Pyrimidine ribonucleoside 5 -phosphates

The promastigotes of donovani were incubated for 24 hours at a concentration of about 10 cells/ml in the presence of (4-3 S)TPP, (4- 5s)tppr, and (U-l C-ribose)TPPR. A radioactive metabolic product was identified which had a retention time and ultraviolet absorption spectrum identical to authentic 4-thio-pyrazolo(3,4-d)pyrimidine ribonucleoside-5 -phosphate (TPPR-MP). 

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. 

Purine and pyrimidine bases + D-riboses —> D-fS-ribonucleosides + phosphate —> D-f-ribonucleotides —> RNA —> Modified RNA... 

Adenylic acids o and b were each converted by trifluoroacetic anhydride into the same 2 3 -cyclic phosphate (13, R = adenin-9-yl) by Brown, Magrath, and Todd. These authors also applied this method to the synthesis of 2 3 -cyclic phosphates (13) of cytidine, uridine, and guanosine. Hydrolysis of these cyclic phosphates gave the corresponding a and b nucleotide mixtures. Markham and Smith found that, during hydrolysis of ribonucleic acid with pancreatic ribonuclease, the 2 3 -cyclic phosphates of the pyrimidine nucleosides are formed as intermediates leading to the ribonucleoside phosphates b. The also showed that 2 3 -cyclic phosphates (13) are formed by very mild, alkaline hydrolysis of ribonucleic acid. The discoveries of these a and b isomers of mononucleotides from... 

In experiments of this type with animal tissues, Rose and Schweigert (2) and Thomson et al. (S) showed that pyrimidine ribonucleosides were converted to DNA nucleotides without cleavage of the JNT-glycosidic bond. As well, Larsson and Neilands (4) performed a similar type of experiment in which P-phosphate and uniformly labeled C-cytidine were administered to rats with regenerating liver. Both substances were incorporated into the liver polynucleotides which, upon isolation, were degraded to their constituent nucleotides for analysis. Their data (Table 16-1) showed that the four nucleotides of RNA had similar specific activities with respect to P, indicating that the labeled phosphate readily equilibrated with the nucleoside phosphate pool during the experimental period. In this experiment, cytidylate derived from RNA and deoxycytidylate derived from DNA had the same P C ratio. This result indicated that both polynucleotide subunits, deoxycytidylate and cytidylate, were derived from a common precursor, evidently a ribonucleotide. 

Semi-empirical potential-energy calculations have been carried out on purine and pyrimidine ribonucleoside 2, 3 -cyclic phosphates and the 5 -phosphates. ... 

A nucleoside consists of a purine or pyrimidine base linked to a pentose, either D-ribose to form a ribonucleo-side or 2-deoxy-D-ribose to form a deoxyribonucleoside. Three major purine bases and their corresponding ribo-nucleosides are adenine/adenosine, guanine/guanosine and hypoxanthine/inosine. The three major pyrimidines and their corresponding ribonucleosides are cytosine/ cytodine, uracil/uradine and thymine/thymidine. A nucleotide such as ATP (Fig. 17-1) is a phosphate or polyphosphate ester of a nucleoside. 

Esterification of dialkyl phosphates is carried out with diethyl azodicarboxylate and triphenylphosphine (equation 46). This condensation is applicable to phosphorylation of pyrimidine nucleosides. The trichloromethanephosphonic acid derivative (69 equation 47) serves as a phosphorylating agent of ribonucleosides, affording the 2 - and 3 -monophosphates. ° Ethyl vinyl ether assisted the dimeric condensation of diethyl hydrogen phosphate giving tetraethyl pyrophosphate (equation 48). ... 

Purine nucleosides are cleaved by the action of purine nucleoside phosphorylase with the liberation of ribose 1-phosphate (Kl, PI). The enzyme is apparently specific for purines. The material from erythrocytes catalyzes the phosphorolysis of purine but not pyrimidine nucleosides (T6.) Purine phosphorylase activity is found widespread in nature and in many animal tissues (FIO). Friedkin and Kalckar investigated an enzyme capable of cleaving purine deoxynucleosides to the aglycone and deoxy-ribose 1-phosphate. They concluded that the enzyme was identical to that which splits purine ribonucleosides (F8, F9). This enzyme is capable of degrading inosine, xanthosine, and guanosine to forms readily attacked by other enzymes. In so doing, it permits living cells to retain the ribose and deoxyribose moieties. 

There are three distinct PRTases in L. donovani (30). One has its major activity with hypoxanthine and guanine a second with adenine and a third with xanthine. Pyrazolo (3,4-d)pyrimidines, such as allopurinol, are efficient substrates for the HGPRtase. Promastigotes accumulate large quantities of allopurinol ribonucleoside-5 -phosphate when exposed to allopurinol (31). A separate PRTase for xanthine is unusual in a eukaryotic cell. XPRTase is also present in L. mexicana and L. amazonensis as well as in four non-pathogenic trypanosomatids (32,33). 

Ribose-purines and ribose-pyrimidines are particularly important because these structures are found in DNA and RNA. DNA and RNA are known as nucleic acids, and they are biopolymers made up of repeating deoxyribofura-nose or ribofuranose units. Nucleic acids are oligo (sugar phosphates) because the backbone of these compounds is phosphorous. This monomeric miit is called a nucleoside, which is a ribose derivative coupled with a purine or a pyrimidine. A ribofuranose with a purine or pyrimidine at Cl is called a ribonucleoside, or simply a nucleoside. 

Several heterocyclic compounds, including purines and pyrimidines, can be coupled to monosaccharides at the anomeric carbon to form a ribonucleoside (or simply a nucleoside). The most common nucleosides are ribose and 2-deoxyribose derivatives containing adenine, guanine, cytosine, thymine, or uracil. The phosphate ester of a nucleoside is called a nucleotide 30, 31, 34, 51, 59,64,66. 

The purine (Pu or R for short) and pyrimidine (Py or Y for short) bases, are connected by a N-glycosidic linkage to the C-T of the pentose sugar, either a ribose in RNA or a 2 -deoxyribose in DNA. The base-(2 -deoxy)ribose unit is called a nucleoside (ribonucleoside or 2 -deoxyribonucleoside). When a nucleoside is phosphorylated either at the 3 -OH or 5 -OH, it is called 3 -nucleotide (Np) or S -nucleotide (pN), respectively. The number of phosphate groups, for example, at the 5 -OH of adenosine, determines whether the nucleotides are designated as adenosine 5 -wo ophosphate (AMP), adenosine 5 -diphosphate (ADP), and... 

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