Thymidine catabolism

Ml. Marsh, J. C., and Seymour, P., Thymidine catabolism by normal and leukemic human leukocytes. /. Clin. Invest. 43, (1964). 

After intravenous administration, about 80-90% of the dose is catabolized in the liver by dihydropyrimidine dehydrogenase (DPD) [38] (Figure 14.3). The formation of the inactive 5-fluoro-5,6-dihydrouracil (5-FUH2) by DPD is the rate-limiting step of 5-FU catabolism [39]. DPD is widely distributed among tissues, with the highest levels found in the liver. Once 5-FU entered tumor cells, its antitumor effect is mainly dependent on the extent of 5-FU anabolism. After two sequential anabolic steps involving thymidine phosphorylase (TP) and thymidine kinase... 

Fig. 14.3 5 -FU catabolism, anabolism and mechanism of action. 5-FUH2, 5-fluoro-5,6-dihydrouracil 5-FdUMP, 5-fluorodeoxyuridine monophosphate TP, thymidine phosphorylase TK, thymidine kinase TS, thymidylate synthase CH2THF, 5,10-methylenetetrahydrofolate.

Fig. lA. Anabolic and catabolic pathways of 5-FU. DPD dihydropyrimidine dehydrogenase, DP di-hydropyrimidinase, pUP beta-ureidopropionase, UP uridine phosphorylase, OPRT orotate phospho-ribosyl transferase, UK uridine kinase, TP thymidine phosphorylase, TK thymidine kinase, RNR ribonucleotide reductase. The three active metabolites (shown in rectangles) are FdUMP (5-fluoro-2 -deoxyuridine 5 -monophosphate) inhibiting TS (thymidylate synthase), and FUTP (5-fluorouridine 5 -triphosphate) and FdUTP (5-fluoro 2 -deoxyuridine 5 -triphosphate) interfering with RNA and DNA, respectively. 

Thymidylate Synthetase and Dihydrofolate Reductase Methylation of deoxyuridine monophosphate (dUMP) to thymidine monophosphate (TMP see Figure 10.8) is essential for the synthesis of DNA, although preformed TMP can be reutUized by salvage from the catabolism of DNA. 

Zinc is involved in many biochemical functions. Several zinc metal-loenzymes have been recognized in the past decade. Zinc is required for each step of cell cycle in microoragnisms and is essential for DNA synthesis. Thymidine kinase, DNA-dependent RNA polymerase, DNA-polymer-ase from various sources, and RNA-dependent DNA polymerase from viruses have been shown to be zinc-dependent enzymes. Zinc also regulates the activity of RNase, thus the catabolism of RNA appears to be zinc dependent. The effect of zinc on protein synthesis may be attributable to its vital role in nucleic acid metabolism. 

There are thirty-three pyrimidine derivatives identified in tobacco and tobacco smoke. The vast majority of these compounds (67%) have been identified in tobacco. Only fourteen are known to exist in tobacco smoke. As previously mentioned, the naturally occurring derivatives of pyrimidine are components of nucleic acids cytosine, thymidine, and uracil. Free pyrimidine and functionalized pyrimidine compounds in tobacco are believed to be formed from the catabolism of various nucleosides (17B21). Several of the pyrimidine-containing compounds in tobacco are agronomic chemical residues while other compounds identified in tobacco smoke are formed from those agronomic residues. 

Biosynthesis and catabolism of DNA and RNA altered Thymidine incorporation into DNA reduced DNA polymerase activity reduced - E.coli P incorporation into RNA reduced... 

Deoxyuridine and thymidine are substrates for pyrimidine nucleoside phosphorylases, but deoxycytidine (and cytidine) is generally regarded as being inert to phosphorolysis (7) Tarris demonstration of deoxycytidine formation from cytosine in extracts of fish milt is an exception to this generalization (8). Catabolism of C3rtosine nucleosides is initiated by deamination to form uracil nucleosides which can be phosphorolyzed. 

Fig. 14-1. Catabolism of deoxyribonucleosides (1) cytidine deaminase (2) adenosine deaminase (3) thymidine phosphotylase (4) purine nucleoside phosphorylase (5) phosphoribomutase (6) deoxyriboaldolase.

Experiments with animal cells have shown that the pyrimidine bases are much less effective DNA precursors than the corresponding deoxy-ribonucleosides, although the interpretation of such experiments is complicated by the rapid catabolism of uracil and thymine which takes place in liver. The incorporation of thymine into DNA in animals (14), or in in vitro systems (15) is slow and contrasts with the much more rapid incorporation of thymidine. 

Thymidine phosphorylase can also use deoxyuridine as substrate [161-163], and the purine nucleoside enzyme can use either the ribonu-cleoside or the deoxyribonucleoside forms of adenine or guanine [115,164], Uridine phosphorylase (EC 2.4.2.3) is a separate entity and will not be considered here, since its regulation is not clearly understood. The four enzymes under consideration are interrelated in function and operate in concert in the regulation of nucleoside catabolism. The mechanisms of their regulation evolved from a number of independent and seemingly devious observations and events, the essence of which may be summarized as follows. 

In S phase the catabolism increases when the total amount of guanosine taken up by the cells exceeds 600 nmol/10 cells, which coincides with a decrease of thymidine incorporation into DNA. The greater importance of catabolic route in Gi cells may be an expression of the lower guanine phosphoribosy1 transferase activity in this stage of the cell cycle. ... 

Further degradation of the nucleosides to the purine bases by the catabolic activity of nucleoside phospho-rylase has not been detected in the assay system used. While the sum of the nucleotides and nucleo sides formed accurately reflects the enzyme activities, inhibition of the conversion of nucleotides to nucleosides by thymidine-5Hriphosphate (TTP) (5) facilitates the assay by measuring the nucleotides only. Indeed, for normal cells, the accumulation of radioactivity was found to be linear with time and protein concentration, in the nucleotide fraction only when TTP was present, while in the sum of the nucleotide and nucleoside fractions in both the presence and the absence of TTP. 

An alternative activation pathway involves the thymidine phosphorylase catalysed conversion of 1 to Floxuridine (FUDR, 4), which is then phosphorylated by thymidine kinase to give 19. The metabolite of 1 - Floxuridine - is itself used as an anti-cancer agent [9]. It was launched in 1970 by Hospira hic [5]. Upon rapid injection, most of Hoxuiidine is catabolized to Fluorouracil hence similar effects on the organism are obtained in this case. On the contrary, when 4 is slowly administered into the arterial blood, it is mostly transformed to 19 thus toxic effects are diminished comparing to 1 [10]. 

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