, ADP, GMP, CMP, UMP are the examples of mononucleotides. 

Pfitzner and Moffatt noted24 that a severe limitation existed for the synthesis of ketopentose nucleosides owing to their instability in certain media, which was attributed to loss of the nitrogenous base by elimination reactions.2... 

The pyrimidine bases, their derivatives, nucleotides, and nucleosides have been extensively studied by NMR techniques including proton magnetic resonance (PMR), carbon-13, nitrogen-15, and fluorine-19 magnetic resonances as well as nitrogen-14 nuclear qxiadrupole resonance (14N NQR). 

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