Thymine deoxyriboside

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...

The most important pyrimidine derivatives are those upon which biological organisms depend. Cytosine 1018 and uracil 1019 are found in ribonucleic acid (RNA) in the form of their ribonucleotides, cytidine 1020 and uridine 1021, while in deoxyribonucleic acid (DNA), cytosine and thymine 1022 are found in the form of their 2 -deoxyribonucleotides, 2 -deoxycytidine 1023 and thymidine 1024. 5-Methylcytosine 1025 is also found to a small extent (c. 5%) in human DNA in the form of its 2 -deoxyriboside 1026, and 5-(hydroxymethyl)cytosine-2 -deoxyriboside 1027 has also been detected in smaller amounts <2005CBI1>. Many variants of cytosine and uracil can be found in RNA including orotic acid 1028 in the form of its ribonucleotide orotidine 1029. Other pyrimidine derivatives to have been isolated from various biological sources include 2 -deoxyuridine 1030, alloxan 1031, and toxopyrimidine (pyramine) 1032 (Figure 2). 

The 5-chloro, 5-bromo and 5-iodo derivatives of uracil are base analogues of thymine and, in cellular systems, can replace the latter in DNA u >. Furthermore, 5-iodo-2 -deoxyuridine is an antiherpes agent currently used for treatment of ocular herpes keratitis. By contrast, 5-fluorouracil can replace uracil in RNA, and, together with 5-fluoro-2 -deoxyuridine, is employed in tumour chemotherapy. All the foregoing are also known mutagens, and 5-bromouracil and its deoxyriboside are widely employed in studies on mutagenesis U9). 

Figure 1.21 Difluorotoluene (36), difluorotoluene deoxyriboside (38), thymine (37), thymidine (39) and adenine (40).

It has been shown that 2,3-difluorotoluene (36) serves as a nonpolar shape mimic of thymine (37) and difluorotoluene deoxyriboside (38) is, surprisingly, an excellent nonpolar nucleoside isostere of thymidine (39) (see Fig. 1.21) [126, 127], Thus, the nucleotide of 36 was incorporated into DNA by several high-fidelity DNA polymerases [126, 128],... 

However, for adenine, guanine, and uracil, the dominant route of anabolism is by way of their ribonucleotide derivatives and traffic along the deoxyribosidic route is not ordinarily significant. Because cytosine is not a substrate for nucleoside phosphorylases, incorporation by the phos-phorylase-kinase route is not possible for this base. The other pyrimidine base of DNA, thymine, is poorly anabolized by both animal and bacterial cells, in spite of the fact that most cells possess thymidine phosphorylase, the action of which is readily reversible. This suggests that ordinarily cellular supplies of deoxyribose 1-phosphate are not available for base anabolism. Experiments are cited below in which it was demonstrated that a significant contribution to the biogenesis of deoxyribose of DNA in E. colt cells did not occur by a route other than ribonucleotide reduction. 

The reduced DNA synthesis observed in vitamin Bn-deficient cells is not only due to the low RNR activity responsible for the synthesis of deoxyribo-nucleotide triphosphates. It is known (Andreeva, 1974) that CHsCbl is involved in the synthesis of the thymine intermediate of DNA. In vitamin Bi2-deflcient cells the DNA content increased upon adding thymine to the medium, which had no effect in the case of Bn -cells (Fig. 5.9, Table 5.3) (Iordan et al, 1979a). It follows, then, that the DNA of propionic acid bacteria is linked with Bi2-coenzymes in two ways through the synthesis of deoxyribosides and through the synthesis of thymine. In addition, we suggest that there is a third type of this connection, mediated by DNA methylation. 

N -Deoxyribosides of cytosine and thymine were prepared biosyntheti-cally from the DNA of E. colt which had been allowed to grow on an N -containing medium (319). When isotopic deoxycjrtidine was injected into rats, it was not incorporated into RNA pyrimidines, which indicated that the transformation of cytidine to deoxycytidine was not reversible (330). However, labeled deoxycytidine served as a precursor of both DNA thymine and cytosine, while labeled thymidine was found solely in thymine. Thus, the conversion of deoxycytidine to DNA thymine was another irreversible step. 

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