Uridylic acid dinucleotides

For uridylic acid dinucleotides (UpU) and short chain polynucleotides (oligo U), a marked protective effect against e aq attack is observed. The reactivity per nucleotide has been measured as a function of pH for UpU and for an oligo U molecule which contains about 20 nucleotides. The results are compared with those for uracil in Figure 5. In the absence... 

Imidazolides of adenylic acid (ImpA) or uridylic acid (ImpU) are polycondensed to oligonucleotides by means of Zn2+ ions. 1673 The resulting phosphordiester bond was found to be of the 2, 5 type. In the reaction of nucleoside 5 -phosphoric acid methyl ester with ImpA in the presence of MgC, 2, 5 -dinucleotides are formed six to nine times more frequently than the corresponding 3, 5 compounds. 63 Polycondensations of ImpA in aqueous solution in the presence of various divalent metal ions lead to short oligo-adenylic acids (pA) (n = 1—5) mainly with 2, 5 -intemucleotide linkages. With Pb2+, for example, the total yield of oligomers was as high as 57%. 1683 1693... 

It would be of interest to halt the hydrolysis when liberation of guanosine and adenosine approaches a maximum, and determine whether the pyrimidine nucleotides are present as a dinucleotide or as the two mononucleotides. It is not clear whether the action of the non-specific phosphatase on an artificially-prepared, equimolecular mixture of the four mononucleotides has been studied (although the individual mononucleotides have been so examined by Bredereck, Beuchelt and Richter ), but Kobayashi has found that guanylic acid is hydrolyzed more readily than adenylic acid which, in turn, is hydrolyzed more readily than the pyrimidine nucleotides. Furthermore, Bredereck, et oZ. have shown that mild chemical hydrolysis of ribosenucleic acid with aqueous pyridine at 100° gives guanylic acid (G) plus a trinucleotide composed of adenylic (A), cytidylic (C), and uridylic (U) acids. On further hydrolysis in aqueous pyridine, adenylic acid is split off. Hence, in ribosenucleic acid, (G) is at one end of the molecule and, in the trinucleotide, (A) is at one end of the molecule. Possible formulas for the tetranucleotide are therefore... 

The first step in de novo pyrimidine biosynthesis is the synthesis of carbamoyl phosphate from bicarbonate and ammonia in a multistep process, requiring the cleavage of two molecules of ATP. This reaction is catalyzed by carbamoyl phosphate synthetase (CPS), and the bicarbonate is phosphorylated by ATP to form carboxyphosphate and ADP (adenine dinucleotide phosphate). Ammonia then reacts with carboxyphosphate to form carbamic acid. The latter is phosphorylated by another molecule of ATP with the mediation of CPS to form carbamoyl phosphate, which reacts with aspartate by aspartate transcarbamoy-lase to form A-carbamoylaspartate. The latter cyclizes to form dihydroorotate, which is then oxidized by NAD-1- to generate orotate. Reaction of orotate with 5-phosphoribosyl-l-pyrophosphate (PRPP), catalyzed by pyrimidine PT, forms the pyrimidine nucleotide orotidylate. This reaction is driven by the hydrolysis of pyrophosphate. Decarboxylatin of orotidylate, catalyzed by orotidylate decarboxylase, forms uridylate (uridine-5 -monophosphate, UMP), a major pyrimidine nucleotide that is a precursor of RNA (Figure 6.53). 

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