Enzyme thymidine kinase

All NRTIs, as exemplified for AZT (Fig. 7), act in a similar fashion following their uptake by the cells, they are phosphorylated successively to their 5 -monophosphate, 5 -diphosphate, and 5 -triphosphate form (De Clercq 2002). Unlike the first phosphorylation step in the metabolic pathway of the acyclic guanosine analogues (see above), which is carried out by a virus-encoded enzyme (thymidine kinase), the first as well as the subsequent phosphorylations of the 2, 3 -dideoxynucleosides are carried out by cellular enzymes, that is, a 2 -deoxynucleoside (e.g., dThd) kinase, a 2 -deoxynucleotide (e.g., dTMP) kinase, and a (2 -deoxy)nucleoside 5 -diphosphate (NDP) kinase. 

Acyclovir (acycloguanosine. Fig. 5.221) is a novel type of nucleoside analogue which becomes achvated only in herpes-infected host cells by a herpes-specific enzyme, thymidine kinase. This enzyme inihates conversion of acyclovir initially to a monophosphate and then to the antiviral triphosphate which inhibits viral DNA polymerase. The host cell polymerase is not inhibited to the same extent, and the antiviral triphosphate is not produced in uninfected cells. Ganciclovir (Fig. 5.22J) is up to 100... 

Aciclovir is a member of a group of nucleoside derivatives termed acyclonucleosides, in that there is an incomplete sugar ring. The structural relationship to 2 -deoxyguanosine should be very clear. Aciclovir is converted into its monophosphate by the viral enzyme thymidine kinase - some viruses also possess enzymes that facilitate their replication in the host cell. The viral enzyme turns out to be much more effective than that of the host cell, and conversion is, therefore, mainly in infected cells. The monophosphate is subsequently converted into the triphosphate hy the host cell enzymes. Aciclovir triphosphate inhibits viral DNA polymerase, much more so than it does the host enzyme, and so terminates DNA replication. 

Few pyrimidine bases are salvaged in human cells. However, the pyrimidine nucleosides uridine and cytidine can be salvaged by uri-dine-cytidine kinase, deoxycytidine can be salvaged by deoxycytidine kinase, and thymidine can be salvaged by the enzyme thymidine kinase. Each of these enzymes catalyzes the phosphorylation of a nucleoside(s) utilizing ATP, and forming UMP, CMP, dCMP, and TMP. 

Cells lacking thymidine kinase (TK- cells) can be isolated by treating cell cultures with high concentrations (30 jug/ml) of 5-bromodeoxyuridine, which kills cells containing the enzyme thymidine kinase due to incorporation of large amounts of the analogue into the cells DNA. 

Mutants lacking, or with much reduced levels of the enzyme thymidine kinase survive as this enyzme is not essential. Thus the cell can make dTMP from dUMP using folic acid as the one carbon donor (Fig. 13.1). 

As with purines, there is indirect evidence from studies in vitro that regenerating tetrathyridia of M. corti can synthesise pyrimidines de novo (315). Furthermore, aspartate transcarbamylase, the first enzyme in the pathway, has been demonstrated in Moniezia benedini (39), while five of the six pathway enzymes have been measured in H. diminuta (326). It appears, therefore, that at least some cestodes have the capacity to synthesise pyrimidines by the biosynthetic route. Little is known of pyrimidine salvage pathways in cestodes, although the key enzyme thymidine kinase has been... 

Correct answer = C. Acyclovir is monophospho- rylated in the cel) by the herpesvirus-coded enzyme, thymidine kinase. Thus, uninfected... 

Thymidylate (dTMP) is formed intracellularly either de novo, in a process of the C(5) methylation of 2 -deoxyuridylate (dUMP), catalyzed by the enzyme thymidylate synthase (TS), or as a product of thymidine salvage via phosphorylation, catalyzed by the enzyme thymidine kinase. The dUMP methylation reaction involves a concerted transfer and reduction of the one-carbon group of... 

There are several points of regulation in the synthesis. For example, dCTP inhibits the salvage reaction catalyzed by deoxycytidine kinase and activates the reaction catalyzed by dCMP deaminase. On the other hand, dTTP inhibits the dCMP deaminase reaction and the enzyme thymidine kinase. 

Zidovudine is a synthetic thymidine analog with potent activity against a broad spectrum of retroviruses including HIV-1, HlV-2, and human T-ceU lymphotrophic viruses (HTLV) 1 and II. The drug has no impact in cells already infected with HIV. It is most effective in activated lymphocytes because the phosphorylating enzyme, thymidine kinase, is S-phase specific. 

In HeLa ceils hydroxyurea is an efficient inhibitor of histone synthesis. This action requires protein synthesis and leads to rapid disappearance of cytoplasmatic histone mRNA The effect is not specific for hydroxyurea since suppression of DNA synthesis by arabino-cytosine or temperature-sensitive mutations yields analogous results. Similarly, the synthesis of some enzymes necessary for DNA replication and active in S-phase is altered by hydroxyurea. Increased activity of ribonucleotide reductase in HeLa and in hamster cells and of the salvage enzyme thymidine kinase in HeLa cells and KB cells has been observed, probably as a consequence of the increased fraction of cells in S-phase. Repression occurs for thymidine kinase in human lymphocytes and for ornithine decarboxylase in Chinese hamster fibroblasts whereas no or only slight effects were seen on ribonucleotide reductase in hamster fibroblasts , on thymidylate synthase in extracts from synchronous mouse cells " , and on DNA polymerase in rabbit kidney cells or HeLa cells . ... 

In mammalian cells, however, two distinct patterns of DNA nthetic activity have been observed in the S phase. In human epithelial adenosarcoma cells (Kasten and Strasser, 1966), rat oral mucosal epithelial cells (Hamilton, 1969), and diploid Chinese hamster cells (Klevecz, 1969) there ate two definite maxima in the DNA synthetic activity. In other established heteroploid lines there appears to be one such maximum (Scharff and Robbins, 1965 Mueller and Kajiwara, 1966 Klevecz, 1969). The depression of DNA synthetic activity in the middle of the S phase of diploid Chinese hamster cells has been correlated with a period of increased synthesis of the enzymes thymidine kinase and lactic dehydrogenase (Klevecz, 1969), whereas heteroploid cells show a unimodal rate curve for DNA synthesis and a uniformly increasing rate of protein synthesis (see Fig. 2). 

Boron-containing nucleosides are considered as promising candidates for BNCT because of their potential to be retained in rapidly dividing tumor cells after 5 -monophosphorylation by phosphory-lating enzymes, thymidine kinase 1 and deoxycytidine kinase. Thymidine kinase 1 is pyrimidine... 

Acyclovir is a synthetic purine nucleoside that exhibits in vitro and in vivo inhibition of a nmnber of human viruses. In particular, acyclovir is active against herpes simplex types 1 (HSV-1) and 2 (HSV-2) viruses, varicella zoster virus (VZV), Epstein-Barr virus (EBV), and cytomegalovirus (CMV). The inhibitory activity of acyclovir is highly selective. The enzyme thymidine kinase of normal cells does not effectively use acyclovir, but thymidine kinase encoded by one of the viruses noted... 

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