Cytosine riboside

Cytosine was first isolated in 1894 from the hydrolyzates of calf thymus. It is one of the four bases in both ribonucleic acid and deoxyribonucleic acids. Hydrolysis via the corresponding nucleotides gives cytosine deoxyriboside (2 -deoxycytidine) and cytosine riboside (cytidine), respectively. 

A molecule in which one of the organic bases is combined with a sugar is called a nucleoside. An example is cytosine riboside ... 

Fig. 11.1.3. Reversed phase HPLC of nucleic acid bases and nucleosides. Chromatographic conditions column, Spherisorb ODS-2, 5 pm (250 x 4.6 mm) mobile phase, 0.05 M monobasic ammonium phosphate, pH 3.5 flow rate, 1.5 ml/min temperature, ambient detection, UV at 260 nm. Peaks C, cytosine U, uracil FU, fluorouracil CR, cytosine riboside A, adenine CdR, cytosine deoxyriboside UR, uracil riboside T, thymine FUR, fluorouracil riboside UdR, uracil deoxyriboside, FUdR, fluorouracil deoxyriboside GR, guanine riboside GdR, guanosine deoxyriboside TdR, thymine deoxyriboside AR, adenine riboside. Reproduced from Miller et al. (1982), with...

Cytidine, Cyd, cytosine riboside, 3-D-ribofumno-syt-cylosine a 3-glycosidic Nucleoside (see) consisting of D-ribose and the pyrimidine base cytosine. M, 243.22, m,p. 220-230 °C (d,), [a]g +34.2°, (c = 2 in water). Cytidine phosphates (see) are metabolically important in all living organisms. 

At the same time that the early studies mentioned above were carried out, it was shown by Loring and Pierce that certain Neurospora mutants utilized uracil and cytosine very poorly in comparison to uridine (uracil riboside) and cytidine (cytosine riboside). These observations led to the suggestion that free pyrimidines were not normal intermediates in nucleic acid biosynthesis. Similar conclusions have been reached by Fries with pyrimidine-deficient strains of the mold Ophiostoma muUiannulatum. ... 

Stokes shift of Sequence 2. This sequence is identical to the normal one except for the replacement of a cytosine one base pair removed from the coumarin site by an abasic riboside (Table 1). 

Until recently the lactol ring structure of 2-desoxy-D-ribose in nucleic acid had been proved conclusively only for the thymidine nucleoside component and in this case it was furanose in form.26 Subsequently Brown and Lythgoe,27 by application of the periodate oxidation procedure to the 2 -desoxy ribosides of guanine, hypoxanthine, cytosine and thymine, afforded proof of the presence of a furanose sugar in each compound. 

The names of nucleosides are derived from the names of the corresponding heterocyclic bases combined with the name of the sugar (ribose or de-oxyribose). In some cases, special names are used for example, cytidine is cytosine-jS-D-riboside. 

Both purine and pyrimidine bases are methylated in RNA. With adenine, the addition of one intact methyl group leads to either the formation of a carbon-to-carbon bond involving the carbon 2 of adenine, or a nitrogen-carbon bond involving the Ne or N of adenine. Guanine is methylated in N, N2, and N7 uracil and cytosine form carbon-to-carbon bonds between CH3 and their carbon 5. Hypoxanthine is methylated on its N dimethyl derivatives of adenine (N6) and guanine (N2) have also been observed. Two methylated bases have been found in DNA 6 methylated aminopurine and 5 methylcytosine. In addition, methyl ribosides have been found in tRNA. 

Two types of rRNA methylases have been purified, one with catalytic properties restricted to rRNA, and another that acts on both rRNA and tRNA. A bacterial preparation capable of methylating either adenine or cytosine of DNA has been purified. But although it is most likely that the enzyme methylating adenine (N-C bond) is different from that methylating cytosine (C-C bond), successful separation has not been accomplished. Almost nothing is known of the enzymes catalyzing 0 methyl riboside formation. 

Since Hammarsten s results showed that not only the pyrimidines of PNA but also those of DNA were labeled after administration of labeled cytidine, it was concluded that this was indirect evidence for the conversion of a pyrimidine riboside to a pyrimidine desoxyriboside. The reason for this was that cytidine could not have been split to cytosine and then reincorporated into desoxycytidine, since it had been shown earlier that the free base, cytosine, could not be utilized for the synthesis of nucleic acid pyrimidines. The possibility should not be overlooked, however, that the above conversion may possibly occur at the nucleotide level thus, cytidylic acid, in which the phosphate is attached to the nucleoside, may be the intermediate in the transformation of PNA to DNA pyrimidines. 

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